Re: [PATCH 2/3] perf/x86/pebs: add workaround for broken OVFL status on HSW

From: Peter Zijlstra
Date: Thu Dec 15 2016 - 12:11:16 EST

On Thu, Dec 15, 2016 at 08:59:56AM -0800, Stephane Eranian wrote:
> On Thu, Dec 15, 2016 at 12:42 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> > On Wed, Dec 14, 2016 at 11:26:49PM -0800, Stephane Eranian wrote:
> >> On Wed, Dec 14, 2016 at 9:55 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> >> >
> >> > Just spotted this again, ping?
> >> >
> >> Ok, on what processor running what command, so I can try and reproduce?
> >
> > For me its more of a correctness issue, i've not actually spotted a
> > problem as such.
> >
> > But every time I read this code it makes me wonder.
> >
> > Supposing that the hardware sets the CTRL overflow flags but hasn't
> > generated the PEBS record yet (or not enough records to reach the PEBS
> > buffer threshold) we still don't want to process these events as if they
> > were !PEBS.
> >
> I am suspicious about the case where you have multiple PEBS events and
> they do not quite fire at the same time but close enough that you may have
> PEBS in-flight by the time you enter handle_irq.
> Last night I ran a simple test on SKL using tip.git:
> $ perf record --e
> cpu/event=0xd0,umask=0x81/upp,cpu/event=0xc0,umask=1/upp,cpu/event=0xd0,umask=0x81/upp
> multichase; perf report -D | fgrep SAMPLE | grep -v 'IP, 0x4' | grep
> -v events
> Basically, looking for samples missing the EXACT tag, i.e., samples
> processed a regular event when I only have PEBS events. Over 8h, I got
> about 3 or 4 such samples. So there is still a condition where we see
> the overflow as regular and not PEBS. So we need to examine that code
> again looking for possible race with PEBS in flight and not having the
> PEBS overflow bits yet.

Isn't that exactly the case I was talking about? and would be avoided by
the proposed patch?

So semantically the counter overflows and then arms PEBS to record a
record on the next event once its armed (and this can be multiple events
after the overflow, since arming takes a while too).

Now, if the chip manages to raise the regular overflow bit during that
time, you get exactly what is described.

meaning we should unconditionally clear the pebs_enabled.