linux-kernel - Re: [kvm-unit-tests Patch 2/5] x86: pmu: Change the minimum value of llc

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Date:   Wed, 25 Oct 2023 05:35:44 -0700
From:   Jim Mattson <jmattson@...gle.com>
To:     "Mi, Dapeng" <dapeng1.mi@...ux.intel.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Zhenyu Wang <zhenyuw@...ux.intel.com>,
        Zhang Xiong <xiong.y.zhang@...el.com>,
        Mingwei Zhang <mizhang@...gle.com>,
        Like Xu <like.xu.linux@...il.com>,
        Dapeng Mi <dapeng1.mi@...el.com>
Subject: Re: [kvm-unit-tests Patch 2/5] x86: pmu: Change the minimum value of
 llc_misses event to 0

On Wed, Oct 25, 2023 at 4:23 AM Mi, Dapeng <dapeng1.mi@...ux.intel.com> wrote:
>
>
> On 10/24/2023 9:03 PM, Jim Mattson wrote:
> > On Tue, Oct 24, 2023 at 12:51 AM Dapeng Mi <dapeng1.mi@...ux.intel.com> wrote:
> >> Along with the CPU HW's upgrade and optimization, the count of LLC
> >> misses event for running loop() helper could be 0 just like seen on
> >> Sapphire Rapids.
> >>
> >> So modify the lower limit of possible count range for LLC misses
> >> events to 0 to avoid LLC misses event test failure on Sapphire Rapids.
> > I'm not convinced that these tests are really indicative of whether or
> > not the PMU is working properly. If 0 is allowed for llc misses, for
> > instance, doesn't this sub-test pass even when the PMU is disabled?
> >
> > Surely, we can do better.
>
>
> Considering the testing workload is just a simple adding loop, it's
> reasonable and possible that it gets a 0 result for LLC misses and
> branch misses events. Yeah, I agree the 0 count makes the results not so
> credible. If we want to avoid these 0 count values, we may have to
> complicate the workload, such as adding flush cache instructions, or
> something like that (I'm not sure if there are instructions which can
> force branch misses). How's your idea about this?

CLFLUSH is probably a good way to ensure cache misses. IBPB may be a
good way to ensure branch mispredictions, or IBRS on parts without
eIBRS.

>
> >
> >> Signed-off-by: Dapeng Mi <dapeng1.mi@...ux.intel.com>
> >> ---
> >>   x86/pmu.c | 2 +-
> >>   1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >> diff --git a/x86/pmu.c b/x86/pmu.c
> >> index 0def28695c70..7443fdab5c8a 100644
> >> --- a/x86/pmu.c
> >> +++ b/x86/pmu.c
> >> @@ -35,7 +35,7 @@ struct pmu_event {
> >>          {"instructions", 0x00c0, 10*N, 10.2*N},
> >>          {"ref cycles", 0x013c, 1*N, 30*N},
> >>          {"llc references", 0x4f2e, 1, 2*N},
> >> -       {"llc misses", 0x412e, 1, 1*N},
> >> +       {"llc misses", 0x412e, 0, 1*N},
> >>          {"branches", 0x00c4, 1*N, 1.1*N},
> >>          {"branch misses", 0x00c5, 0, 0.1*N},
> >>   }, amd_gp_events[] = {
> >> --
> >> 2.34.1
> >>