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

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Date:   Thu, 11 Mar 2021 10:41:00 -0500
From:   "Liang, Kan" <kan.liang@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     mingo@...nel.org, linux-kernel@...r.kernel.org, acme@...nel.org,
        tglx@...utronix.de, bp@...en8.de, namhyung@...nel.org,
        jolsa@...hat.com, ak@...ux.intel.com, yao.jin@...ux.intel.com,
        alexander.shishkin@...ux.intel.com, adrian.hunter@...el.com
Subject: Re: [PATCH V2 16/25] perf/x86: Register hybrid PMUs



On 3/11/2021 7:34 AM, Peter Zijlstra wrote:
> On Wed, Mar 10, 2021 at 08:37:52AM -0800, kan.liang@...ux.intel.com 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