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Message-ID: <20200601122807.630619283@goodmis.org>
Date: Mon, 01 Jun 2020 08:27:34 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: linux-kernel@...r.kernel.org
Cc: Ingo Molnar <mingo@...nel.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Tom Zanussi <zanussi@...nel.org>
Subject: [for-next][PATCH 05/12] tracing: Add histogram-design document
From: Tom Zanussi <zanussi@...nel.org>
Add a new Documentation/trace/histogram-design.rst file describing the
ftrace histogram low-level design, meant to help developers trying to
understand the internals when extending or making use of the hist
triggers for higher-level tools.
This documentation refers to the hist_debug files implemented by
'tracing: Add hist_debug trace event files for histogram debugging' so
users wishing to try out the test examples here should make sure
CONFIG_HIST_TRIGGERS_DEBUG is enabled.
Link: http://lkml.kernel.org/r/256b29c3274bb89a10157c4a8d1a8bce7e74849e.1585941485.git.zanussi@kernel.org
Signed-off-by: Tom Zanussi <zanussi@...nel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@...dmis.org>
---
Documentation/trace/histogram-design.rst | 2115 ++++++++++++++++++++++
1 file changed, 2115 insertions(+)
create mode 100644 Documentation/trace/histogram-design.rst
diff --git a/Documentation/trace/histogram-design.rst b/Documentation/trace/histogram-design.rst
new file mode 100644
index 000000000000..c246753f0ffc
--- /dev/null
+++ b/Documentation/trace/histogram-design.rst
@@ -0,0 +1,2115 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+======================
+Histogram Design Notes
+======================
+
+:Author: Tom Zanussi <zanussi@...nel.org>
+
+This document attempts to provide a description of how the ftrace
+histograms work and how the individual pieces map to the data
+structures used to implement them in trace_events_hist.c and
+tracing_map.c.
+
+Note: All the ftrace histogram command examples assume the working
+ directory is the ftrace /tracing directory. For example::
+
+ # cd /sys/kernel/debug/tracing
+
+Also, the histogram output displayed for those commands will be
+generally be truncated - only enough to make the point is displayed.
+
+'hist_debug' trace event files
+==============================
+
+If the kernel is compiled with CONFIG_HIST_TRIGGERS_DEBUG set, an
+event file named 'hist_debug' will appear in each event's
+subdirectory. This file can be read at any time and will display some
+of the hist trigger internals described in this document. Specific
+examples and output will be described in test cases below.
+
+Basic histograms
+================
+
+First, basic histograms. Below is pretty much the simplest thing you
+can do with histograms - create one with a single key on a single
+event and cat the output::
+
+ # echo 'hist:keys=pid' >> events/sched/sched_waking/trigger
+
+ # cat events/sched/sched_waking/hist
+
+ { pid: 18249 } hitcount: 1
+ { pid: 13399 } hitcount: 1
+ { pid: 17973 } hitcount: 1
+ { pid: 12572 } hitcount: 1
+ ...
+ { pid: 10 } hitcount: 921
+ { pid: 18255 } hitcount: 1444
+ { pid: 25526 } hitcount: 2055
+ { pid: 5257 } hitcount: 2055
+ { pid: 27367 } hitcount: 2055
+ { pid: 1728 } hitcount: 2161
+
+ Totals:
+ Hits: 21305
+ Entries: 183
+ Dropped: 0
+
+What this does is create a histogram on the sched_waking event using
+pid as a key and with a single value, hitcount, which even if not
+explicitly specified, exists for every histogram regardless.
+
+The hitcount value is a per-bucket value that's automatically
+incremented on every hit for the given key, which in this case is the
+pid.
+
+So in this histogram, there's a separate bucket for each pid, and each
+bucket contains a value for that bucket, counting the number of times
+sched_waking was called for that pid.
+
+Each histogram is represented by a hist_data struct.
+
+To keep track of each key and value field in the histogram, hist_data
+keeps an array of these fields named fields[]. The fields[] array is
+an array containing struct hist_field representations of each
+histogram val and key in the histogram (variables are also included
+here, but are discussed later). So for the above histogram we have one
+key and one value; in this case the one value is the hitcount value,
+which all histograms have, regardless of whether they define that
+value or not, which the above histogram does not.
+
+Each struct hist_field contains a pointer to the ftrace_event_field
+from the event's trace_event_file along with various bits related to
+that such as the size, offset, type, and a hist_field_fn_t function,
+which is used to grab the field's data from the ftrace event buffer
+(in most cases - some hist_fields such as hitcount don't directly map
+to an event field in the trace buffer - in these cases the function
+implementation gets its value from somewhere else). The flags field
+indicates which type of field it is - key, value, variable, variable
+reference, etc., with value being the default.
+
+The other important hist_data data structure in addition to the
+fields[] array is the tracing_map instance created for the histogram,
+which is held in the .map member. The tracing_map implements the
+lock-free hash table used to implement histograms (see
+kernel/trace/tracing_map.h for much more discussion about the
+low-level data structures implementing the tracing_map). For the
+purposes of this discussion, the tracing_map contains a number of
+buckets, each bucket corresponding to a particular tracing_map_elt
+object hashed by a given histogram key.
+
+Below is a diagram the first part of which describes the hist_data and
+associated key and value fields for the histogram described above. As
+you can see, there are two fields in the fields array, one val field
+for the hitcount and one key field for the pid key.
+
+Below that is a diagram of a run-time snapshot of what the tracing_map
+might look like for a given run. It attempts to show the
+relationships between the hist_data fields and the tracing_map
+elements for a couple hypothetical keys and values.
+
+ +------------------+
+ | hist_data |
+ +------------------+ +----------------+
+ | .fields[] |---->| val = hitcount |----------------------------+
+ +----------------+ +----------------+ |
+ | .map | | .size | |
+ +----------------+ +--------------+ |
+ | .offset | |
+ +--------------+ |
+ | .fn() | |
+ +--------------+ |
+ . |
+ . |
+ . |
+ +----------------+ <--- n_vals |
+ | key = pid |----------------------------|--+
+ +----------------+ | |
+ | .size | | |
+ +--------------+ | |
+ | .offset | | |
+ +--------------+ | |
+ | .fn() | | |
+ +----------------+ <--- n_fields | |
+ | unused | | |
+ +----------------+ | |
+ | | | |
+ +--------------+ | |
+ | | | |
+ +--------------+ | |
+ | | | |
+ +--------------+ | |
+ n_keys = n_fields - n_vals | |
+ | |
+The hist_data n_vals and n_fields delineate the extent of the fields[] | |
+array and separate keys from values for the rest of the code. | |
+ | |
+Below is a run-time representation of the tracing_map part of the | |
+histogram, with pointers from various parts of the fields[] array | |
+to corresponding parts of the tracing_map. | |
+ | |
+The tracing_map consists of an array of tracing_map_entrys and a set | |
+of preallocated tracing_map_elts (abbreviated below as map_entry and | |
+map_elt). The total number of map_entrys in the hist_data.map array = | |
+map->max_elts (actually map->map_size but only max_elts of those are | |
+used. This is a property required by the map_insert() algorithm). | |
+ | |
+If a map_entry is unused, meaning no key has yet hashed into it, its | |
+.key value is 0 and its .val pointer is NULL. Once a map_entry has | |
+been claimed, the .key value contains the key's hash value and the | |
+.val member points to a map_elt containing the full key and an entry | |
+for each key or value in the map_elt.fields[] array. There is an | |
+entry in the map_elt.fields[] array corresponding to each hist_field | |
+in the histogram, and this is where the continually aggregated sums | |
+corresponding to each histogram value are kept. | |
+ | |
+The diagram attempts to show the relationship between the | |
+hist_data.fields[] and the map_elt.fields[] with the links drawn | |
+between diagrams:: | |
+ | |
+ +-----------+ | |
+ | hist_data | | |
+ +-----------+ | |
+ | .fields | | |
+ +---------+ +-----------+ | |
+ | .map |---->| map_entry | | |
+ +---------+ +-----------+ | |
+ | .key |---> 0 | |
+ +---------+ | |
+ | .val |---> NULL | |
+ +-----------+ | |
+ | map_entry | | |
+ +-----------+ | |
+ | .key |---> pid = 999 | |
+ +---------+ +-----------+ | |
+ | .val |--->| map_elt | | |
+ +---------+ +-----------+ | |
+ . | .key |---> full key * | |
+ . +---------+ +---------------+ | |
+ . | .fields |--->| .sum (val) |<-+ |
+ +-----------+ +---------+ | 2345 | | |
+ | map_entry | +---------------+ | |
+ +-----------+ | .offset (key) |<----+
+ | .key |---> 0 | 0 | | |
+ +---------+ +---------------+ | |
+ | .val |---> NULL . | |
+ +-----------+ . | |
+ | map_entry | . | |
+ +-----------+ +---------------+ | |
+ | .key | | .sum (val) or | | |
+ +---------+ +---------+ | .offset (key) | | |
+ | .val |--->| map_elt | +---------------+ | |
+ +-----------+ +---------+ | .sum (val) or | | |
+ | map_entry | | .offset (key) | | |
+ +-----------+ +---------------+ | |
+ | .key |---> pid = 4444 | |
+ +---------+ +-----------+ | |
+ | .val | | map_elt | | |
+ +---------+ +-----------+ | |
+ | .key |---> full key * | |
+ +---------+ +---------------+ | |
+ | .fields |--->| .sum (val) |<-+ |
+ +---------+ | 65523 | |
+ +---------------+ |
+ | .offset (key) |<----+
+ | 0 |
+ +---------------+
+ .
+ .
