Re: [PATCH/RFC v3] perf core: Allow setting up max frame stack depth via sysctl

[Frederic Weisbecker]

On Mon, Apr 25, 2016 at 09:29:28PM -0300, Arnaldo Carvalho de Melo wrote:
> commit cd544af4f7fede01cb512d52bb3efe62aa19271d
> Author: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
> 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@xxxxxxxxx>
>     Acked-by: Alexei Starovoitov <ast@xxxxxxxxxx>
>     Acked-by: David Ahern <dsahern@xxxxxxxxx>

Reviewed-by: Frederic Weisbecker <fweisbec@xxxxxxxxx>

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.