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

[Frederic Weisbecker]

On Wed, Apr 27, 2016 at 09:53:58AM -0300, Arnaldo Carvalho de Melo wrote:
> 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.

There is no rush though. The sysfs limit will probably be enough for most users. Unless
someone requested it?