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

[Alexei Starovoitov]

On Mon, Apr 25, 2016 at 09:29:28PM -0300, Arnaldo Carvalho de Melo wrote:
> Em Mon, Apr 25, 2016 at 05:07:26PM -0700, Alexei Starovoitov escreveu:
> > > +	{
> > > +		.procname	= "perf_event_max_stack",
> > > +		.data		= NULL, /* filled in by handler */
> > > +		.maxlen		= sizeof(sysctl_perf_event_max_stack),
> > > +		.mode		= 0644,
> > > +		.proc_handler	= perf_event_max_stack_handler,
> > > +		.extra1		= &zero,
> > > +	},
>  
> > you need to define a max value otherwise perf_callchain_entry__sizeof
> > will overflow. Sure it's root only facility, but still not nice.
> > 1M? Anything above 1M stack frames would be insane anyway.
> > The rest looks good. Thanks!
> 
> Something else? ;-)

all looks good to me. Thanks a bunch!

> 
> 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>

yep :)
hopefully Brendan can give it another spin.