+ .
+ +---------------+
+ | .sum (val) or |
+ | .offset (key) |
+ +---------------+
+ | .sum (val) or |
+ | .offset (key) |
+ +---------------+
+
+Abbreviations used in the diagrams::
+
+ hist_data = struct hist_trigger_data
+ hist_data.fields = struct hist_field
+ fn = hist_field_fn_t
+ map_entry = struct tracing_map_entry
+ map_elt = struct tracing_map_elt
+ map_elt.fields = struct tracing_map_field
+
+Whenever a new event occurs and it has a hist trigger associated with
+it, event_hist_trigger() is called. event_hist_trigger() first deals
+with the key: for each subkey in the key (in the above example, there
+is just one subkey corresponding to pid), the hist_field that
+represents that subkey is retrieved from hist_data.fields[] and the
+hist_field_fn_t fn() associated with that field, along with the
+field's size and offset, is used to grab that subkey's data from the
+current trace record.
+
+Once the complete key has been retrieved, it's used to look that key
+up in the tracing_map. If there's no tracing_map_elt associated with
+that key, an empty one is claimed and inserted in the map for the new
+key. In either case, the tracing_map_elt associated with that key is
+returned.
+
+Once a tracing_map_elt available, hist_trigger_elt_update() is called.
+As the name implies, this updates the element, which basically means
+updating the element's fields. There's a tracing_map_field associated
+with each key and value in the histogram, and each of these correspond
+to the key and value hist_fields created when the histogram was
+created. hist_trigger_elt_update() goes through each value hist_field
+and, as for the keys, uses the hist_field's fn() and size and offset
+to grab the field's value from the current trace record. Once it has
+that value, it simply adds that value to that field's
+continually-updated tracing_map_field.sum member. Some hist_field
+fn()s, such as for the hitcount, don't actually grab anything from the
+trace record (the hitcount fn() just increments the counter sum by 1),
+but the idea is the same.
+
+Once all the values have been updated, hist_trigger_elt_update() is
+done and returns. Note that there are also tracing_map_fields for
+each subkey in the key, but hist_trigger_elt_update() doesn't look at
+them or update anything - those exist only for sorting, which can
+happen later.
+
+Basic histogram test
+--------------------
+
+This is a good example to try. It produces 3 value fields and 2 key
+fields in the output::
+
+ # echo 'hist:keys=common_pid,call_site.sym:values=bytes_req,bytes_alloc,hitcount' >> events/kmem/kmalloc/trigger
+
+To see the debug data, cat the kmem/kmalloc's 'hist_debug' file. It
+will show the trigger info of the histogram it corresponds to, along
+with the address of the hist_data associated with the histogram, which
+will become useful in later examples. It then displays the number of
+total hist_fields associated with the histogram along with a count of
+how many of those correspond to keys and how many correspond to values.
+
+It then goes on to display details for each field, including the
+field's flags and the position of each field in the hist_data's
+fields[] array, which is useful information for verifying that things
+internally appear correct or not, and which again will become even
+more useful in further examples::
+
+ # cat events/kmem/kmalloc/hist_debug
+
+ # event histogram
+ #
+ # trigger info: hist:keys=common_pid,call_site.sym:vals=hitcount,bytes_req,bytes_alloc:sort=hitcount:size=2048 [active]
+ #
+
+ hist_data: 000000005e48c9a5
+
+ n_vals: 3
+ n_keys: 2
+ n_fields: 5
+
+ val fields:
+
+ hist_data->fields[0]:
+ flags:
+ VAL: HIST_FIELD_FL_HITCOUNT
+ type: u64
+ size: 8
+ is_signed: 0
+
+ hist_data->fields[1]:
+ flags:
+ VAL: normal u64 value
+ ftrace_event_field name: bytes_req
+ type: size_t
+ size: 8
+ is_signed: 0
+
+ hist_data->fields[2]:
+ flags:
+ VAL: normal u64 value
+ ftrace_event_field name: bytes_alloc
+ type: size_t
+ size: 8
+ is_signed: 0
+
+ key fields:
+
+ hist_data->fields[3]:
+ flags:
+ HIST_FIELD_FL_KEY
+ ftrace_event_field name: common_pid
+ type: int
+ size: 8
+ is_signed: 1
+
+ hist_data->fields[4]:
+ flags:
+ HIST_FIELD_FL_KEY
+ ftrace_event_field name: call_site
+ type: unsigned long
+ size: 8
+ is_signed: 0
+
+The commands below can be used to clean things up for the next test::
+
+ # echo '!hist:keys=common_pid,call_site.sym:values=bytes_req,bytes_alloc,hitcount' >> events/kmem/kmalloc/trigger
+
+Variables
+=========
+
+Variables allow data from one hist trigger to be saved by one hist
+trigger and retrieved by another hist trigger. For example, a trigger
+on the sched_waking event can capture a timestamp for a particular
+pid, and later a sched_switch event that switches to that pid event
+can grab the timestamp and use it to calculate a time delta between
+the two events::
+
+ # echo 'hist:keys=pid:ts0=common_timestamp.usecs' >>
+ events/sched/sched_waking/trigger
+
+ # echo 'hist:keys=next_pid:wakeup_lat=common_timestamp.usecs-$ts0' >>
+ events/sched/sched_switch/trigger
+
+In terms of the histogram data structures, variables are implemented
+as another type of hist_field and for a given hist trigger are added
+to the hist_data.fields[] array just after all the val fields. To
+distinguish them from the existing key and val fields, they're given a
+new flag type, HIST_FIELD_FL_VAR (abbreviated FL_VAR) and they also
+make use of a new .var.idx field member in struct hist_field, which
+maps them to an index in a new map_elt.vars[] array added to the
+map_elt specifically designed to store and retrieve variable values.
+The diagram below shows those new elements and adds a new variable
+entry, ts0, corresponding to the ts0 variable in the sched_waking
+trigger above.
+
+sched_waking histogram
+----------------------
+
+ +------------------+
+ | hist_data |<-------------------------------------------------------+
+ +------------------+ +-------------------+ |
+ | .fields[] |-->| val = hitcount | |
+ +----------------+ +-------------------+ |
+ | .map | | .size | |
+ +----------------+ +-----------------+ |
+ | .offset | |
+ +-----------------+ |
+ | .fn() | |
+ +-----------------+ |
+ | .flags | |
+ +-----------------+ |
+ | .var.idx | |
+ +-------------------+ |
+ | var = ts0 | |
+ +-------------------+ |
+ | .size | |
+ +-----------------+ |
+ | .offset | |
+ +-----------------+ |
+ | .fn() | |
+ +-----------------+ |
+ | .flags & FL_VAR | |
+ +-----------------+ |
+ | .var.idx |----------------------------+-+ |
+ +-----------------+ | | |
+ . | | |
+ . | | |
+ . | | |
+ +-------------------+ <--- n_vals | | |
+ | key = pid | | | |
+ +-------------------+ | | |
+ | .size | | | |
+ +-----------------+ | | |
+ | .offset | | | |
+ +-----------------+ | | |
+ | .fn() | | | |
+ +-----------------+ | | |
+ | .flags & FL_KEY | | | |
+ +-----------------+ | | |
+ | .var.idx | | | |
+ +-------------------+ <--- n_fields | | |
+ | unused | | | |
+ +-------------------+ | | |
+ | | | | |
+ +-----------------+ | | |
+ | | | | |
+ +-----------------+ | | |
+ | | | | |
+ +-----------------+ | | |
+ | | | | |
+ +-----------------+ | | |
+ | | | | |
+ +-----------------+ | | |
+ n_keys = n_fields - n_vals | | |
+ | | |
+ | | |
+This is very similar to the basic case. In the above diagram, we can | | |
+see a new .flags member has been added to the struct hist_field | | |
+struct, and a new entry added to hist_data.fields representing the ts0 | | |
+variable. For a normal val hist_field, .flags is just 0 (modulo | | |
+modifier flags), but if the value is defined as a variable, the .flags | | |
+contains a set FL_VAR bit. | | |
+ | | |
+As you can see, the ts0 entry's .var.idx member contains the index | | |
+into the tracing_map_elts' .vars[] array containing variable values. | | |
+This idx is used whenever the value of the variable is set or read. | | |
+The map_elt.vars idx assigned to the given variable is assigned and | | |
+saved in .var.idx by create_tracing_map_fields() after it calls | | |
+tracing_map_add_var(). | | |
+ | | |
+Below is a representation of the histogram at run-time, which | | |
+populates the map, along with correspondence to the above hist_data and | | |
+hist_field data structures. | | |
+ | | |
+The diagram attempts to show the relationship between the | | |
+hist_data.fields[] and the map_elt.fields[] and map_elt.vars[] with | | |
+the links drawn between diagrams. For each of the map_elts, you can | | |
+see that the .fields[] members point to the .sum or .offset of a key | | |
+or val and the .vars[] members point to the value of a variable. The | | |
