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Date: Wed, 26 Aug 2009 22:28:27 +0200
From: Ingo Molnar <mingo@...e.hu>
To: David Miller <davem@...emloft.net>
Cc: nhorman@...driver.com, rostedt@...dmis.org, fweisbec@...il.com,
billfink@...dspring.com, netdev@...r.kernel.org, brice@...i.com,
gallatin@...i.com
Subject: Re: Receive side performance issue with multi-10-GigE and NUMA
* Ingo Molnar <mingo@...e.hu> wrote:
> * David Miller <davem@...emloft.net> wrote:
>
> > From: Ingo Molnar <mingo@...e.hu>
> > Date: Wed, 26 Aug 2009 21:08:30 +0200
> >
> > > Sigh, no. Please re-read the past discussions about this.
> > > trace_skb_sources.c is a hack and should be converted to generic
> > > tracepoints. Is there anything in it that cannot be expressed in
> > > terms of TRACE_EVENT()?
> >
> > Neil explained why he needed to implement it this way in his reply
> > to Steven Rostedt. I attach it here for your convenience.
>
> thanks. The argument is invalid:
>
> > > BTW, why not just do this as events? Or was this just a easy way
> > > to communicate with the user space tools?
> >
> > Thats exactly why I did it. the idea is for me to now write a
> > user space tool that lets me analyze the events and ajust process
> > scheduling to optimize the rx path. Neil
>
> All tooling (in fact _more_ tooling) can be done based on generic,
> TRACE_EVENT() based tracepoints. Generic tracepoints are far more
> available, have a generalized format with format parsers and user
> tooling implemented, etc. etc.
To expand on the 'etc. etc.'.
Right now we already have once TRACE_EVENT() based generic
tracepoint for skbs - the skb_free one in
include/trace/events/skb.h.
Here's a list of examples of what that single generic tracepoint
allows us to do, which Neil's kernel/trace/trace_skb_sources.c code
cannot do:
- structured format/field description:
aldebaran:~> cat /debug/tracing/events/skb/kfree_skb/format
name: kfree_skb
ID: 603
format:
field:unsigned short common_type; offset:0; size:2;
field:unsigned char common_flags; offset:2; size:1;
field:unsigned char common_preempt_count; offset:3; size:1;
field:int common_pid; offset:4; size:4;
field:int common_tgid; offset:8; size:4;
field:void * skbaddr; offset:16; size:8;
field:unsigned short protocol; offset:24; size:2;
field:void * location; offset:32; size:8;
print fmt: "skbaddr=%p protocol=%u location=%p", REC->skbaddr, REC->protocol, REC->location
The advantages of that are numerous: we have a user-space parser
for that, so new tracepoints or changes to tracepoints can be
propagated across the tooling automatically. (see below examples
about how this works in practice)
- perfcounters integration:
- it's enumerated and visible in the list of tracepoints:
aldebaran:~> perf list 2>&1 | grep skb
skb:kfree_skb [Tracepoint event]
- the tracepoint can be used for statistics (perf stat):
aldebaran:~> perf stat -e skb:kfree_skb -a sleep 1
Performance counter stats for 'sleep 1':
- noise analysis:
aldebaran:~> perf stat --repeat 10 -e skb:kfree_skb -a sleep 1
Performance counter stats for 'sleep 1' (10 runs):
25 skb:kfree_skb ( +- 7.692% )
- the tracepoint can be used for profiling:
aldebaran:~> perf top -e skb:kfree_skb -c 1
------------------------------------------------------------------------------
PerfTop: 334 irqs/sec kernel: 0.3% [1 skb:kfree_skb], (all, 16 CPUs)
------------------------------------------------------------------------------
samples pcnt RIP kernel function
______ _______ _____ ________________ _______________
23.00 - 100.0% - ffffffff81266828 : store_bind
- can be used to do call-graph profiling that captures kernel
and user-space call-graphs as well:
aldebaran:~> perf record --call-graph -e skb:kfree_skb -c 1 -f -a sleep 1
[ perf record: Captured and wrote 0.035 MB perf.data (~1547 samples) ]
aldebaran:~> perf report
...
# Samples: 4102
#
# Overhead Command Shared Object Symbol
# ........ ............... ........................................................................................................ ......
