Re: [tip:perf/urgent] perf: Fix SIGIO handling

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

On Wed, 2011-06-01 at 15:41 -0400, Vince Weaver wrote:
> On Sat, 28 May 2011, tip-bot for Peter Zijlstra wrote:
> 
> > Commit-ID:  f506b3dc0ec454a16d40cab9ee5d75435b39dc50
> > Gitweb:     http://git.kernel.org/tip/f506b3dc0ec454a16d40cab9ee5d75435b39dc50
> > Author:     Peter Zijlstra <a.p.zijlstra@chello.nl>
> > AuthorDate: Thu, 26 May 2011 17:02:53 +0200
> > Committer:  Ingo Molnar <mingo@elte.hu>
> > CommitDate: Sat, 28 May 2011 17:04:59 +0200
> > 
> > perf: Fix SIGIO handling
> > 
> > Vince noticed that unless we mmap() a buffer, SIGIO gets lost. So
> > explicitly push the wakeup (including signals) when requested.
> > 
> > Reported-by: Vince Weaver <vweaver1@eecs.utk.edu>
> > Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> > Cc: <stable@kernel.org>
> > Link: http://lkml.kernel.org/n/tip-2euus3f3x3dyvdk52cjxw8zu@git.kernel.org
> > Signed-off-by: Ingo Molnar <mingo@elte.hu>
> > ---
> >  kernel/events/core.c |    8 ++++++++
> >  1 files changed, 8 insertions(+), 0 deletions(-)
> > 
> > diff --git a/kernel/events/core.c b/kernel/events/core.c
> > index c09767f..d863b3c 100644
> > --- a/kernel/events/core.c
> > +++ b/kernel/events/core.c
> > @@ -5028,6 +5028,14 @@ static int __perf_event_overflow(struct perf_event *event, int nmi,
> >  	else
> >  		perf_event_output(event, nmi, data, regs);
> >  
> > +	if (event->fasync && event->pending_kill) {
> > +		if (nmi) {
> > +			event->pending_wakeup = 1;
> > +			irq_work_queue(&event->pending);
> > +		} else
> > +			perf_event_wakeup(event);
> > +	}
> > +
> >  	return ret;
> >  }
> 
> there is something strange about this commit.  
> 
> With it I get many more overflows than expected (on the order of 40x more)
> when I am using PERF_IOC_REFRESH,1 to restart the sampled counter
> from the end of the signal handler.
> 
> Also if you use PERF_IOC_REFRESH with a value other than 1, the value
> seems to be ignored and all of the counts are POLL_IN rather than
> POLL_HUP as I'd expect.

OK, so what semantics do we expect?

Currently (with this patch) when a SIGIO is registered, every event
overflow (sample) generates a POLL_IN SIGIO, except when event_limit
disables the counter, in which case its a POLL_HUP.

Without the patch, we used to send POLL_IN on every wakeup that would
wake poll() and POLL_HUP every time event_limit was reached.

event_limit is incremented using IOC_REFRESH, when its non-zero its
decremented on every overflow, and when it reaches 0 again it fires
SIGIO-POLL_HUP and disables the event.