+arrows between the two diagrams show the linkages between those | | |
+tracing_map members and the field definitions in the corresponding | | |
+hist_data fields[] members. | | |
+ | | |
+ +-----------+ | | |
+ | hist_data | | | |
+ +-----------+ | | |
+ | .fields | | | |
+ +---------+ +-----------+ | | |
+ | .map |---->| map_entry | | | |
+ +---------+ +-----------+ | | |
+ | .key |---> 0 | | |
+ +---------+ | | |
+ | .val |---> NULL | | |
+ +-----------+ | | |
+ | map_entry | | | |
+ +-----------+ | | |
+ | .key |---> pid = 999 | | |
+ +---------+ +-----------+ | | |
+ | .val |--->| map_elt | | | |
+ +---------+ +-----------+ | | |
+ . | .key |---> full key * | | |
+ . +---------+ +---------------+ | | |
+ . | .fields |--->| .sum (val) | | | |
+ . +---------+ | 2345 | | | |
+ . +--| .vars | +---------------+ | | |
+ . | +---------+ | .offset (key) | | | |
+ . | | 0 | | | |
+ . | +---------------+ | | |
+ . | . | | |
+ . | . | | |
+ . | . | | |
+ . | +---------------+ | | |
+ . | | .sum (val) or | | | |
+ . | | .offset (key) | | | |
+ . | +---------------+ | | |
+ . | | .sum (val) or | | | |
+ . | | .offset (key) | | | |
+ . | +---------------+ | | |
+ . | | | |
+ . +---------------->+---------------+ | | |
+ . | ts0 |<--+ | |
+ . | 113345679876 | | | |
+ . +---------------+ | | |
+ . | unused | | | |
+ . | | | | |
+ . +---------------+ | | |
+ . . | | |
+ . . | | |
+ . . | | |
+ . +---------------+ | | |
+ . | unused | | | |
+ . | | | | |
+ . +---------------+ | | |
+ . | unused | | | |
+ . | | | | |
+ . +---------------+ | | |
+ . | | |
+ +-----------+ | | |
+ | map_entry | | | |
+ +-----------+ | | |
+ | .key |---> pid = 4444 | | |
+ +---------+ +-----------+ | | |
+ | .val |--->| map_elt | | | |
+ +---------+ +-----------+ | | |
+ . | .key |---> full key * | | |
+ . +---------+ +---------------+ | | |
+ . | .fields |--->| .sum (val) | | | |
+ +---------+ | 2345 | | | |
+ +--| .vars | +---------------+ | | |
+ | +---------+ | .offset (key) | | | |
+ | | 0 | | | |
+ | +---------------+ | | |
+ | . | | |
+ | . | | |
+ | . | | |
+ | +---------------+ | | |
+ | | .sum (val) or | | | |
+ | | .offset (key) | | | |
+ | +---------------+ | | |
+ | | .sum (val) or | | | |
+ | | .offset (key) | | | |
+ | +---------------+ | | |
+ | | | |
+ | +---------------+ | | |
+ +---------------->| ts0 |<--+ | |
+ | 213499240729 | | |
+ +---------------+ | |
+ | unused | | |
+ | | | |
+ +---------------+ | |
+ . | |
+ . | |
+ . | |
+ +---------------+ | |
+ | unused | | |
+ | | | |
+ +---------------+ | |
+ | unused | | |
+ | | | |
+ +---------------+ | |
+ | |
+For each used map entry, there's a map_elt pointing to an array of | |
+.vars containing the current value of the variables associated with | |
+that histogram entry. So in the above, the timestamp associated with | |
+pid 999 is 113345679876, and the timestamp variable in the same | |
+.var.idx for pid 4444 is 213499240729. | |
+ | |
+sched_switch histogram | |
+---------------------- | |
+ | |
+The sched_switch histogram paired with the above sched_waking | |
+histogram is shown below. The most important aspect of the | |
+sched_switch histogram is that it references a variable on the | |
+sched_waking histogram above. | |
+ | |
+The histogram diagram is very similar to the others so far displayed, | |
+but it adds variable references. You can see the normal hitcount and | |
+key fields along with a new wakeup_lat variable implemented in the | |
+same way as the sched_waking ts0 variable, but in addition there's an | |
+entry with the new FL_VAR_REF (short for HIST_FIELD_FL_VAR_REF) flag. | |
+ | |
+Associated with the new var ref field are a couple of new hist_field | |
+members, var.hist_data and var_ref_idx. For a variable reference, the | |
+var.hist_data goes with the var.idx, which together uniquely identify | |
+a particular variable on a particular histogram. The var_ref_idx is | |
+just the index into the var_ref_vals[] array that caches the values of | |
+each variable whenever a hist trigger is updated. Those resulting | |
+values are then finally accessed by other code such as trace action | |
+code that uses the var_ref_idx values to assign param values. | |
+ | |
+The diagram below describes the situation for the sched_switch | |
+histogram referred to before: | |
+ | |
+ # echo 'hist:keys=next_pid:wakeup_lat=common_timestamp.usecs-$ts0' >> | |
+ events/sched/sched_switch/trigger | |
+ | |
+ +------------------+ | |
+ | hist_data | | |
+ +------------------+ +-----------------------+ | |
+ | .fields[] |-->| val = hitcount | | |
+ +----------------+ +-----------------------+ | |
+ | .map | | .size | | |
+ +----------------+ +---------------------+ | |
+ +--| .var_refs[] | | .offset | | |
+ | +----------------+ +---------------------+ | |
+ | | .fn() | | |
+ | var_ref_vals[] +---------------------+ | |
+ | +-------------+ | .flags | | |
+ | | $ts0 |<---+ +---------------------+ | |
+ | +-------------+ | | .var.idx | | |
+ | | | | +---------------------+ | |
+ | +-------------+ | | .var.hist_data | | |
+ | | | | +---------------------+ | |
+ | +-------------+ | | .var_ref_idx | | |
+ | | | | +-----------------------+ | |
+ | +-------------+ | | var = wakeup_lat | | |
+ | . | +-----------------------+ | |
+ | . | | .size | | |
+ | . | +---------------------+ | |
+ | +-------------+ | | .offset | | |
+ | | | | +---------------------+ | |
+ | +-------------+ | | .fn() | | |
+ | | | | +---------------------+ | |
+ | +-------------+ | | .flags & FL_VAR | | |
+ | | +---------------------+ | |
+ | | | .var.idx | | |
+ | | +---------------------+ | |
+ | | | .var.hist_data | | |
+ | | +---------------------+ | |
+ | | | .var_ref_idx | | |
+ | | +---------------------+ | |
+ | | . | |
+ | | . | |
+ | | . | |
+ | | +-----------------------+ <--- n_vals | |
+ | | | key = pid | | |
+ | | +-----------------------+ | |
+ | | | .size | | |
+ | | +---------------------+ | |
+ | | | .offset | | |
+ | | +---------------------+ | |
+ | | | .fn() | | |
+ | | +---------------------+ | |
+ | | | .flags | | |
+ | | +---------------------+ | |
+ | | | .var.idx | | |
+ | | +-----------------------+ <--- n_fields | |
+ | | | unused | | |
+ | | +-----------------------+ | |
+ | | | | | |
+ | | +---------------------+ | |
+ | | | | | |
+ | | +---------------------+ | |
+ | | | | | |
+ | | +---------------------+ | |
+ | | | | | |
+ | | +---------------------+ | |
+ | | | | | |
+ | | +---------------------+ | |
+ | | n_keys = n_fields - n_vals | |
+ | | | |
+ | | | |
+ | | +-----------------------+ | |
+ +---------------------->| var_ref = $ts0 | | |
+ | +-----------------------+ | |
+ | | .size | | |
+ | +---------------------+ | |
+ | | .offset | | |
+ | +---------------------+ | |
+ | | .fn() | | |
+ | +---------------------+ | |
+ | | .flags & FL_VAR_REF | | |
+ | +---------------------+ | |
+ | | .var.idx |--------------------------+ |
+ | +---------------------+ |
+ | | .var.hist_data |----------------------------+
+ | +---------------------+
+ +---| .var_ref_idx |
+ +---------------------+
+
+Abbreviations used in the diagrams::
+
+ hist_data = struct hist_trigger_data
+ hist_data.fields = struct hist_field
+ fn = hist_field_fn_t
+ FL_KEY = HIST_FIELD_FL_KEY
+ FL_VAR = HIST_FIELD_FL_VAR
+ FL_VAR_REF = HIST_FIELD_FL_VAR_REF
+
+When a hist trigger makes use of a variable, a new hist_field is
+created with flag HIST_FIELD_FL_VAR_REF. For a VAR_REF field, the
+var.idx and var.hist_data take the same values as the referenced
+variable, as well as the referenced variable's size, type, and
+is_signed values. The VAR_REF field's .name is set to the name of the
+variable it references. If a variable reference was created using the
+explicit system.event.$var_ref notation, the hist_field's system and
+event_name variabls are also set.
+
+So, in order to handle an event for the sched_switch histogram,
+because we have a reference to a variable on another histogram, we
+need to resolve all variable references first. This is done via the
+resolve_var_refs() calls made from event_hist_trigger(). What this
+does is grabs the var_refs[] array from the hist_data representing the
+sched_switch histogram. For each one of those, the referenced
+variable's var.hist_data along with the current key is used to look up
+the corresponding tracing_map_elt in that histogram. Once found, the
+referenced variable's var.idx is used to look up the variable's value
+using tracing_map_read_var(elt, var.idx), which yields the value of
+the variable for that element, ts0 in the case above. Note that both
+the hist_fields representing both the variable and the variable
+reference have the same var.idx, so this is straightforward.
