Re: [PATCH] perf/x86/amd: Do not WARN on every IRQ

[Jirka Hladky]

Hi Breno,

I'm definitively voting for using WARN_ON_ONCE - in the current
implementation, we are getting thousands of the same warnings and Call
Traces, causing the system to become unusable.

>Anyway, please let me know whatever is your preferred way and I will submit a v2.
@Peter Zijlstra and @Sandipan - could you please comment on the
preferred implementation of the patch?

THANK YOU
Jirka

On Wed, Sep 13, 2023 at 6:24 PM Breno Leitao <leitao@xxxxxxxxxx> wrote:
>
> Hi Peter,
>
> On Fri, Jun 16, 2023 at 03:29:54PM +0200, Peter Zijlstra wrote:
> > On Fri, Jun 16, 2023 at 04:53:15AM -0700, Breno Leitao wrote:
> > > On some systems, the Performance Counter Global Status Register is
> > > coming with reserved bits set, which causes the system to be unusable
> > > if a simple `perf top` runs. The system hits the WARN() thousands times
> > > while perf runs.
> > >
> > > WARNING: CPU: 18 PID: 20608 at arch/x86/events/amd/core.c:944 amd_pmu_v2_handle_irq+0x1be/0x2b0
> > >
> > > This happens because the "Performance Counter Global Status Register"
> > > (PerfCntGlobalStatus) MSR has bit 7 set. Bit 7 should be reserved according
> > > to the documentation (Figure 13-12 from "AMD64 Architecture Programmer’s
> > > Manual, Volume 2: System Programming, 24593"[1]
> >
> > Would it then not make more sense to mask out bit7 before:
> >
> > +     status &= ~AMD_PMU_V2_GLOBAL_STATUS_RESERVED;
> >       if (!status)
> >               goto done;
>
> Instead of masking `status` against AMD_PMU_V2_GLOBAL_STATUS_RESERVED
> (AMD64_NUM_COUNTERS?), I opted for using the `amd_pmu_global_cntr_mask`
> global variable because it seems to represent what the loop below is
> iterating over:
>
>         /* PMC Enable and Overflow bits for PerfCntrGlobal* registers */
>         static u64 amd_pmu_global_cntr_mask __read_mostly;
>
> Also, I think we want to WARN_ON_ONCE() if we see this problem. Right
> now, it warns at every time we call this function, which makes the
> machine unusable, but, warning it once could be helpful to figure out
> there is something wrong with the machine/firmware.
>
> Anyway, please let me know whatever is your preferred way and I will
> submit a v2.
>


-- 
-Jirka