Re: [RFC PATCH v3 0/3] x86/perf/amd: AMD PMC counters and NMI latency

[Cyrill Gorcunov]

On Tue, Apr 02, 2019 at 03:03:02PM +0200, Peter Zijlstra wrote:
> On Mon, Apr 01, 2019 at 09:46:33PM +0000, Lendacky, Thomas wrote:
> > This patch series addresses issues with increased NMI latency in newer
> > AMD processors that can result in unknown NMI messages when PMC counters
> > are active.
> > 
> > The following fixes are included in this series:
> > 
> > - Resolve a race condition when disabling an overflowed PMC counter,
> >   specifically when updating the PMC counter with a new value.
> > - Resolve handling of active PMC counter overflows in the perf NMI
> >   handler and when to report that the NMI is not related to a PMC.
> > - Remove earlier workaround for spurious NMIs by re-ordering the
> >   PMC stop sequence to disable the PMC first and then remove the PMC
> >   bit from the active_mask bitmap. As part of disabling the PMC, the
> >   code will wait for an overflow to be reset.
> > 
> > The last patch re-works the order of when the PMC is removed from the
> > active_mask. There was a comment from a long time ago about having
> > to clear the bit in active_mask before disabling the counter because
> > the perf NMI handler could re-enable the PMC again. Looking at the
> > handler today, I don't see that as possible, hence the reordering. The
> > question will be whether the Intel PMC support will now have issues.
> > There is still support for using x86_pmu_handle_irq() in the Intel
> > core.c file. Did Intel have any issues with spurious NMIs in the past?
> > Peter Z, any thoughts on this?
> 
> I can't remember :/ I suppose we'll see if anything pops up after these
> here patches. At least then we get a chance to properly document things.
> 
> > Also, I couldn't completely get rid of the "running" bit because it
> > is used by arch/x86/events/intel/p4.c. An old commit comment that
> > seems to indicate the p4 code suffered the spurious interrupts:
> > 03e22198d237 ("perf, x86: Handle in flight NMIs on P4 platform").
> > So maybe that partially answers my previous question...
> 
> Yeah, the P4 code is magic, and I don't have any such machines left, nor
> do I think does Cyrill who wrote much of that.

It was so long ago :) What I remember from the head is some of the counters
were borken on hardware level so that I had to use only one counter instead
of two present in the system. And there were spurious NMIs too. I think
we can move this "running" bit to per-cpu base declared inside p4 code
only, so get rid of it from cpu_hw_events?

> I have vague memories of the P4 thing crashing with Vince's perf_fuzzer,
> but maybe I'm wrong.

No, you're correct. p4 was crashing many times before we manage to make
it more-less stable. The main problem though that to find working p4 box
is really a problem.

> Ideally we'd find a willing victim to maintain that thing, or possibly
> just delete it, dunno if anybody still cares.

As to me, I would rather mark this p4pmu code as deprecated, until there
is *real* need for its support.

> 
> Anyway, I like these patches, but I cannot apply since you send them
> base64 encoded and my script chokes on that.