Re: [RFC PATCH 6/7] perf, x86: large PEBS interrupt threshold

[Andi Kleen]

On Wed, May 28, 2014 at 07:05:31PM +0200, Peter Zijlstra wrote:
> On Wed, May 28, 2014 at 09:08:47AM -0700, Andi Kleen wrote:
> > On Wed, May 28, 2014 at 05:35:48PM +0200, Peter Zijlstra wrote:
> > > On Wed, May 28, 2014 at 07:58:31AM -0700, Andi Kleen wrote:
> 
> > > > When a PEBS counter overflows it clears its respective GLOBAL_STATUS bit
> > > > automatically.
> > > 
> > > That's an ambiguous statement; did you mean to say a PEBS enabled
> > > counter will not raise its bit in GLOBAL_STATUS on counter overflow?
> > 
> > Let's revisit how PEBS works:
> > 
> > - The counter overflows and sets the GLOBAL_STATUS bit
> > - The PEBS assist is armed
> > - The counter triggers again
> > - The PEBS assist fires and delivers a PEBS record
> > - Finally it clears the GLOBAL_STATUS
> > - When the threshold is reached it raises an PMI
> > 
> > So the GLOBAL_STATUS bit is visible between the first overflow and the end 
> > of the PEBS record delivery.
> 
> OK, so that's something entirely different from what you initially said,
> but it is what I thought it did -- you said that it clears on overflow
> but it clears after recording.

Fair enough. I should have said PEBS assist.

> If we get the PMI (where denoted) we can actually reconstruct which
> event triggered, by looking at which bit(s) flipped between the recorded
> state and the current state (due to E coming before F)

Normally when the PMI PEBS handler runs the GLOBAL_STATUS is already cleared
(as the PEBS assist will execute concurrently during the NMI entry)
Looking at the status won't help you much, it only has the PEBS bit 
set.

I don't think we need to do anything. It's a very unlikely situation
in normal operation, as the counter period is very long compared
to the race window.  When it happens very rarely we can ignore it.

It can happen mainly when you program two counters to count exactly
the same thing with the same threshold, but why would you do that?

I guess what would make sense is to add some debug counter somewhere
for this situation (more than one bit set)

-Andi
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