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

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Date: Thu, 20 Jun 2024 09:02:15 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: kan.liang@...ux.intel.com
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 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.

>  	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;

?