+
+Variable and variable reference test
+------------------------------------
+
+This example creates a variable on the sched_waking event, ts0, and
+uses it in the sched_switch trigger. The sched_switch trigger also
+creates its own variable, wakeup_lat, but nothing yet uses it::
+
+ # echo 'hist:keys=pid:ts0=common_timestamp.usecs' >> events/sched/sched_waking/trigger
+
+ # echo 'hist:keys=next_pid:wakeup_lat=common_timestamp.usecs-$ts0' >> events/sched/sched_switch/trigger
+
+Looking at the sched_waking 'hist_debug' output, in addition to the
+normal key and value hist_fields, in the val fields section we see a
+field with the HIST_FIELD_FL_VAR flag, which indicates that that field
+represents a variable. Note that in addition to the variable name,
+contained in the var.name field, it includes the var.idx, which is the
+index into the tracing_map_elt.vars[] array of the actual variable
+location. Note also that the output shows that variables live in the
+same part of the hist_data->fields[] array as normal values::
+
+ # cat events/sched/sched_waking/hist_debug
+
+ # event histogram
+ #
+ # trigger info: hist:keys=pid:vals=hitcount:ts0=common_timestamp.usecs:sort=hitcount:size=2048:clock=global [active]
+ #
+
+ hist_data: 000000009536f554
+
+ n_vals: 2
+ n_keys: 1
+ n_fields: 3
+
+ val fields:
+
+ hist_data->fields[0]:
+ flags:
+ VAL: HIST_FIELD_FL_HITCOUNT
+ type: u64
+ size: 8
+ is_signed: 0
+
+ hist_data->fields[1]:
+ flags:
+ HIST_FIELD_FL_VAR
+ var.name: ts0
+ var.idx (into tracing_map_elt.vars[]): 0
+ type: u64
+ size: 8
+ is_signed: 0
+
+ key fields:
+
+ hist_data->fields[2]:
+ flags:
+ HIST_FIELD_FL_KEY
+ ftrace_event_field name: pid
+ type: pid_t
+ size: 8
+ is_signed: 1
+
+Moving on to the sched_switch trigger hist_debug output, in addition
+to the unused wakeup_lat variable, we see a new section displaying
+variable references. Variable references are displayed in a separate
+section because in addition to to being logically separate from
+variables and values, they actually live in a separate hist_data
+array, var_refs[].
+
+In this example, the sched_switch trigger has a reference to a
+variable on the sched_waking trigger, $ts0. Looking at the details,
+we can see that the var.hist_data value of the referenced variable
+matches the previously displayed sched_waking trigger, and the var.idx
+value matches the previously displayed var.idx value for that
+variable. Also displayed is the var_ref_idx value for that variable
+reference, which is where the value for that variable is cached for
+use when the trigger is invoked::
+
+ # cat events/sched/sched_switch/hist_debug
+
+ # event histogram
+ #
+ # trigger info: hist:keys=next_pid:vals=hitcount:wakeup_lat=common_timestamp.usecs-$ts0:sort=hitcount:size=2048:clock=global [active]
+ #
+
+ hist_data: 00000000f4ee8006
+
+ n_vals: 2
+ n_keys: 1
+ n_fields: 3
+
+ val fields:
+
+ hist_data->fields[0]:
+ flags:
+ VAL: HIST_FIELD_FL_HITCOUNT
+ type: u64
+ size: 8
+ is_signed: 0
+
+ hist_data->fields[1]:
+ flags:
+ HIST_FIELD_FL_VAR
+ var.name: wakeup_lat
+ var.idx (into tracing_map_elt.vars[]): 0
+ type: u64
+ size: 0
+ is_signed: 0
+
+ key fields:
+
+ hist_data->fields[2]:
+ flags:
+ HIST_FIELD_FL_KEY
+ ftrace_event_field name: next_pid
+ type: pid_t
+ size: 8
+ is_signed: 1
+
+ variable reference fields:
+
+ hist_data->var_refs[0]:
+ flags:
+ HIST_FIELD_FL_VAR_REF
+ name: ts0
+ var.idx (into tracing_map_elt.vars[]): 0
+ var.hist_data: 000000009536f554
+ var_ref_idx (into hist_data->var_refs[]): 0
+ type: u64
+ size: 8
+ is_signed: 0
+
+The commands below can be used to clean things up for the next test::
+
+ # echo '!hist:keys=next_pid:wakeup_lat=common_timestamp.usecs-$ts0' >> events/sched/sched_switch/trigger
+
+ # echo '!hist:keys=pid:ts0=common_timestamp.usecs' >> events/sched/sched_waking/trigger
+
+Actions and Handlers
+====================
+
+Adding onto the previous example, we will now do something with that
+wakeup_lat variable, namely send it and another field as a synthetic
+event.
+
+The onmatch() action below basically says that whenever we have a
+sched_switch event, if we have a matching sched_waking event, in this
+case if we have a pid in the sched_waking histogram that matches the
+the next_pid field on this sched_switch event, we retrieve the
+variables specified in the wakeup_latency() trace action, and use
+them to generate a new wakeup_latency event into the trace stream.
+
+Note that the way the trace handlers such as wakeup_latency() (which
+could equivalently be written trace(wakeup_latency,$wakeup_lat,next_pid)
+are implemented, the parameters specified to the trace handler must be
+variables. In this case, $wakeup_lat is obviously a variable, but
+next_pid isn't, since it's just naming a field in the sched_switch
+trace event. Since this is something that almost every trace() and
+save() action does, a special shortcut is implemented to allow field
+names to be used directly in those cases. How it works is that under
+the covers, a temporary variable is created for the named field, and
+this variable is what is actually passed to the trace handler. In the
+code and documentation, this type of variable is called a 'field
+variable'.
+
+Fields on other trace event's histograms can be used as well. In that
+case we have to generate a new histogram and an unfortunately named
+'synthetic_field' (the use of synthetic here has nothing to do with
+synthetic events) and use that special histogram field as a variable.
+
+The diagram below illustrates the new elements described above in the
+context of the sched_switch histogram using the onmatch() handler and
+the trace() action.
+
+First, we define the wakeup_latency synthetic event::
+
+ # echo 'wakeup_latency u64 lat; pid_t pid' >> synthetic_events
+
+Next, the sched_waking hist trigger as before::
+
+ # echo 'hist:keys=pid:ts0=common_timestamp.usecs' >>
+ events/sched/sched_waking/trigger
+
+Finally, we create a hist trigger on the sched_switch event that
+generates a wakeup_latency() trace event. In this case we pass
+next_pid into the wakeup_latency synthetic event invocation, which
+means it will be automatically converted into a field variable::
+
+ # echo 'hist:keys=next_pid:wakeup_lat=common_timestamp.usecs-$ts0: \
+ onmatch(sched.sched_waking).wakeup_latency($wakeup_lat,next_pid)' >>
+ /sys/kernel/debug/tracing/events/sched/sched_switch/trigger
+
+The diagram for the sched_switch event is similar to previous examples
+but shows the additional field_vars[] array for hist_data and shows
+the linkages between the field_vars and the variables and references
+created to implement the field variables. The details are discussed
+below::
+
+ +------------------+
+ | hist_data |
+ +------------------+ +-----------------------+
+ | .fields[] |-->| val = hitcount |
+ +----------------+ +-----------------------+
+ | .map | | .size |
+ +----------------+ +---------------------+
+ +---| .field_vars[] | | .offset |
+ | +----------------+ +---------------------+
+ |+--| .var_refs[] | | .offset |
+ || +----------------+ +---------------------+
+ || | .fn() |
+ || var_ref_vals[] +---------------------+
+ || +-------------+ | .flags |
+ || | $ts0 |<---+ +---------------------+
+ || +-------------+ | | .var.idx |
+ || | $next_pid |<-+ | +---------------------+
+ || +-------------+ | | | .var.hist_data |
+ ||+>| $wakeup_lat | | | +---------------------+
+ ||| +-------------+ | | | .var_ref_idx |
+ ||| | | | | +-----------------------+
+ ||| +-------------+ | | | var = wakeup_lat |
+ ||| . | | +-----------------------+
+ ||| . | | | .size |
+ ||| . | | +---------------------+
+ ||| +-------------+ | | | .offset |
+ ||| | | | | +---------------------+
+ ||| +-------------+ | | | .fn() |
+ ||| | | | | +---------------------+
+ ||| +-------------+ | | | .flags & FL_VAR |
+ ||| | | +---------------------+
+ ||| | | | .var.idx |
+ ||| | | +---------------------+
+ ||| | | | .var.hist_data |
+ ||| | | +---------------------+
+ ||| | | | .var_ref_idx |
+ ||| | | +---------------------+
+ ||| | | .
+ ||| | | .
+ ||| | | .
+ ||| | | .
+ ||| +--------------+ | | .
+ +-->| field_var | | | .
+ || +--------------+ | | .
+ || | var | | | .
+ || +------------+ | | .
+ || | val | | | .
+ || +--------------+ | | .
+ || | field_var | | | .
+ || +--------------+ | | .
+ || | var | | | .
+ || +------------+ | | .
+ || | val | | | .
+ || +------------+ | | .
+ || . | | .
+ || . | | .
