Re: [regression 2.6.39-rc2][bisected] "perf, x86: P4 PMU - Readproper MSR register to catch" and NMIs

From: Cyrill Gorcunov
Date: Thu Apr 14 2011 - 05:27:39 EST

On Thu, Apr 14, 2011 at 12:05 PM, Ingo Molnar <mingo@xxxxxxx> wrote:
>> If there is no counters overflowed I believe we should not poke LVTPC until
>> we sure NMI comes from it (and counter overflow is the only sign that NMI
>> came from LVTPC as far as I may say, and I see also a possibility for race if
>> counter signal reaches LVTPC and it is being processed inside apic chip
>> {which might take some time too before real NMI signal appears in cpu} and as
>> result hard to tell what we get in output -- double nmi again or something
>> else).
> Well, we unmasked unconditionally before. If we unmask conditionally now, we
> risk not unmasking. We risk a completely stuck PMU (there wont ever come *any*
> NMI from it if we ever forget to unmask) versus spurious NMIs.
> Maybe we can do it - but it will need a lot of testing on a lot of CPU types to
> make sure there's no other CPU quirks in this area ...
> So unless the conditional unmasking fixes a real bug (in kgdb or elsewhere)
> lets unmask unconditionally now to fix the P4 regression in .39 - and queue up
> a *separate* patch that moves it even further down and makes it conditional -
> but queue that up for .40.
> Thanks,
>        Ingo

OK. Ingo I'll send a patch from Don with all tested-by (including me) and my ack
as only get back home. (I don't mind if Don beat me on this ;)
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