linux-kernel - Re: PEBS bug on HSW: "Unexpected number of pebs records 10" (was: Re: [GIT PULL] perf changes for v3.12)

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Message-ID: <20130910152845.GI31370@twins.programming.kicks-ass.net>
Date:	Tue, 10 Sep 2013 17:28:45 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	eranian@...il.com
Cc:	Ingo Molnar <mingo@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andi Kleen <andi@...stfloor.org>
Subject: Re: PEBS bug on HSW: "Unexpected number of pebs records 10" (was:
 Re: [GIT PULL] perf changes for v3.12)

On Tue, Sep 10, 2013 at 07:15:19AM -0700, Stephane Eranian wrote:
> The threshold is where to generate the interrupt. It does not mean
> where to stop PEBS recording.

It does, since we don't set a reset value. So once a PEBS assist
happens, that counter stops until we reprogram it in the PMI.

> So it is possible that in HSW, we may
> get into a situation where it takes time to get to the handler to stop
> the PMU. I don't know how given we use NMI. Well, unless we were
> already servicing an NMI at the time. But given that we stop the PMU
> almost immediately in the handler, I don't see how that would
> possible.  The other oddity in HSW is that we clear the NMI on entry
> to the handler and not at the end. I never gotten an good explanation
> as to why that was necessary. So maybe it is related...

Even if the PMI was delayed that counter would never do another entry.
So I really don't see how we can have more than nr_counters PEBS entries
in the buffer.
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