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

[Andi Kleen]

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:
> > > In particular, the 0x90 offset (IA32_PERF_GLOBAL_STATUS). Intel once
> > > confirmed to me that that is a direct copy of the similarly named MSR at
> > > the time of the PEBS assist.
> > 
> > That is correct.
> > 
> > > 
> > > This is a problem, since if multiple counters overflow multiple bits
> > > will be set and its (afaict) ambiguous which event is for which counter.
> > 
> > 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.

> 
> Because if it does raise it, but then clears it again, its raised for a
> short while and might be observed.

That's correct. 

But it's no different than with threshold 1 and relatively rare.

-andi
-- 
ak@xxxxxxxxxxxxxxx -- Speaking for myself only
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