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

[Arnaldo Carvalho de Melo <acme@kernel.org>]

Em Tue, Apr 26, 2016 at 11:58:10PM +0200, Frederic Weisbecker escreveu:
> 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

Yeah, I'll work on that too.

> 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 for reviewieng it!

- Arnaldo