+ || . | | +-----------------------+ <--- n_vals
+ || +--------------+ | | | key = pid |
+ || | field_var | | | +-----------------------+
+ || +--------------+ | | | .size |
+ || | var |--+| +---------------------+
+ || +------------+ ||| | .offset |
+ || | val |-+|| +---------------------+
+ || +------------+ ||| | .fn() |
+ || ||| +---------------------+
+ || ||| | .flags |
+ || ||| +---------------------+
+ || ||| | .var.idx |
+ || ||| +---------------------+ <--- n_fields
+ || |||
+ || ||| n_keys = n_fields - n_vals
+ || ||| +-----------------------+
+ || |+->| var = next_pid |
+ || | | +-----------------------+
+ || | | | .size |
+ || | | +---------------------+
+ || | | | .offset |
+ || | | +---------------------+
+ || | | | .flags & FL_VAR |
+ || | | +---------------------+
+ || | | | .var.idx |
+ || | | +---------------------+
+ || | | | .var.hist_data |
+ || | | +-----------------------+
+ || +-->| val for next_pid |
+ || | | +-----------------------+
+ || | | | .size |
+ || | | +---------------------+
+ || | | | .offset |
+ || | | +---------------------+
+ || | | | .fn() |
+ || | | +---------------------+
+ || | | | .flags |
+ || | | +---------------------+
+ || | | | |
+ || | | +---------------------+
+ || | |
+ || | |
+ || | | +-----------------------+
+ +|------------------|-|>| var_ref = $ts0 |
+ | | | +-----------------------+
+ | | | | .size |
+ | | | +---------------------+
+ | | | | .offset |
+ | | | +---------------------+
+ | | | | .fn() |
+ | | | +---------------------+
+ | | | | .flags & FL_VAR_REF |
+ | | | +---------------------+
+ | | +---| .var_ref_idx |
+ | | +-----------------------+
+ | | | var_ref = $next_pid |
+ | | +-----------------------+
+ | | | .size |
+ | | +---------------------+
+ | | | .offset |
+ | | +---------------------+
+ | | | .fn() |
+ | | +---------------------+
+ | | | .flags & FL_VAR_REF |
+ | | +---------------------+
+ | +-----| .var_ref_idx |
+ | +-----------------------+
+ | | var_ref = $wakeup_lat |
+ | +-----------------------+
+ | | .size |
+ | +---------------------+
+ | | .offset |
+ | +---------------------+
+ | | .fn() |
+ | +---------------------+
+ | | .flags & FL_VAR_REF |
+ | +---------------------+
+ +------------------------| .var_ref_idx |
+ +---------------------+
+
+As you can see, for a field variable, two hist_fields are created: one
+representing the variable, in this case next_pid, and one to actually
+get the value of the field from the trace stream, like a normal val
+field does. These are created separately from normal variable
+creation and are saved in the hist_data->field_vars[] array. See
+below for how these are used. In addition, a reference hist_field is
+also created, which is needed to reference the field variables such as
+$next_pid variable in the trace() action.
+
+Note that $wakeup_lat is also a variable reference, referencing the
+value of the expression common_timestamp-$ts0, and so also needs to
+have a hist field entry representing that reference created.
+
+When hist_trigger_elt_update() is called to get the normal key and
+value fields, it also calls update_field_vars(), which goes through
+each field_var created for the histogram, and available from
+hist_data->field_vars and calls val->fn() to get the data from the
+current trace record, and then uses the var's var.idx to set the
+variable at the var.idx offset in the appropriate tracing_map_elt's
+variable at elt->vars[var.idx].
+
+Once all the variables have been updated, resolve_var_refs() can be
+called from event_hist_trigger(), and not only can our $ts0 and
+$next_pid references be resolved but the $wakeup_lat reference as
+well. At this point, the trace() action can simply access the values
+assembled in the var_ref_vals[] array and generate the trace event.
+
+The same process occurs for the field variables associated with the
+save() action.
+
+Abbreviations used in the diagram::
+
+ hist_data = struct hist_trigger_data
+ hist_data.fields = struct hist_field
+ field_var = struct field_var
+ fn = hist_field_fn_t
+ FL_KEY = HIST_FIELD_FL_KEY
+ FL_VAR = HIST_FIELD_FL_VAR
+ FL_VAR_REF = HIST_FIELD_FL_VAR_REF
+
+trace() action field variable test
+----------------------------------
+
+This example adds to the previous test example by finally making use
+of the wakeup_lat variable, but in addition also creates a couple of
+field variables that then are all passed to the wakeup_latency() trace
+action via the onmatch() handler.
+
+First, we create the wakeup_latency synthetic event::
+
+ # echo 'wakeup_latency u64 lat; pid_t pid; char comm[16]' >> synthetic_events
+
+Next, the sched_waking trigger from previous examples::
+
+ # echo 'hist:keys=pid:ts0=common_timestamp.usecs' >> events/sched/sched_waking/trigger
+
+Finally, as in the previous test example, we calculate and assign the
+wakeup latency using the $ts0 reference from the sched_waking trigger
+to the wakeup_lat variable, and finally use it along with a couple
+sched_switch event fields, next_pid and next_comm, to generate a
+wakeup_latency trace event. The next_pid and next_comm event fields
+are automatically converted into field variables for this purpose::
+
+ # echo 'hist:keys=next_pid:wakeup_lat=common_timestamp.usecs-$ts0:onmatch(sched.sched_waking).wakeup_latency($wakeup_lat,next_pid,next_comm)' >> /sys/kernel/debug/tracing/events/sched/sched_switch/trigger
+
+The sched_waking hist_debug output shows the same data as in the
+previous test example::
+
+ # cat events/sched/sched_waking/hist_debug
+
+ # event histogram
+ #
+ # trigger info: hist:keys=pid:vals=hitcount:ts0=common_timestamp.usecs:sort=hitcount:size=2048:clock=global [active]
+ #
+
+ hist_data: 00000000d60ff61f
+
+ n_vals: 2
+ n_keys: 1
+ n_fields: 3
+
+ val fields:
+
+ hist_data->fields[0]:
+ flags:
+ VAL: HIST_FIELD_FL_HITCOUNT
+ type: u64
+ size: 8
+ is_signed: 0
+
+ hist_data->fields[1]:
+ flags:
+ HIST_FIELD_FL_VAR
+ var.name: ts0
+ var.idx (into tracing_map_elt.vars[]): 0
+ type: u64
+ size: 8
+ is_signed: 0
+
+ key fields:
+
+ hist_data->fields[2]:
+ flags:
+ HIST_FIELD_FL_KEY
+ ftrace_event_field name: pid
+ type: pid_t
+ size: 8
+ is_signed: 1
+
+The sched_switch hist_debug output shows the same key and value fields
+as in the previous test example - note that wakeup_lat is still in the
+val fields section, but that the new field variables are not there -
+although the field variables are variables, they're held separately in
+the hist_data's field_vars[] array. Although the field variables and
+the normal variables are located in separate places, you can see that
+the actual variable locations for those variables in the
+tracing_map_elt.vars[] do have increasing indices as expected:
+wakeup_lat takes the var.idx = 0 slot, while the field variables for
+next_pid and next_comm have values var.idx = 1, and var.idx = 2. Note
+also that those are the same values displayed for the variable
+references corresponding to those variables in the variable reference
+fields section. Since there are two triggers and thus two hist_data
+addresses, those addresses also need to be accounted for when doing
+the matching - you can see that the first variable refers to the 0
+var.idx on the previous hist trigger (see the hist_data address
+associated with that trigger), while the second variable refers to the
+0 var.idx on the sched_switch hist trigger, as do all the remaining
+variable references.
+
+Finally, the action tracking variables section just shows the system
+and event name for the onmatch() handler::
+
+ # cat events/sched/sched_switch/hist_debug
+
+ # event histogram
+ #
+ # trigger info: hist:keys=next_pid:vals=hitcount:wakeup_lat=common_timestamp.usecs-$ts0:sort=hitcount:size=2048:clock=global:onmatch(sched.sched_waking).wakeup_latency($wakeup_lat,next_pid,next_comm) [active]
+ #
+
+ hist_data: 0000000008f551b7
+
+ n_vals: 2
+ n_keys: 1
+ n_fields: 3
+
+ val fields:
+
+ hist_data->fields[0]:
+ flags:
+ VAL: HIST_FIELD_FL_HITCOUNT
+ type: u64
+ size: 8
+ is_signed: 0
+
+ hist_data->fields[1]:
+ flags:
+ HIST_FIELD_FL_VAR
+ var.name: wakeup_lat
+ var.idx (into tracing_map_elt.vars[]): 0
+ type: u64
+ size: 0
+ is_signed: 0
+
+ key fields:
+
+ hist_data->fields[2]:
+ flags:
+ HIST_FIELD_FL_KEY
+ ftrace_event_field name: next_pid
+ type: pid_t
+ size: 8
+ is_signed: 1
+
+ variable reference fields:
+
+ hist_data->var_refs[0]:
+ flags:
+ HIST_FIELD_FL_VAR_REF
+ name: ts0
+ var.idx (into tracing_map_elt.vars[]): 0
+ var.hist_data: 00000000d60ff61f
+ var_ref_idx (into hist_data->var_refs[]): 0
+ type: u64
+ size: 8
+ is_signed: 0
+
+ hist_data->var_refs[1]:
+ flags:
+ HIST_FIELD_FL_VAR_REF
+ name: wakeup_lat
+ var.idx (into tracing_map_elt.vars[]): 0
+ var.hist_data: 0000000008f551b7
+ var_ref_idx (into hist_data->var_refs[]): 1
+ type: u64
+ size: 0
+ is_signed: 0
+
+ hist_data->var_refs[2]:
+ flags:
+ HIST_FIELD_FL_VAR_REF
+ name: next_pid
+ var.idx (into tracing_map_elt.vars[]): 1
+ var.hist_data: 0000000008f551b7
+ var_ref_idx (into hist_data->var_refs[]): 2
+ type: pid_t
+ size: 4
+ is_signed: 0
+
+ hist_data->var_refs[3]:
+ flags:
+ HIST_FIELD_FL_VAR_REF
+ name: next_comm
+ var.idx (into tracing_map_elt.vars[]): 2
+ var.hist_data: 0000000008f551b7
+ var_ref_idx (into hist_data->var_refs[]): 3
+ type: char[16]
+ size: 256
+ is_signed: 0
+
+ field variables:
+
+ hist_data->field_vars[0]:
+
+ field_vars[0].var:
+ flags:
+ HIST_FIELD_FL_VAR
+ var.name: next_pid
+ var.idx (into tracing_map_elt.vars[]): 1
+
+ field_vars[0].val:
+ ftrace_event_field name: next_pid
+ type: pid_t
+ size: 4
+ is_signed: 1
+
+ hist_data->field_vars[1]:
+
+ field_vars[1].var:
+ flags:
+ HIST_FIELD_FL_VAR
+ var.name: next_comm
+ var.idx (into tracing_map_elt.vars[]): 2
+
+ field_vars[1].val:
+ ftrace_event_field name: next_comm
+ type: char[16]
+ size: 256
+ is_signed: 0
+
+ action tracking variables (for onmax()/onchange()/onmatch()):
+
+ hist_data->actions[0].match_data.event_system: sched
+ hist_data->actions[0].match_data.event: sched_waking
+
+The commands below can be used to clean things up for the next test::
+
+ # echo '!hist:keys=next_pid:wakeup_lat=common_timestamp.usecs-$ts0:onmatch(sched.sched_waking).wakeup_latency($wakeup_lat,next_pid,next_comm)' >> /sys/kernel/debug/tracing/events/sched/sched_switch/trigger
+
+ # echo '!hist:keys=pid:ts0=common_timestamp.usecs' >> events/sched/sched_waking/trigger
+
+ # echo '!wakeup_latency u64 lat; pid_t pid; char comm[16]' >> synthetic_events
+
+action_data and the trace() action
+----------------------------------
+
+As mentioned above, when the trace() action generates a synthetic
+event, all the parameters to the synthetic event either already are
+variables or are converted into variables (via field variables), and
+finally all those variable values are collected via references to them
+into a var_ref_vals[] array.
