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[PATCH 2/2] x86, perf: Allow zero PEBS status with only single active event

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  [PATCH 2/2] x86, perf: Allow zero PEBS status with only single active event Andi Kleen <andi@firstfloor.org> - 2015-12-03 22:30 +0100
    Re: [PATCH 2/2] x86, perf: Allow zero PEBS status with only single  active event Peter Zijlstra <peterz@infradead.org> - 2015-12-03 22:30 +0100

#1283392 — [PATCH 2/2] x86, perf: Allow zero PEBS status with only single active event

From: Andi Kleen <ak@linux.intel.com>

Normally we drop PEBS events with a zero status field. But when
there is only a single PEBS event active we can assume the
PEBS record is for that event. The PEBS buffer is always flushed
when PEBS events are disabled, so there is no risk of mishandling
state PEBS records this way.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
---
 arch/x86/kernel/cpu/perf_event_intel_ds.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/arch/x86/kernel/cpu/perf_event_intel_ds.c b/arch/x86/kernel/cpu/perf_event_intel_ds.c
index da294de..1259065 100644
--- a/arch/x86/kernel/cpu/perf_event_intel_ds.c
+++ b/arch/x86/kernel/cpu/perf_event_intel_ds.c
@@ -1235,6 +1235,18 @@ static void intel_pmu_drain_pebs_nhm(struct pt_regs *iregs)
 		pebs_status = p->status & cpuc->pebs_enabled;
 		pebs_status &= (1ULL << x86_pmu.max_pebs_events) - 1;
 
+		/*
+		 * On some CPUs the PEBS status can be zero when PEBS is
+		 * racing with clearing of GLOBAL_STATUS.
+		 *
+		 * Normally we would drop that record, but in the
+		 * case when there is only a single active PEBS event
+		 * we can assume it's for that event.
+		 */
+		if (!pebs_status && cpuc->pebs_enabled &&
+			!(cpuc->pebs_enabled & (cpuc->pebs_enabled-1)))
+			pebs_status = cpuc->pebs_enabled;
+
 		bit = find_first_bit((unsigned long *)&pebs_status,
 					x86_pmu.max_pebs_events);
 		if (bit >= x86_pmu.max_pebs_events)
-- 
2.4.3

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#1283402 — Re: [PATCH 2/2] x86, perf: Allow zero PEBS status with only single active event

On Thu, Dec 03, 2015 at 01:22:20PM -0800, Andi Kleen wrote:
> +		/*
> +		 * On some CPUs the PEBS status can be zero when PEBS is
> +		 * racing with clearing of GLOBAL_STATUS.
> +		 *
> +		 * Normally we would drop that record, but in the
> +		 * case when there is only a single active PEBS event
> +		 * we can assume it's for that event.
> +		 */
> +		if (!pebs_status && cpuc->pebs_enabled &&
> +			!(cpuc->pebs_enabled & (cpuc->pebs_enabled-1)))
> +			pebs_status = cpuc->pebs_enabled;
> +
>  		bit = find_first_bit((unsigned long *)&pebs_status,
>  					x86_pmu.max_pebs_events);
>  		if (bit >= x86_pmu.max_pebs_events)


Ah! I think this is the comment I just asked for.


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