linux-kernel - Re: [PATCH V3] cpufreq: Call transition notifier only once for each policy

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Date:   Fri, 22 Mar 2019 11:49:57 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Rafael Wysocki <rjw@...ysocki.net>,
        Russell King <linux@...linux.org.uk>,
        "David S. Miller" <davem@...emloft.net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        linux-pm@...r.kernel.org,
        Vincent Guittot <vincent.guittot@...aro.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        sparclinux@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH V3] cpufreq: Call transition notifier only once for each
 policy

On 21-03-19, 12:45, Peter Zijlstra wrote:
> On Wed, Mar 20, 2019 at 10:22:23AM +0530, Viresh Kumar wrote:
> > diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
> > index 3fae23834069..b2fe665878f7 100644
> > --- a/arch/x86/kernel/tsc.c
> > +++ b/arch/x86/kernel/tsc.c
> > @@ -958,10 +958,15 @@ static int time_cpufreq_notifier(struct notifier_block *nb, unsigned long val,
> >  	struct cpufreq_freqs *freq = data;
> >  	unsigned long *lpj;
> >  
> > +	if (WARN_ON_ONCE(cpumask_weight(freq->policy->related_cpus) != 1)) {
> > +		mark_tsc_unstable("cpufreq changes: related CPUs affected");
> 
> I suspect this is a big fat nop, but it won't hurt.
> 
> > +		return 0;
> > +	}
> > +
> >  	lpj = &boot_cpu_data.loops_per_jiffy;
> >  #ifdef CONFIG_SMP
> >  	if (!(freq->flags & CPUFREQ_CONST_LOOPS))
> > -		lpj = &cpu_data(freq->cpu).loops_per_jiffy;
> > +		lpj = &cpu_data(freq->policy->cpu).loops_per_jiffy;
> >  #endif
> >  
> >  	if (!ref_freq) {
> > @@ -977,7 +982,7 @@ static int time_cpufreq_notifier(struct notifier_block *nb, unsigned long val,
> >  		if (!(freq->flags & CPUFREQ_CONST_LOOPS))
> >  			mark_tsc_unstable("cpufreq changes");
> >  
> > -		set_cyc2ns_scale(tsc_khz, freq->cpu, rdtsc());
> > +		set_cyc2ns_scale(tsc_khz, freq->policy->cpu, rdtsc());
> >  	}
> >  
> >  	return 0;
> 
> Just wondering, since we say x86 cpufreq handlers will only have a
> single CPU here,
> 
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index 65e4559eef2f..1ac8c710cccc 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -6649,10 +6649,8 @@ static void kvm_hyperv_tsc_notifier(void)
> >  }
> >  #endif
> >  
> > -static int kvmclock_cpufreq_notifier(struct notifier_block *nb, unsigned long val,
> > -				     void *data)
> > +static void __kvmclock_cpufreq_notifier(struct cpufreq_freqs *freq, int cpu)
> >  {
> > -	struct cpufreq_freqs *freq = data;
> >  	struct kvm *kvm;
> >  	struct kvm_vcpu *vcpu;
> >  	int i, send_ipi = 0;
> > @@ -6696,17 +6694,12 @@ static int kvmclock_cpufreq_notifier(struct notifier_block *nb, unsigned long va
> >  	 *
> >  	 */
> >  
> > -	if (val == CPUFREQ_PRECHANGE && freq->old > freq->new)
> > -		return 0;
> > -	if (val == CPUFREQ_POSTCHANGE && freq->old < freq->new)
> > -		return 0;
> > -
> > -	smp_call_function_single(freq->cpu, tsc_khz_changed, freq, 1);
> > +	smp_call_function_single(cpu, tsc_khz_changed, freq, 1);
> >  
> >  	spin_lock(&kvm_lock);
> >  	list_for_each_entry(kvm, &vm_list, vm_list) {
> >  		kvm_for_each_vcpu(i, vcpu, kvm) {
> > -			if (vcpu->cpu != freq->cpu)
> > +			if (vcpu->cpu != cpu)
> >  				continue;
> >  			kvm_make_request(KVM_REQ_CLOCK_UPDATE, vcpu);
> >  			if (vcpu->cpu != smp_processor_id())
> > @@ -6728,8 +6721,24 @@ static int kvmclock_cpufreq_notifier(struct notifier_block *nb, unsigned long va
> >  		 * guest context is entered kvmclock will be updated,
> >  		 * so the guest will not see stale values.
> >  		 */
> > -		smp_call_function_single(freq->cpu, tsc_khz_changed, freq, 1);
> > +		smp_call_function_single(cpu, tsc_khz_changed, freq, 1);
> >  	}
> > +}
> > +
> > +static int kvmclock_cpufreq_notifier(struct notifier_block *nb, unsigned long val,
> > +				     void *data)
> > +{
> > +	struct cpufreq_freqs *freq = data;
> > +	int cpu;
> > +
> > +	if (val == CPUFREQ_PRECHANGE && freq->old > freq->new)
> > +		return 0;
> > +	if (val == CPUFREQ_POSTCHANGE && freq->old < freq->new)
> > +		return 0;
> > +
> > +	for_each_cpu(cpu, freq->policy->cpus)
> > +		__kvmclock_cpufreq_notifier(freq, cpu);
> > +
> >  	return 0;
> >  }
> >  
> 
> Then why to we pretend otherwise here?

My intention was to not add any bug here because of lack of my
knowledge of the architecture in question and so I tried to be safe.

If you guys think the behavior should be same here as of the tsc, then
we can add similar checks here.

-- 
viresh