+
+The values in the var_ref_vals[] array, however, don't necessarily
+follow the same ordering as the synthetic event params. To address
+that, struct action_data contains another array, var_ref_idx[] that
+maps the trace action params to the var_ref_vals[] values. Below is a
+diagram illustrating that for the wakeup_latency() synthetic event::
+
+ +------------------+ wakeup_latency()
+ | action_data | event params var_ref_vals[]
+ +------------------+ +-----------------+ +-----------------+
+ | .var_ref_idx[] |--->| $wakeup_lat idx |---+ | |
+ +----------------+ +-----------------+ | +-----------------+
+ | .synth_event | | $next_pid idx |---|-+ | $wakeup_lat val |
+ +----------------+ +-----------------+ | | +-----------------+
+ . | +->| $next_pid val |
+ . | +-----------------+
+ . | .
+ +-----------------+ | .
+ | | | .
+ +-----------------+ | +-----------------+
+ +--->| $wakeup_lat val |
+ +-----------------+
+
+Basically, how this ends up getting used in the synthetic event probe
+function, trace_event_raw_event_synth(), is as follows::
+
+ for each field i in .synth_event
+ val_idx = .var_ref_idx[i]
+ val = var_ref_vals[val_idx]
+
+action_data and the onXXX() handlers
+------------------------------------
+
+The hist trigger onXXX() actions other than onmatch(), such as onmax()
+and onchange(), also make use of and internally create hidden
+variables. This information is contained in the
+action_data.track_data struct, and is also visible in the hist_debug
+output as will be described in the example below.
+
+Typically, the onmax() or onchange() handlers are used in conjunction
+with the save() and snapshot() actions. For example::
+
+ # echo 'hist:keys=next_pid:wakeup_lat=common_timestamp.usecs-$ts0: \
+ onmax($wakeup_lat).save(next_comm,prev_pid,prev_prio,prev_comm)' >>
+ /sys/kernel/debug/tracing/events/sched/sched_switch/trigger
+
+or::
+
+ # echo 'hist:keys=next_pid:wakeup_lat=common_timestamp.usecs-$ts0: \
+ onmax($wakeup_lat).snapshot()' >>
+ /sys/kernel/debug/tracing/events/sched/sched_switch/trigger
+
+save() action field variable test
+---------------------------------
+
+For this example, instead of generating a synthetic event, the save()
+action is used to save field values whenever an onmax() handler
+detects that a new max latency has been hit. As in the previous
+example, the values being saved are also field values, but in this
+case, are kept in a separate hist_data array named save_vars[].
+
+As in previous test examples, we set up the sched_waking trigger::
+
+ # echo 'hist:keys=pid:ts0=common_timestamp.usecs' >> events/sched/sched_waking/trigger
+
+In this case, however, we set up the sched_switch trigger to save some
+sched_switch field values whenever we hit a new maximum latency. For
+both the onmax() handler and save() action, variables will be created,
+which we can use the hist_debug files to examine::
+
+ # echo 'hist:keys=next_pid:wakeup_lat=common_timestamp.usecs-$ts0:onmax($wakeup_lat).save(next_comm,prev_pid,prev_prio,prev_comm)' >> events/sched/sched_switch/trigger
+
+The sched_waking hist_debug output shows the same data as in the
+previous test examples::
+
+ # cat events/sched/sched_waking/hist_debug
+
+ #
+ # trigger info: hist:keys=pid:vals=hitcount:ts0=common_timestamp.usecs:sort=hitcount:size=2048:clock=global [active]
+ #
+
+ hist_data: 00000000e6290f48
+
+ n_vals: 2
+ n_keys: 1
+ n_fields: 3
+
+ val fields:
+
+ hist_data->fields[0]:
+ flags:
+ VAL: HIST_FIELD_FL_HITCOUNT
+ type: u64
+ size: 8
+ is_signed: 0
+
+ hist_data->fields[1]:
+ flags:
+ HIST_FIELD_FL_VAR
+ var.name: ts0
+ var.idx (into tracing_map_elt.vars[]): 0
+ type: u64
+ size: 8
+ is_signed: 0
+
+ key fields:
+
+ hist_data->fields[2]:
+ flags:
+ HIST_FIELD_FL_KEY
+ ftrace_event_field name: pid
+ type: pid_t
+ size: 8
+ is_signed: 1
+
+The output of the sched_switch trigger shows the same val and key
+values as before, but also shows a couple new sections.
+
+First, the action tracking variables section now shows the
+actions[].track_data information describing the special tracking
+variables and references used to track, in this case, the running
+maximum value. The actions[].track_data.var_ref member contains the
+reference to the variable being tracked, in this case the $wakeup_lat
+variable. In order to perform the onmax() handler function, there
+also needs to be a variable that tracks the current maximum by getting
+updated whenever a new maximum is hit. In this case, we can see that
+an autogenerated veriable named ' __max' has been created and is
+visible in the actions[].track_data.track_var variable.
+
+Finally, in the new 'save action variables' section, we can see that
+the 4 params to the save() function have resulted in 4 field variables
+being created for the purposes of saving the values of the named
+fields when the max is hit. These variables are kept in a separate
+save_vars[] array off of hist_data, so are displayed in a separate
+section::
+
+ # cat events/sched/sched_switch/hist_debug
+
+ # event histogram
+ #
+ # trigger info: hist:keys=next_pid:vals=hitcount:wakeup_lat=common_timestamp.usecs-$ts0:sort=hitcount:size=2048:clock=global:onmax($wakeup_lat).save(next_comm,prev_pid,prev_prio,prev_comm) [active]
+ #
+
+ hist_data: 0000000057bcd28d
+
+ n_vals: 2
+ n_keys: 1
+ n_fields: 3
+
+ val fields:
+
+ hist_data->fields[0]:
+ flags:
+ VAL: HIST_FIELD_FL_HITCOUNT
+ type: u64
+ size: 8
+ is_signed: 0
+
+ hist_data->fields[1]:
+ flags:
+ HIST_FIELD_FL_VAR
+ var.name: wakeup_lat
+ var.idx (into tracing_map_elt.vars[]): 0
+ type: u64
+ size: 0
+ is_signed: 0
+
+ key fields:
+
+ hist_data->fields[2]:
+ flags:
+ HIST_FIELD_FL_KEY
+ ftrace_event_field name: next_pid
+ type: pid_t
+ size: 8
+ is_signed: 1
+
+ variable reference fields:
+
+ hist_data->var_refs[0]:
+ flags:
+ HIST_FIELD_FL_VAR_REF
+ name: ts0
+ var.idx (into tracing_map_elt.vars[]): 0
+ var.hist_data: 00000000e6290f48
+ var_ref_idx (into hist_data->var_refs[]): 0
+ type: u64
+ size: 8
+ is_signed: 0
+
+ hist_data->var_refs[1]:
+ flags:
+ HIST_FIELD_FL_VAR_REF
+ name: wakeup_lat
+ var.idx (into tracing_map_elt.vars[]): 0
+ var.hist_data: 0000000057bcd28d
+ var_ref_idx (into hist_data->var_refs[]): 1
+ type: u64
+ size: 0
+ is_signed: 0
+
+ action tracking variables (for onmax()/onchange()/onmatch()):
+
+ hist_data->actions[0].track_data.var_ref:
+ flags:
+ HIST_FIELD_FL_VAR_REF
+ name: wakeup_lat
+ var.idx (into tracing_map_elt.vars[]): 0
+ var.hist_data: 0000000057bcd28d
+ var_ref_idx (into hist_data->var_refs[]): 1
+ type: u64
+ size: 0
+ is_signed: 0
+
+ hist_data->actions[0].track_data.track_var:
+ flags:
+ HIST_FIELD_FL_VAR
+ var.name: __max
+ var.idx (into tracing_map_elt.vars[]): 1
+ type: u64
+ size: 8
+ is_signed: 0
+
+ save action variables (save() params):
+
+ hist_data->save_vars[0]:
+
+ save_vars[0].var:
+ flags:
+ HIST_FIELD_FL_VAR
+ var.name: next_comm
+ var.idx (into tracing_map_elt.vars[]): 2
+
+ save_vars[0].val:
+ ftrace_event_field name: next_comm
+ type: char[16]
+ size: 256
+ is_signed: 0
+
+ hist_data->save_vars[1]:
+
+ save_vars[1].var:
+ flags:
+ HIST_FIELD_FL_VAR
+ var.name: prev_pid
+ var.idx (into tracing_map_elt.vars[]): 3
+
+ save_vars[1].val:
+ ftrace_event_field name: prev_pid
+ type: pid_t
+ size: 4
+ is_signed: 1
+
+ hist_data->save_vars[2]:
+
+ save_vars[2].var:
+ flags:
+ HIST_FIELD_FL_VAR
+ var.name: prev_prio
+ var.idx (into tracing_map_elt.vars[]): 4
+
+ save_vars[2].val:
+ ftrace_event_field name: prev_prio
+ type: int
+ size: 4
+ is_signed: 1
+
+ hist_data->save_vars[3]:
+
+ save_vars[3].var:
+ flags:
+ HIST_FIELD_FL_VAR
+ var.name: prev_comm
+ var.idx (into tracing_map_elt.vars[]): 5
+
+ save_vars[3].val:
+ ftrace_event_field name: prev_comm
+ type: char[16]
+ size: 256
+ is_signed: 0
+
+The commands below can be used to clean things up for the next test::
+
+ # echo '!hist:keys=next_pid:wakeup_lat=common_timestamp.usecs-$ts0:onmax($wakeup_lat).save(next_comm,prev_pid,prev_prio,prev_comm)' >> events/sched/sched_switch/trigger
+
+ # echo '!hist:keys=pid:ts0=common_timestamp.usecs' >> events/sched/sched_waking/trigger
+
+A couple special cases
+======================
+
+While the above covers the basics of the histogram internals, there
+are a couple of special cases that should be discussed, since they
+tend to creae even more confusion. Those are field variables on other
+histograms, and aliases, both described below through example tests
+using the hist_debug files.
