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

[Alexei Starovoitov]

On Wed, Apr 20, 2016 at 07:47:30PM -0300, Arnaldo Carvalho de Melo wrote:
> 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-by: Brendan Gregg <brendan.d.gregg@xxxxxxxxx>
> Link: http://lkml.kernel.org/n/tip-cgls6uuncwjtq969tys1j6b0@xxxxxxxxxxxxxx
> Signed-off-by: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>

Nice. I like it. That's a great approach to hard problem.
Java guys will be happy too.
Please also adjust two places in kernel/bpf/stackmap.c

> +	{
> +		.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,

zero seems to be the wrong minimum. I think it should be at least 2 to
to fit user/kernel tags ?
Probably needs to define max as well.