Re: [RFC][PATCH] perf_events: Fix FORK events

[Ingo Molnar <mingo@elte.hu>]

* Peter Zijlstra <peterz@infradead.org> wrote:

> Hi,
> 
> While looking into a problem reported by Pekka, I noticed that I wasn't
> receiving any FORK events for a workload that did fork (see attachment).

ah yes, my old testcase for forks.

> After making the below change, stuff started working again, thing is, I'm 
> not sure why.

In case it matters: that workload of 4x fork (and the whole fork events 
mechanism) was always very sensitive to the precise timing of when a child 
and a parent does what, in the final dance of wait(), do_exit(), 
release_task(), etc. when a task exits. Especially on SMP systems.

> The main change is sending the FORK event to the parent instead of the 
> child thread, however perf_event_fork() is at the end of copy_process(), 
> which is after perf_event_init_task() which inherits all the counters, so 
> it should all have worked as it was.
> 
> We could of course just slam the commit in and not worry about it, but
> that just doesn't feel right.

Would be nice to figure it out ...

	Ingo