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.gr...@gmail.com>
> Link: http://lkml.kernel.org/n/tip-cgls6uuncwjtq969tys1j...@git.kernel.org
> Signed-off-by: Arnaldo Carvalho de Melo <a...@redhat.com>
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.
