linux-kernel - Re: [PATCH 5/5] ftrace: Update dynamic ftrace calls only if necessary

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Date:	Fri, 18 Mar 2016 10:27:23 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Jiri Olsa <jolsa@...nel.org>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Namhyung Kim <namhyung@...nel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Arnaldo Carvalho de Melo <acme@...nel.org>
Subject: Re: [PATCH 5/5] ftrace: Update dynamic ftrace calls only if
 necessary


Patches 4 and 5 seem agnostic to patches 1-3. I'll pull 4,5 into my
tree and 1-3 can go though tip. I'd like to run theses through my tests.

OK?

-- Steve


On Wed, 16 Mar 2016 15:34:33 +0100
Jiri Olsa <jolsa@...nel.org> wrote:

> Currently dynamic ftrace calls are updated any time
> the ftrace_ops is un/registered. If we do  this update
> only when it's needed, we save lot of time for perf
> system wide ftrace function sampling/counting.
> 
> The reason is that for system wide sampling/counting,
> perf creates event for each cpu in the system.
> 
> Each event then registers separate copy of ftrace_ops,
> which ends up in FTRACE_UPDATE_CALLS updates. On servers
> with many cpus that means serious stall (240 cpus server):
> 
> Counting:
>   # time ./perf stat -e ftrace:function -a sleep 1
> 
>    Performance counter stats for 'system wide':
> 
>               370,663      ftrace:function
> 
>           1.401427505 seconds time elapsed
> 
>   real    3m51.743s
>   user    0m0.023s
>   sys     3m48.569s
> 
> Sampling:
>   # time ./perf record -e ftrace:function -a sleep 1
>   [ perf record: Woken up 0 times to write data ]
>   Warning:
>   Processed 141200 events and lost 5 chunks!
> 
>   [ perf record: Captured and wrote 10.703 MB perf.data (135950 samples) ]
> 
>   real    2m31.429s
>   user    0m0.213s
>   sys     2m29.494s
> 
> There's no reason to do the FTRACE_UPDATE_CALLS update
> for each event in perf case, because all the ftrace_ops
> always share the same filter, so the updated calls are
> always the same.
> 
> It's required that only first ftrace_ops registration
> does the FTRACE_UPDATE_CALLS update (also sometimes
> the second if the first one used the trampoline), but
> the rest can be only cheaply linked into the ftrace_ops
> list.
> 
> Counting:
>   # time ./perf stat -e ftrace:function -a sleep 1
> 
>    Performance counter stats for 'system wide':
> 
>              398,571      ftrace:function
> 
>          1.377503733 seconds time elapsed
> 
>   real    0m2.787s
>   user    0m0.005s
>   sys     0m1.883s
> 
> Sampling:
>   # time ./perf record -e ftrace:function -a sleep 1
>   [ perf record: Woken up 0 times to write data ]
>   Warning:
>   Processed 261730 events and lost 9 chunks!
> 
>   [ perf record: Captured and wrote 19.907 MB perf.data (256293 samples) ]
> 
>   real    1m31.948s
>   user    0m0.309s
>   sys     1m32.051s
> 
> Signed-off-by: Jiri Olsa <jolsa@...nel.org>
> ---