#
88.44% distccd 3641efb1d0 [.] 0x00003641efb1d0
3.07% Xorg 3641ed6590 [.] 0x00003641ed6590
2.51% at-spi-registry 3642a0db50 [.] 0x00003642a0db50
2.24% sshd /lib64/libc-2.8.so [.] __libc_read
0.73% sshd 7f71d4e69590 [.] 0x007f71d4e69590
0.63% init [kernel] [k] store_bind
0.56% sshd /lib64/libc-2.8.so [.] __recvmsg
0.49% gnome-settings- 3642a0db8b [.] 0x00003642a0db8b
0.39% sshd /lib64/libc-2.8.so [.] __GI___libc_connect
0.39% sshd /lib64/libc-2.8.so [.] __sendto_nocancel
0.15% id /lib64/libc-2.8.so [.] __GI___libc_connect
|
|--50.00%-- get_mapping
| __nscd_get_map_ref
|
--50.00%-- __nscd_open_socket
0.10% metacity 3641ed6590 [.] 0x00003641ed6590
0.07% gdm-simple-gree 3642a0db8b [.] 0x00003642a0db8b
|
|--66.67%-- 0x3641ed65cb
|
--33.33%-- 0x3642a0db8b
0.05% bash /lib64/libc-2.8.so [.] __GI___libc_connect
|
|--50.00%-- get_mapping
| __nscd_get_map_ref
|
--50.00%-- __nscd_open_socket
0.05% :3129 /lib64/libc-2.8.so [.] __GI___libc_connect
|
|--50.00%-- get_mapping
| __nscd_get_map_ref
|
--50.00%-- __nscd_open_socket
0.05% :3098 /lib64/libc-2.8.so [.] __GI___libc_connect
|
|--50.00%-- get_mapping
| __nscd_get_map_ref
|
--50.00%-- __nscd_open_socket
0.02% init [kernel] [k] bind_con_driver
0.02% gnome-power-man 3642a0db50 [.] 0x00003642a0db50
0.02% cc1 /opt/crosstool/gcc-4.2.2-glibc-2.3.6/i686-unknown-linux-gnu/libexec/gcc/i686-unknown-linux-gnu/4.2.2/cc1 [.] num_positive
- can be used to capture traces to user-space and analyze them
there:
aldebaran:/home/mingo> perf record -e skb:kfree_skb:r -c 1 -R -f -a sleep 10
[ perf record: Captured and wrote 4.426 MB perf.data (~193365 samples) ]
aldebaran:/home/mingo> perf trace
version = 0.5B6
init-0 [000] 0.000000: kfree_skb: skbaddr=0xffff8801bcc15300 protocol=2048 location=0xffffffff81461c94
Xorg-4411 [000] 0.000000: kfree_skb: skbaddr=0xffff8801bb955a00 protocol=0 location=0xffffffff814e8aff
at-spi-registry-4948 [000] 0.000000: kfree_skb: skbaddr=0xffff8801bb955a00 protocol=0 location=0xffffffff814e8aff
...
- generic tracepoints can be available with lots of other
tracepoints at once - while the skb_sources plugin is exclusive.
(no other plugin can be active at the same time) Generic
tracepoints have separate toggles - any sub-set of tracepoints
can be active at any time.
- per tracepoint filter expressions support, such as:
aldebaran:/debug/tracing/events/skb/kfree_skb> echo 'protocol == 0 && common_pid == 123' > filter
aldebaran:/debug/tracing/events/skb/kfree_skb> cat filter protocol == 0 && common_pid == 123
protocol == 0 && common_pid == 123
When this filter is modified, the kernel creates a (safe) list of
(atomically evaluatable) predicaments from the expression and the
data is filtered before it's traced.
The filter engine works in process, softirq, IRQ, NMI and any
other context and is very fast as well. (no parsing overhead in
the fastpath - we pre-parse the expression and break it down.)
In other words, generic tracepoints are _vastly_ superior to the
skb_sources plugin, and this fact is obvious to all tracing
developers, that's why every tracing developer who commented on this
thread asked (in a rather befuddled way) "why not TRACE_EVENT()?".
And note that the above examples were based on a _single_ existing
generic tracepoint of very limited utility - and still it already
allowed a lot of interesting data to be captured. If we had a more
comprehensive set of skb tracepoints, a whole lot of interesting
possibilities would open up ...
All in one, we dont do new ftrace plugins that can be done via
generic tracepoints - we only limit ftrace plugins to vastly
different things like the function tracer or the latency tracer.
That's why we have things like a tracing tree and a review process,
to address such issues before patches get committed.
David, please sort this out before sending any bits in this area to
Linus, Neil's response is basically "i want it this way" which is
not really acceptable - the maintainers of kernel/trace/* dont want
it this way, for very good technical reasons.
The skb_sources hack should be converted to a proper
TRACE_EVENT(skb_dequeue) tracepoint. Also, as we offered it on the
onset, we'd be glad to help out with the conversion. I can do a
patch if nobody volunteers.
Plus we'd like to encourage more TRACE_EVENT() networking
tracepoints like the existing skb_free. They are a great tool.
Ingo
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