Re: perf: child events not killed on release paths, survive indefinitely

[Peter Zijlstra]

On Fri, Jul 18, 2014 at 01:32:39PM +0100, Mark Rutland wrote:
> Hi all,
> 
> Sheetal reported a weird issue on arm where events which have been
> closed seem to stay around and compete for HW counters if an application
> has forked between the events being opened and them being closed.
> 
> I've reproduced this in mainline and linux-next and this seems to be a
> generic issue; the below test case fires on my x86-64 workstation as
> well as on arm and arm64.
> 
> The problem is the way we (don't) handle child events when releasing a
> parent in perf_release and perf_event_release_kernel. We call put_event
> on the parent when it is released, but this will exit early having done
> nothing because each child will have incremented the parent refcount
> when initialised from perf_event_init_task. We don't seem to do anything
> about the children in this case.
> 
> Thus the parent event can't be killed until all the children have first
> been killed. As the only places references to them exist are the
> parent's child_list and their respective tasks' hardware
> perf_event_context, they'll only get killed when their respective tasks
> exit (I confirmed this with some printks in hw_perf_event_destroy and
> put_event). Until that happens they remain in their respective contexts
> and continue to compete for HW counters, adversely affecting events
> opened later.
> 
> I'm not sure what the best way of handling this is. We need to clean up
> the children when the last possible user of the event is gone, but it
> looks to me like we'd need to have a separate child_refcount or
> reader_refcount to be able to tell when the events are still useful and
> when the only references which remain are internal.
> 
> Any ideas?

Jiri was recently poking at that:

lkml.kernel.org/r/1405079782-8139-3-git-send-email-jolsa@xxxxxxxxxx

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