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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Date:	Mon, 16 Sep 2013 17:41:46 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	eranian@...il.com
Cc:	Peter Zijlstra <peterz@...radead.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)


* Stephane Eranian <eranian@...glemail.com> wrote:

> Hi,
> 
> Some updates on this problem.
> I have been running tests all week-end long on my HSW.
> I can reproduce the problem. What I know:
> 
> - It is not linked with callchain
> - The extra entries are valid
> - The reset values are still zeroes
> - The problem does not happen on SNB with the same test case
> - The PMU state looks sane when that happens.
> - The problem occurs even when restricting to one CPU/core (taskset -c 0-3)
> 
> So it seems like the threshold is ignored. But I don't understand where 
> there reset values are coming from. So it looks more like a bug in 
> micro-code where under certain circumstances multiple entries get 
> written.

Either multiple entries are written, or the PMI/NMI is not asserted as it 
should be?

> Something must be happening with the interrupt or HT. I will disable HT 
> next and also disable the NMI watchdog.

Yes, interaction with the NMI watchdog events might also be possible.

If it's truly just the threshold that is broken occasionally in a 
statistically insignificant manner then the bug is relatively benign and 
we could work it around in the kernel by ignoring excess entries.

In that case we should probably not annoy users with the scary kernel 
warning and instead increase a debug count somewhere so that it's still 
detectable.

Thanks,

	Ingo
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