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Date: Wed, 4 Mar 2009 12:00:15 -0500
From: Mathieu Desnoyers <compudj@...stal.dyndns.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
Andrew Morton <akpm@...ux-foundation.org>,
Peter Zijlstra <peterz@...radead.org>,
Frederic Weisbecker <fweisbec@...il.com>,
Theodore Tso <tytso@....edu>,
Arjan van de Ven <arjan@...radead.org>,
Pekka Paalanen <pq@....fi>,
Arnaldo Carvalho de Melo <acme@...hat.com>,
"H. Peter Anvin" <hpa@...or.com>, Martin Bligh <mbligh@...gle.com>,
"Frank Ch. Eigler" <fche@...hat.com>,
Tom Zanussi <tzanussi@...il.com>,
Masami Hiramatsu <mhiramat@...hat.com>,
KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
Jason Baron <jbaron@...hat.com>,
Christoph Hellwig <hch@...radead.org>,
Jiaying Zhang <jiayingz@...gle.com>,
Eduard - Gabriel Munteanu <eduard.munteanu@...ux360.ro>,
mrubin@...gle.com, md@...gle.com
Subject: Re: [PATCH 0/5] [RFC] binary reading of ftrace ring buffers
* Steven Rostedt (rostedt@...dmis.org) wrote:
>
> RFC only, not for pulling, unless everyone is fine with these :-)
>
> After telling the folks from Google that I had splice code that I needed
> to get working, I finally got around to doing it.
>
> Not only did I get the splice code working, but I also made a perl
> script (might want to cover you eyes if you look at that code) that
> can automagically create the output from the binary files reading
> the format arguments in the /debugfs/tracing/events/<subsys>/<event>/format
> file.
>
> From previous patches, we have in include/trace/sched_event_types.h:
>
> #undef TRACE_SYSTEM
> #define TRACE_SYSTEM sched
>
> TRACE_EVENT_FORMAT(sched_switch,
> TPPROTO(struct rq *rq, struct task_struct *prev,
> struct task_struct *next),
> TPARGS(rq, prev, next),
> TPFMT("task %s:%d ==> %s:%d",
> prev->comm, prev->pid, next->comm, next->pid),
> TRACE_STRUCT(
> TRACE_FIELD(pid_t, prev_pid, prev->pid)
> TRACE_FIELD(int, prev_prio, prev->prio)
> TRACE_FIELD_SPECIAL(char next_comm[TASK_COMM_LEN],
> next_comm,
> TPCMD(memcpy(TRACE_ENTRY->next_comm,
> next->comm,
> TASK_COMM_LEN)))
> TRACE_FIELD(pid_t, next_pid, next->pid)
> TRACE_FIELD(int, next_prio, next->prio)
> ),
> TPRAWFMT("prev %d:%d ==> next %s:%d:%d")
> );
>
I fear that putting these user-visible format strings in tracepoint
header files will create a big maintainability issue.
I'll post the LTTng patchset in a jiffy, where the format string
awareness is done in a tracer-specific module. I don't understand why
Peter Z. is not yelling against your tracepoints modifications : they
are actually presenting to userspace an interface that is meant to
eventually change.
I used a separate layer for format string presentation for this very
purpose : I don't want to tie the kernel code instrumentation
(tracepoints) to any kind of user-visible API.
Mathieu
> By making that, we automagically get this file:
>
> # cat /debug/tracing/events/sched/sched_switch/format
> name: sched_switch
> ID: 29
> format:
> field:unsigned char type; offset:0; size:1;
> field:unsigned char flags; offset:1; size:1;
> field:unsigned char preempt_count; offset:2; size:1;
> field:int pid; offset:4; size:4;
> field:int tgid; offset:8; size:4;
>
> field:pid_t prev_pid; offset:12; size:4;
> field:int prev_prio; offset:16; size:4;
> field special:char next_comm[TASK_COMM_LEN]; offset:20; size:16;
> field:pid_t next_pid; offset:36; size:4;
> field:int next_prio; offset:40; size:4;
>
> print fmt: "prev %d:%d ==> next %s:%d:%d"
>
>
> Now with this patch set, we create a way to read the ftrace ring buffers
> directly, as a binary page. Splice has been used such that the user
> could splice the ring buffers without need to copy the pages.
> The pages are taken from the ring buffers and can be placed directly
> into files, without extra copies.
>
> # ls /debug/tracing/binary_buffers/
> 0 1 2 3 4 5 6 7
>
> One can either just use the read/write to grab live data from these
> buffers, or they could use splice. I have a simple file that reads
> this buffers using splice. Note, it only runs on one file, you
> can make multiple copies to run more. The ring buffers in ftrace
> are per cpu and they are not dependent on each other.
>
> Also, if there is no data in the buffer, it returns -EAGAIN.
>
> # find /debug/tracing/events -name 'type' | while read f; do
> > echo raw > $f; done
>
> # find /debug/tracing/events -name 'enable' | while read f; do
> > echo 1 > $f; done
>
> # ./splice /debug/tracing/binary_buffers/0 /tmp/buf-0
>
> Yes you can run multiple instances of this on different buffers.
>
> # ./rb-read.pl /tmp/buf-0
>
> produces:
>
> 0 [000] 7071.936459: (irq_handler_entry) irq 48
> 0 [000] 7071.936462: (irq_handler_exit) irq 48 ret 1
> 0 [000] 7071.988801: (sched_signal_send) sig: 14 task 0
> 0 [000] 7071.988813: (sched_wakeup) task 0 success=1
> 0 [000] 7071.988823: (sched_switch) prev 0:140 ==> next ntpd:6582272:0
> 3303 [000] 7071.988916: (sched_switch) prev 0:120 ==> next swap:7497072:0
> 0 [000] 7072.020370: (sched_wakeup) task 0 success=1
> 0 [000] 7072.020407: (sched_switch) prev 0:140 ==> next even:808416116:0
> 16 [000] 7071.936157: (sched_switch) prev 0:115 ==> next swap:7497072:0
>
> Notice the "(sched_switch)" lines.
>
> The splice C code:
> http://www.kernel.org/pub/linux/kernel/people/rostedt/splice.c
>
> The perl script to parse:
> http://www.kernel.org/pub/linux/kernel/people/rostedt/rb-read.pl
>
> And yes I know, they are both ugly :-p
>
> The following patches are in:
>
> git://git.kernel.org/pub/scm/linux/kernel/git/rostedt/linux-2.6-trace.git
>
> branch: rfc/splice/tip/tracing/ftrace
>
>
> Steven Rostedt (5):
> ring-buffer: reset write field for ring_buffer_read_page
> ring-buffer: fix ring_buffer_read_page
> ring-buffer: replace sizeof of event header with offsetof
> ring-buffer: make ring_buffer_read_page read from start on partial page
> tracing: add binary buffer files for use with splice
>
> ----
> include/linux/ring_buffer.h | 7 +-
> kernel/trace/ring_buffer.c | 118 +++++++++++++------
> kernel/trace/trace.c | 274 +++++++++++++++++++++++++++++++++++++++++-
> kernel/trace/trace.h | 1 +
> 4 files changed, 357 insertions(+), 43 deletions(-)
> --
>
--
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
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