Re: [PATCH V2 16/25] perf/x86: Register hybrid PMUs

[Liang, Kan]

On 3/11/2021 7:34 AM, Peter Zijlstra wrote:
> On Wed, Mar 10, 2021 at 08:37:52AM -0800, kan.liang@xxxxxxxxxxxxxxx wrote:
> > @@ -2092,9 +2105,37 @@ static int __init init_hw_perf_events(void)
> >   	if (err)
> >   		goto out1;
> >   
> > -	err = perf_pmu_register(&pmu, "cpu", PERF_TYPE_RAW);
> > -	if (err)
> > -		goto out2;
> > +	if (!is_hybrid()) {
> > +		err = perf_pmu_register(&pmu, "cpu", PERF_TYPE_RAW);
> > +		if (err)
> > +			goto out2;
> > +	} else {
> > +		u8 cpu_type = get_hybrid_cpu_type(smp_processor_id());
> > +		struct x86_hybrid_pmu *hybrid_pmu;
> > +		int i;
> > +
> > +		for (i = 0; i < x86_pmu.num_hybrid_pmus; i++) {
> > +			hybrid_pmu = &x86_pmu.hybrid_pmu[i];
> > +
> > +			hybrid_pmu->pmu = pmu;
> > +			hybrid_pmu->pmu.type = -1;
> > +			hybrid_pmu->pmu.attr_update = x86_pmu.attr_update;
> > +			hybrid_pmu->pmu.capabilities |= PERF_PMU_CAP_HETEROGENEOUS_CPUS;
> > +
> > +			/* Only register the PMU for the boot CPU */
>
> Why ?!
> > AFAICT we could register them all here. That instantly fixes that
> CPU_STARTING / CPU_DEAD fail elsewhere in this patch.

It's possible that all CPUs of a certain type all offline, but I cannot 
know the information here, because the boot CPU is the only online CPU. 
I don't know the status of the other CPUs.

If we unconditionally register all PMUs, users may see a PMU in 
/sys/devices, but they cannot use it, because there is no available CPU.
Is it acceptable that registering an empty PMU?


Thanks,
Kan