Re: [PATCH/RFC v3] perf core: Allow setting up max frame stack depth via sysctl
From: Arnaldo Carvalho de Melo
Date: Wed Apr 27 2016 - 08:54:08 EST
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@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
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