linux-kernel - Re: [PATCH] perf/core: fix the bug in the event multiplexing

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Message-ID: <1bb556dc-8adc-63ea-75b3-ad0596f1b89e@amperemail.onmicrosoft.com>
Date:   Wed, 9 Aug 2023 17:17:26 +0800
From:   Shijie Huang <shijie@...eremail.onmicrosoft.com>
To:     Oliver Upton <oliver.upton@...ux.dev>,
        Huang Shijie <shijie@...amperecomputing.com>
Cc:     maz@...nel.org, james.morse@....com, suzuki.poulose@....com,
        yuzenghui@...wei.com, catalin.marinas@....com, will@...nel.org,
        pbonzini@...hat.com, peterz@...radead.org, ingo@...hat.com,
        acme@...nel.org, mark.rutland@....com,
        alexander.shishkin@...ux.intel.com, jolsa@...nel.org,
        namhyung@...nel.org, irogers@...gle.com,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-perf-users@...r.kernel.org, patches@...erecomputing.com,
        zwang@...erecomputing.com
Subject: Re: [PATCH] perf/core: fix the bug in the event multiplexing

Hi Oliver,

在 2023/8/9 16:25, Oliver Upton 写道:
> Hi Huang,
>
> On Wed, Aug 09, 2023 at 09:39:53AM +0800, Huang Shijie wrote:
>> 2.) Root cause.
>> 	There is only 7 counters in my arm64 platform:
>> 	  (one cycle counter) + (6 normal counters)
>>
>> 	In 1.3 above, we will use 10 event counters.
>> 	Since we only have 7 counters, the perf core will trigger
>>         	event multiplexing in hrtimer:
>> 	     merge_sched_in() -->perf_mux_hrtimer_restart() -->
>> 	     perf_rotate_context().
>>
>>         In the perf_rotate_context(), it does not restore some PMU registers
>>         as context_switch() does.  In context_switch():
>>               kvm_sched_in()  --> kvm_vcpu_pmu_restore_guest()
>>               kvm_sched_out() --> kvm_vcpu_pmu_restore_host()
>>
>>         So we got wrong result.
> This is a rather vague description of the problem. AFAICT, the
> issue here is on VHE systems we wind up getting the EL0 count
> enable/disable bits backwards when entering the guest, which is
> corroborated by the data you have below.
>
>> +void arch_perf_rotate_pmu_set(void)
>> +{
>> +	if (is_guest())
>> +		kvm_vcpu_pmu_restore_guest(NULL);
>> +	else
>> +		kvm_vcpu_pmu_restore_host(NULL);
>> +}
>> +
> This sort of hook is rather nasty, and I'd strongly prefer a solution
> that's confined to KVM. I don't think the !is_guest() branch is
> necessary at all. Regardless of how the pmu context is changed, we need
> to go through vcpu_put() before getting back out to userspace.
>
> We can check for a running vCPU (ick) from kvm_set_pmu_events() and either
> do the EL0 bit flip there or make a request on the vCPU to call
> kvm_vcpu_pmu_restore_guest() immediately before reentering the guest.
> I'm slightly leaning towards the latter, unless anyone has a better idea
> here.

Thanks a lot, I will check the code about the latter one, and try to fix 
it again.


Thanks

Huang Shijie