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

[Ingo Molnar]

* Stephane Eranian <eranian@xxxxxxxxxxxxxx> wrote:

> On Tue, Sep 10, 2013 at 6:38 AM, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
> >
> > * Stephane Eranian <eranian@xxxxxxxxxxxxxx> wrote:
> >
> >> Hi,
> >>
> >> Ok, so I am able to reproduce the problem using a simpler
> >> test case with a simple multithreaded program where
> >> #threads >> #CPUs.
> >
> > Does it go away if you use 'perf record --all-cpus'?
> >
> Haven't tried that yet.
> 
> But I verified the DS pointers:
> init:
> CPU6 pebs base=ffff8808262de000 index=ffff8808262de000
> intr=ffff8808262de0c0 max=ffff8808262defc0
> crash:
> CPU6 pebs base=ffff8808262de000 index=ffff8808262de9c0
> intr=ffff8808262de0c0 max=ffff8808262defc0
> 
> Neither the base nor the max are modified.
> The index simply goes beyond the threshold but that's not a bug.
> It is 12 after the threshold of 1, so total 13 is my new crash report.
> 
> Two things to try:
> - measure only one thread/core
> - move the threshold a bit farther away (to get 2 or 3 entries)
> 
> The threshold is where to generate the interrupt. It does not mean where 
> to stop PEBS recording. 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...

Do you mean:

        if (!x86_pmu.late_ack)
                apic_write(APIC_LVTPC, APIC_DM_NMI);

AFAICS that means the opposite: that we clear the NMI late, i.e. shortly 
before return, after we've processed the PMU.

Do the symptoms change if you remove the x86_pmu.late_ack setting line 
from:

        case 60: /* Haswell Client */
        case 70:
        case 71:
        case 63:
        case 69:
                x86_pmu.late_ack = true;

?

Thanks,

	Ingo
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