+
+Test of field variables on other histograms
+-------------------------------------------
+
+This example is similar to the previous examples, but in this case,
+the sched_switch trigger references a hist trigger field on another
+event, namely the sched_waking event. In order to accomplish this, a
+field variable is created for the other event, but since an existing
+histogram can't be used, as existing histograms are immutable, a new
+histogram with a matching variable is created and used, and we'll see
+that reflected in the hist_debug output shown below.
+
+First, we create the wakeup_latency synthetic event. Note the
+addition of the prio field::
+
+ # echo 'wakeup_latency u64 lat; pid_t pid; int prio' >> synthetic_events
+
+As in previous test examples, we set up the sched_waking trigger::
+
+ # echo 'hist:keys=pid:ts0=common_timestamp.usecs' >> events/sched/sched_waking/trigger
+
+Here we set up a hist trigger on sched_switch to send a wakeup_latency
+event using an onmatch handler naming the sched_waking event. Note
+that the third param being passed to the wakeup_latency() is prio,
+which is a field name that needs to have a field variable created for
+it. There isn't however any prio field on the sched_switch event so
+it would seem that it wouldn't be possible to create a field variable
+for it. The matching sched_waking event does have a prio field, so it
+should be possible to make use of it for this purpose. The problem
+with that is that it's not currently possible to define a new variable
+on an existing histogram, so it's not possible to add a new prio field
+variable to the existing sched_waking histogram. It is however
+possible to create an additional new 'matching' sched_waking histogram
+for the same event, meaning that it uses the same key and filters, and
+define the new prio field variable on that.
+
+Here's the sched_switch trigger::
+
+ # echo 'hist:keys=next_pid:wakeup_lat=common_timestamp.usecs-$ts0:onmatch(sched.sched_waking).wakeup_latency($wakeup_lat,next_pid,prio)' >> events/sched/sched_switch/trigger
+
+And here's the output of the hist_debug information for the
+sched_waking hist trigger. Note that there are two histograms
+displayed in the output: the first is the normal sched_waking
+histogram we've seen in the previous examples, and the second is the
+special histogram we created to provide the prio field variable.
+
+Looking at the second histogram below, we see a variable with the name
+synthetic_prio. This is the field variable created for the prio field
+on that sched_waking histogram::
+
+ # cat events/sched/sched_waking/hist_debug
+
+ # event histogram
+ #
+ # trigger info: hist:keys=pid:vals=hitcount:ts0=common_timestamp.usecs:sort=hitcount:size=2048:clock=global [active]
+ #
+
+ hist_data: 00000000349570e4
+
+ n_vals: 2
+ n_keys: 1
+ n_fields: 3
+
+ val fields:
+
+ hist_data->fields[0]:
+ flags:
+ VAL: HIST_FIELD_FL_HITCOUNT
+ type: u64
+ size: 8
+ is_signed: 0
+
+ hist_data->fields[1]:
+ flags:
+ HIST_FIELD_FL_VAR
+ var.name: ts0
+ var.idx (into tracing_map_elt.vars[]): 0
+ type: u64
+ size: 8
+ is_signed: 0
+
+ key fields:
+
+ hist_data->fields[2]:
+ flags:
+ HIST_FIELD_FL_KEY
+ ftrace_event_field name: pid
+ type: pid_t
+ size: 8
+ is_signed: 1
+
+
+ # event histogram
+ #
+ # trigger info: hist:keys=pid:vals=hitcount:synthetic_prio=prio:sort=hitcount:size=2048 [active]
+ #
+
+ hist_data: 000000006920cf38
+
+ n_vals: 2
+ n_keys: 1
+ n_fields: 3
+
+ val fields:
+
+ hist_data->fields[0]:
+ flags:
+ VAL: HIST_FIELD_FL_HITCOUNT
+ type: u64
+ size: 8
+ is_signed: 0
+
+ hist_data->fields[1]:
+ flags:
+ HIST_FIELD_FL_VAR
+ ftrace_event_field name: prio
+ var.name: synthetic_prio
+ var.idx (into tracing_map_elt.vars[]): 0
+ type: int
+ size: 4
+ is_signed: 1
+
+ key fields:
+
+ hist_data->fields[2]:
+ flags:
+ HIST_FIELD_FL_KEY
+ ftrace_event_field name: pid
+ type: pid_t
+ size: 8
+ is_signed: 1
+
+Looking at the sched_switch histogram below, we can see a reference to
+the synthetic_prio variable on sched_waking, and looking at the
+associated hist_data address we see that it is indeed associated with
+the new histogram. Note also that the other references are to a
+normal variable, wakeup_lat, and to a normal field variable, next_pid,
+the details of which are in the field variables section::
+
+ # cat events/sched/sched_switch/hist_debug
+
+ # event histogram
+ #
+ # trigger info: hist:keys=next_pid:vals=hitcount:wakeup_lat=common_timestamp.usecs-$ts0:sort=hitcount:size=2048:clock=global:onmatch(sched.sched_waking).wakeup_latency($wakeup_lat,next_pid,prio) [active]
+ #
+
+ hist_data: 00000000a73b67df
+
+ n_vals: 2
+ n_keys: 1
+ n_fields: 3
+
+ val fields:
+
+ hist_data->fields[0]:
+ flags:
+ VAL: HIST_FIELD_FL_HITCOUNT
+ type: u64
+ size: 8
+ is_signed: 0
+
+ hist_data->fields[1]:
+ flags:
+ HIST_FIELD_FL_VAR
+ var.name: wakeup_lat
+ var.idx (into tracing_map_elt.vars[]): 0
+ type: u64
+ size: 0
+ is_signed: 0
+
+ key fields:
+
+ hist_data->fields[2]:
+ flags:
+ HIST_FIELD_FL_KEY
+ ftrace_event_field name: next_pid
+ type: pid_t
+ size: 8
+ is_signed: 1
+
+ variable reference fields:
+
+ hist_data->var_refs[0]:
+ flags:
+ HIST_FIELD_FL_VAR_REF
+ name: ts0
+ var.idx (into tracing_map_elt.vars[]): 0
+ var.hist_data: 00000000349570e4
+ var_ref_idx (into hist_data->var_refs[]): 0
+ type: u64
+ size: 8
+ is_signed: 0
+
+ hist_data->var_refs[1]:
+ flags:
+ HIST_FIELD_FL_VAR_REF
+ name: wakeup_lat
+ var.idx (into tracing_map_elt.vars[]): 0
+ var.hist_data: 00000000a73b67df
+ var_ref_idx (into hist_data->var_refs[]): 1
+ type: u64
+ size: 0
+ is_signed: 0
+
+ hist_data->var_refs[2]:
+ flags:
+ HIST_FIELD_FL_VAR_REF
+ name: next_pid
+ var.idx (into tracing_map_elt.vars[]): 1
+ var.hist_data: 00000000a73b67df
+ var_ref_idx (into hist_data->var_refs[]): 2
+ type: pid_t
+ size: 4
+ is_signed: 0
+
+ hist_data->var_refs[3]:
+ flags:
+ HIST_FIELD_FL_VAR_REF
+ name: synthetic_prio
+ var.idx (into tracing_map_elt.vars[]): 0
+ var.hist_data: 000000006920cf38
+ var_ref_idx (into hist_data->var_refs[]): 3
+ type: int
+ size: 4
+ is_signed: 1
+
+ field variables:
+
+ hist_data->field_vars[0]:
+
+ field_vars[0].var:
+ flags:
+ HIST_FIELD_FL_VAR
+ var.name: next_pid
+ var.idx (into tracing_map_elt.vars[]): 1
+
+ field_vars[0].val:
+ ftrace_event_field name: next_pid
+ type: pid_t
+ size: 4
+ is_signed: 1
+
+ action tracking variables (for onmax()/onchange()/onmatch()):
+
+ hist_data->actions[0].match_data.event_system: sched
+ hist_data->actions[0].match_data.event: sched_waking
+
+The commands below can be used to clean things up for the next test::
+
+ # echo '!hist:keys=next_pid:wakeup_lat=common_timestamp.usecs-$ts0:onmatch(sched.sched_waking).wakeup_latency($wakeup_lat,next_pid,prio)' >> events/sched/sched_switch/trigger
+
+ # echo '!hist:keys=pid:ts0=common_timestamp.usecs' >> events/sched/sched_waking/trigger
+
+ # echo '!wakeup_latency u64 lat; pid_t pid; int prio' >> synthetic_events
+
+Alias test
+----------
+
+This example is very similar to previous examples, but demonstrates
+the alias flag.
