linux-kernel - Re: [RESEND PATCH 01/12] perf/x86/intel: Support the PEBS event mask

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Date: Thu, 20 Jun 2024 11:58:42 -0400
From: "Liang, Kan" <kan.liang@...ux.intel.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: mingo@...nel.org, acme@...nel.org, namhyung@...nel.org,
 irogers@...gle.com, adrian.hunter@...el.com,
 alexander.shishkin@...ux.intel.com, linux-kernel@...r.kernel.org,
 ak@...ux.intel.com, eranian@...gle.com
Subject: Re: [RESEND PATCH 01/12] perf/x86/intel: Support the PEBS event mask



On 2024-06-20 3:02 a.m., Peter Zijlstra wrote:
> On Tue, Jun 18, 2024 at 08:10:33AM -0700, kan.liang@...ux.intel.com wrote:
>> diff --git a/arch/x86/events/intel/ds.c b/arch/x86/events/intel/ds.c
>> index e010bfed8417..a0104c82baed 100644
>> --- a/arch/x86/events/intel/ds.c
>> +++ b/arch/x86/events/intel/ds.c
> 
>> @@ -2157,6 +2157,7 @@ static void intel_pmu_drain_pebs_nhm(struct pt_regs *iregs, struct perf_sample_d
>>  	void *base, *at, *top;
>>  	short counts[INTEL_PMC_IDX_FIXED + MAX_FIXED_PEBS_EVENTS] = {};
>>  	short error[INTEL_PMC_IDX_FIXED + MAX_FIXED_PEBS_EVENTS] = {};
>> +	int max_pebs_events = hweight64(x86_pmu.pebs_events_mask);
> 
> Consider pebs_events_mask = 0xf0, then max_pebs_events becomes 4.

The intel_pmu_drain_pebs_nhm() is a NHM specific function. It's
impossible that there is a pebs_events_mask = 0xf0.

There are only 4 counters. The mask should always be 0xf.
> 
>>  	int bit, i, size;
>>  	u64 mask;
>>  
>> @@ -2168,8 +2169,8 @@ static void intel_pmu_drain_pebs_nhm(struct pt_regs *iregs, struct perf_sample_d
>>  
>>  	ds->pebs_index = ds->pebs_buffer_base;
>>  
>> -	mask = (1ULL << x86_pmu.max_pebs_events) - 1;
>> -	size = x86_pmu.max_pebs_events;
>> +	mask = x86_pmu.pebs_events_mask;
>> +	size = max_pebs_events;
> 
> This is wrong.. you have 8 bits to iterate, of which only the top 4 are
> set.
> 
>>  	if (x86_pmu.flags & PMU_FL_PEBS_ALL) {
>>  		mask |= ((1ULL << x86_pmu.num_counters_fixed) - 1) << INTEL_PMC_IDX_FIXED;
>>  		size = INTEL_PMC_IDX_FIXED + x86_pmu.num_counters_fixed;
>> @@ -2208,8 +2209,8 @@ static void intel_pmu_drain_pebs_nhm(struct pt_regs *iregs, struct perf_sample_d
>>  			pebs_status = p->status = cpuc->pebs_enabled;
>>  
>>  		bit = find_first_bit((unsigned long *)&pebs_status,
>> -					x86_pmu.max_pebs_events);
>> -		if (bit >= x86_pmu.max_pebs_events)
>> +					max_pebs_events);
>> +		if (bit >= max_pebs_events)
>>  			continue;
> 
> But the bit check here then truncates the thing to the lower 4 bits
> which are all 0.
> 
> Should this not be something like:
> 
> 		if (!(pebs_event_mask & (1 << bit)))
> 			continue;
> 
> ?
> 

For the existing hardware, I don't think it's necessary. The counters
are contiguous.

Thanks,
Kan