Re: [RFC Patch] perf_event: fix a cgroup switch warning

[Peter Zijlstra <peterz@infradead.org>]

On Mon, May 13, 2019 at 05:27:47PM -0700, Cong Wang wrote:
> We have been consistently triggering the warning
> WARN_ON_ONCE(cpuctx->cgrp) in perf_cgroup_switch() for a rather
> long time, although we still have no clue on how to reproduce it.
> 
> Looking into the code, it seems the only possibility here is that
> the process calling perf_event_open() with a cgroup target exits
> before the process in the target cgroup exits but after it gains
> CPU to run. This is because we use the atomic counter
> perf_cgroup_events as an indication of whether cgroup perf event
> has enabled or not, which is inaccurate, illustrated as below:
> 
> CPU 0					CPU 1
> // open perf events with a cgroup
> // target for all CPU's
> perf_event_open():
>   account_event_cpu()
>   // perf_cgroup_events == 1
> 				// Schedule in a process in the target cgroup
> 				perf_cgroup_switch()
> perf_event_release_kernel():
>   unaccount_event_cpu()
>   // perf_cgroup_events == 0
> 				// schedule out
> 				// but perf_cgroup_sched_out() is skipped
> 				// cpuctx->cgrp left as non-NULL

				which implies we observed:
				'perf_cgroup_events == 0'

> 				// schedule in another process
> 				perf_cgroup_switch() // WARN triggerred

				which implies we observed:
				'perf_cgroup_events == 1'


Which is impossible. It _might_ have been possible if the out and in
happened on different CPUs. But then I'm not sure that is enough to
trigger the problem.

> The proposed fix is kinda ugly,

Yes :-)

> Suggestions? Thoughts?

At perf_event_release time, when it is the last cgroup event, there
should not be any cgroup events running anymore, so ideally
perf_cgroup_switch() would not set state.

Furthermore; list_update_cgroup_event() will actually clear cpuctx->cgrp
on removal of the last cgroup event.

Also; perf_cgroup_switch() will WARN when there are not in fact any
cgroup events at all. I would expect that WARN to trigger too in your
scneario. But you're not seeing that?

I do however note that that check seems racy; we do that without holding
the ctx_lock.