Re: [PATCH -v2] perf, x86: try to handle unknown nmis with runningperfctrs

[Robert Richter]

On 16.08.10 12:27:06, Cyrill Gorcunov wrote:
> On Mon, Aug 16, 2010 at 04:48:36PM +0200, Peter Zijlstra wrote:
> > I liked the one without funny timestamps in better, the whole timestamps
> > thing just feels too fragile.
> > 
> 
> Me too, the former Roberts patch (if I'm not missing something) looks good
> to me.
> 
> >
> > Relying on handled > 1 to arm the back-to-back filter seems doable.

Peter, I will rip out the timestamp code from the -v2 patch. My first
patch does not deal with a 2-1-0 sequence, so it has false positives.
We do not necessarily need the timestamps if back-to-back nmis are
rare. Without using timestamps the statistically lost ratio for
unknown nmis will be as the ratio for back-to-back nmis, with
timestamps we could catch almost every unknown nmi. So if we encounter
problems we could still implement timestamp code on top.

> It's doable _but_ I think there is nothing we can do, there is no
> way (at least I known of) to check if there is latched nmi from
> perf counters. We only can assume that if there multiple counters
> overflowed most probably the next unknown nmi has the same nature,
> ie it came from perf.

As said, I think with timestamps we could be able to detect 100% of
the unknown nmis. I guess we get now more than 90% with mutliple
counters, and 100% with a single counter running. So, this is already
more than a simple improvement.

> Yes, we can loose real unknown nmi in this
> case but I think this is justified trade off. If an user need
> a precise counting of unknown nmis he should not arm perf events
> at all, if there an user with nmi button (guys where did you get this
> magic buttuns? i need one ;) he better to not arm perf events too
> otherwise he might have to click twice
> 
> (and of course we should keep in mind Andi's proposal but it
>  is a next step I think).

Yes, this patch is the first step, now we can change the nmi handler
priority. The perf handler must not have the lowest priority anymore.

> > (Also, you didn't deal with the TSC going backwards..)

Does this also happen in the case of a back-to-back nmi? I don't know
the conditions for a backward running TSC. Maybe, if an nmi is
retriggered the TSC wont be adjusted by a negative offset, I don't
know...

-Robert

-- 
Advanced Micro Devices, Inc.
Operating System Research Center

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