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Date: Tue, 26 Apr 2016 23:58:10 +0200
From: Frederic Weisbecker <fweisbec@gmail.com>
To: Arnaldo Carvalho de Melo <arnaldo.melo@gmail.com>
Cc: Alexei Starovoitov <alexei.starovoitov@gmail.com>,
        David Ahern <dsahern@gmail.com>, Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@kernel.org>,
        Adrian Hunter <adrian.hunter@intel.com>,
        Brendan Gregg <brendan.d.gregg@gmail.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Alexei Starovoitov <ast@kernel.org>, He Kuang <hekuang@huawei.com>,
        Jiri Olsa <jolsa@redhat.com>, Masami Hiramatsu <mhiramat@kernel.org>,
        Milian Wolff <milian.wolff@kdab.com>,
        Namhyung Kim <namhyung@kernel.org>,
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Subject: Re: [PATCH/RFC v3] perf core: Allow setting up max frame stack depth
 via sysctl
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On Mon, Apr 25, 2016 at 09:29:28PM -0300, Arnaldo Carvalho de Melo wrote:
> commit cd544af4f7fede01cb512d52bb3efe62aa19271d
> Author: Arnaldo Carvalho de Melo <acme@redhat.com>
> Date:   Thu Apr 21 12:28:50 2016 -0300
> 
>     perf core: Allow setting up max frame stack depth via sysctl
>     
>     The default remains 127, which is good for most cases, and not even hit
>     most of the time, but then for some cases, as reported by Brendan, 1024+
>     deep frames are appearing on the radar for things like groovy, ruby.
>     
>     And in some workloads putting a _lower_ cap on this may make sense. One
>     that is per event still needs to be put in place tho.
>     
>     The new file is:
>     
>       # cat /proc/sys/kernel/perf_event_max_stack
>       127
>     
>     Chaging it:
>     
>       # echo 256 > /proc/sys/kernel/perf_event_max_stack
>       # cat /proc/sys/kernel/perf_event_max_stack
>       256
>     
>     But as soon as there is some event using callchains we get:
>     
>       # echo 512 > /proc/sys/kernel/perf_event_max_stack
>       -bash: echo: write error: Device or resource busy
>       #
>     
>     Because we only allocate the callchain percpu data structures when there
>     is a user, which allows for changing the max easily, its just a matter
>     of having no callchain users at that point.
>     
>     Reported-and-Tested-by: Brendan Gregg <brendan.d.gregg@gmail.com>
>     Acked-by: Alexei Starovoitov <ast@kernel.org>
>     Acked-by: David Ahern <dsahern@gmail.com>

Reviewed-by: Frederic Weisbecker <fweisbec@gmail.com>

I first thought that this should be a tunable per event instead of a global sysctl
but then I realized that we still need a root-only-tunable maximum limit value to oppose
against any future per event limit and this sysctl value does the job.

Nice patch!

Thanks.