+
+First, we create the wakeup_latency synthetic event::
+
+ # echo 'wakeup_latency u64 lat; pid_t pid; char comm[16]' >> synthetic_events
+
+Next, we create a sched_waking trigger similar to previous examples,
+but in this case we save the pid in the waking_pid variable::
+
+ # echo 'hist:keys=pid:waking_pid=pid:ts0=common_timestamp.usecs' >> events/sched/sched_waking/trigger
+
+For the sched_switch trigger, instead of using $waking_pid directly in
+the wakeup_latency synthetic event invocation, we create an alias of
+$waking_pid named $woken_pid, and use that in the synthetic event
+invocation instead::
+
+ # echo 'hist:keys=next_pid:woken_pid=$waking_pid:wakeup_lat=common_timestamp.usecs-$ts0:onmatch(sched.sched_waking).wakeup_latency($wakeup_lat,$woken_pid,next_comm)' >> events/sched/sched_switch/trigger
+
+Looking at the sched_waking hist_debug output, in addition to the
+normal fields, we can see the waking_pid variable::
+
+ # cat events/sched/sched_waking/hist_debug
+
+ # event histogram
+ #
+ # trigger info: hist:keys=pid:vals=hitcount:waking_pid=pid,ts0=common_timestamp.usecs:sort=hitcount:size=2048:clock=global [active]
+ #
+
+ hist_data: 00000000a250528c
+
+ n_vals: 3
+ n_keys: 1
+ n_fields: 4
+
+ val fields:
+
+ hist_data->fields[0]:
+ flags:
+ VAL: HIST_FIELD_FL_HITCOUNT
+ type: u64
+ size: 8
+ is_signed: 0
+
+ hist_data->fields[1]:
+ flags:
+ HIST_FIELD_FL_VAR
+ ftrace_event_field name: pid
+ var.name: waking_pid
+ var.idx (into tracing_map_elt.vars[]): 0
+ type: pid_t
+ size: 4
+ is_signed: 1
+
+ hist_data->fields[2]:
+ flags:
+ HIST_FIELD_FL_VAR
+ var.name: ts0
+ var.idx (into tracing_map_elt.vars[]): 1
+ type: u64
+ size: 8
+ is_signed: 0
+
+ key fields:
+
+ hist_data->fields[3]:
+ flags:
+ HIST_FIELD_FL_KEY
+ ftrace_event_field name: pid
+ type: pid_t
+ size: 8
+ is_signed: 1
+
+The sched_switch hist_debug output shows that a variable named
+woken_pid has been created but that it also has the
+HIST_FIELD_FL_ALIAS flag set. It also has the HIST_FIELD_FL_VAR flag
+set, which is why it appears in the val field section.
+
+Despite that implementation detail, an alias variable is actually more
+like a variable reference; in fact it can be thought of as a reference
+to a reference. The implementation copies the var_ref->fn() from the
+variable reference being referenced, in this case, the waking_pid
+fn(), which is hist_field_var_ref() and makes that the fn() of the
+alias. The hist_field_var_ref() fn() requires the var_ref_idx of the
+variable reference it's using, so waking_pid's var_ref_idx is also
+copied to the alias. The end result is that when the value of alias
+is retrieved, in the end it just does the same thing the original
+reference would have done and retrieves the same value from the
+var_ref_vals[] array. You can verify this in the output by noting
+that the var_ref_idx of the alias, in this case woken_pid, is the same
+as the var_ref_idx of the reference, waking_pid, in the variable
+reference fields section.
+
+Additionally, once it gets that value, since it is also a variable, it
+then saves that value into its var.idx. So the var.idx of the
+woken_pid alias is 0, which it fills with the value from var_ref_idx 0
+when its fn() is called to update itself. You'll also notice that
+there's a woken_pid var_ref in the variable refs section. That is the
+reference to the woken_pid alias variable, and you can see that it
+retrieves the value from the same var.idx as the woken_pid alias, 0,
+and then in turn saves that value in its own var_ref_idx slot, 3, and
+the value at this position is finally what gets assigned to the
+$woken_pid slot in the trace event invocation::
+
+ # cat events/sched/sched_switch/hist_debug
+
+ # event histogram
+ #
+ # trigger info: hist:keys=next_pid:vals=hitcount:woken_pid=$waking_pid,wakeup_lat=common_timestamp.usecs-$ts0:sort=hitcount:size=2048:clock=global:onmatch(sched.sched_waking).wakeup_latency($wakeup_lat,$woken_pid,next_comm) [active]
+ #
+
+ hist_data: 0000000055d65ed0
+
+ n_vals: 3
+ n_keys: 1
+ n_fields: 4
+
+ val fields:
+
+ hist_data->fields[0]:
+ flags:
+ VAL: HIST_FIELD_FL_HITCOUNT
+ type: u64
+ size: 8
+ is_signed: 0
+
+ hist_data->fields[1]:
+ flags:
+ HIST_FIELD_FL_VAR
+ HIST_FIELD_FL_ALIAS
+ var.name: woken_pid
+ var.idx (into tracing_map_elt.vars[]): 0
+ var_ref_idx (into hist_data->var_refs[]): 0
+ type: pid_t
+ size: 4
+ is_signed: 1
+
+ hist_data->fields[2]:
+ flags:
+ HIST_FIELD_FL_VAR
+ var.name: wakeup_lat
+ var.idx (into tracing_map_elt.vars[]): 1
+ type: u64
+ size: 0
+ is_signed: 0
+
+ key fields:
+
+ hist_data->fields[3]:
+ flags:
+ HIST_FIELD_FL_KEY
+ ftrace_event_field name: next_pid
+ type: pid_t
+ size: 8
+ is_signed: 1
+
+ variable reference fields:
+
+ hist_data->var_refs[0]:
+ flags:
+ HIST_FIELD_FL_VAR_REF
+ name: waking_pid
+ var.idx (into tracing_map_elt.vars[]): 0
+ var.hist_data: 00000000a250528c
+ var_ref_idx (into hist_data->var_refs[]): 0
+ type: pid_t
+ size: 4
+ is_signed: 1
+
+ hist_data->var_refs[1]:
+ flags:
+ HIST_FIELD_FL_VAR_REF
+ name: ts0
+ var.idx (into tracing_map_elt.vars[]): 1
+ var.hist_data: 00000000a250528c
+ var_ref_idx (into hist_data->var_refs[]): 1
+ type: u64
+ size: 8
+ is_signed: 0
+
+ hist_data->var_refs[2]:
+ flags:
+ HIST_FIELD_FL_VAR_REF
+ name: wakeup_lat
+ var.idx (into tracing_map_elt.vars[]): 1
+ var.hist_data: 0000000055d65ed0
+ var_ref_idx (into hist_data->var_refs[]): 2
+ type: u64
+ size: 0
+ is_signed: 0
+
+ hist_data->var_refs[3]:
+ flags:
+ HIST_FIELD_FL_VAR_REF
+ name: woken_pid
+ var.idx (into tracing_map_elt.vars[]): 0
+ var.hist_data: 0000000055d65ed0
+ var_ref_idx (into hist_data->var_refs[]): 3
+ type: pid_t
+ size: 4
+ is_signed: 1
+
+ hist_data->var_refs[4]:
+ flags:
+ HIST_FIELD_FL_VAR_REF
+ name: next_comm
+ var.idx (into tracing_map_elt.vars[]): 2
+ var.hist_data: 0000000055d65ed0
+ var_ref_idx (into hist_data->var_refs[]): 4
+ type: char[16]
+ size: 256
+ is_signed: 0
+
+ field variables:
+
+ hist_data->field_vars[0]:
+
+ field_vars[0].var:
+ flags:
+ HIST_FIELD_FL_VAR
+ var.name: next_comm
+ var.idx (into tracing_map_elt.vars[]): 2
+
+ field_vars[0].val:
+ ftrace_event_field name: next_comm
+ type: char[16]
+ size: 256
+ is_signed: 0
+
+ action tracking variables (for onmax()/onchange()/onmatch()):
+
+ hist_data->actions[0].match_data.event_system: sched
+ hist_data->actions[0].match_data.event: sched_waking
+
+The commands below can be used to clean things up for the next test::
+
+ # echo '!hist:keys=next_pid:woken_pid=$waking_pid:wakeup_lat=common_timestamp.usecs-$ts0:onmatch(sched.sched_waking).wakeup_latency($wakeup_lat,$woken_pid,next_comm)' >> events/sched/sched_switch/trigger
+
+ # echo '!hist:keys=pid:ts0=common_timestamp.usecs' >> events/sched/sched_waking/trigger
+
+ # echo '!wakeup_latency u64 lat; pid_t pid; char comm[16]' >> synthetic_events
--
2.26.2
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