linux-kernel - Re: [PATCH 2/3] KVM: x86/vPMU: Reuse perf_event to avoid unnecessary pmc_reprogram

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Date:   Tue, 1 Oct 2019 20:18:45 +0800
From:   Like Xu <like.xu@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        rkrcmar@...hat.com, sean.j.christopherson@...el.com,
        vkuznets@...hat.com, Jim Mattson <jmattson@...gle.com>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        ak@...ux.intel.com, wei.w.wang@...el.com, kan.liang@...el.com,
        like.xu@...el.com, ehankland@...gle.com, arbel.moshe@...cle.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] KVM: x86/vPMU: Reuse perf_event to avoid unnecessary
 pmc_reprogram_counter

Hi Peter,

On 2019/10/1 16:22, Peter Zijlstra wrote:
> On Mon, Sep 30, 2019 at 03:22:56PM +0800, Like Xu wrote:
>> diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c
>> index 46875bbd0419..74bc5c42b8b5 100644
>> --- a/arch/x86/kvm/pmu.c
>> +++ b/arch/x86/kvm/pmu.c
>> @@ -140,6 +140,35 @@ static void pmc_reprogram_counter(struct kvm_pmc *pmc, u32 type,
>>   	clear_bit(pmc->idx, (unsigned long*)&pmc_to_pmu(pmc)->reprogram_pmi);
>>   }
>>   
>> +static void pmc_pause_counter(struct kvm_pmc *pmc)
>> +{
>> +	if (!pmc->perf_event)
>> +		return;
>> +
>> +	pmc->counter = pmc_read_counter(pmc);
>> +
>> +	perf_event_disable(pmc->perf_event);
>> +
>> +	/* reset count to avoid redundant accumulation */
>> +	local64_set(&pmc->perf_event->count, 0);
> 
> Yuck, don't frob in data structures you don't own.

Yes, it's reasonable. Thanks.

> 
> Just like you exported the IOC_PERIOD thing, so too is there a
> IOC_RESET.
> 
> Furthermore; wth do you call pmc_read_counter() _before_ doing
> perf_event_disable() ? Doing it the other way around is much cheaper,
> even better, you can use perf_event_count() after disable.

Yes, it's much better and let me apply this.

> 
>> +}
>> +
>> +static bool pmc_resume_counter(struct kvm_pmc *pmc)
>> +{
>> +	if (!pmc->perf_event)
>> +		return false;
>> +
>> +	/* recalibrate sample period and check if it's accepted by perf core */
>> +	if (perf_event_period(pmc->perf_event,
>> +			(-pmc->counter) & pmc_bitmask(pmc)))
>> +		return false;
> 
> I'd do the reset here, but then you have 3 function in a row that do
> perf_event_ctx_lock()+perf_event_ctx_unlock(), which is rather
> expensive.

Calling pmc_pause_counter() is not always followed by calling 
pmc_resume_counter(). The former may be called multiple times before the 
later is called, so if we do not reset event->count in the 
pmc_pause_counter(), it will be repeatedly accumulated into pmc->counter 
which is a functional error.

> 
>> +
>> +	/* reuse perf_event to serve as pmc_reprogram_counter() does*/
>> +	perf_event_enable(pmc->perf_event);
>> +	clear_bit(pmc->idx, (unsigned long *)&pmc_to_pmu(pmc)->reprogram_pmi);
>> +	return true;
>> +}
>