linux-kernel - Re: [tip:perf/core] perf/x86/intel: Clean-up/reduce PEBS code

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Message-ID: <20130916072431.GV26785@twins.programming.kicks-ass.net>
Date:	Mon, 16 Sep 2013 09:24:31 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	mingo@...nel.org, hpa@...or.com, linux-kernel@...r.kernel.org,
	tglx@...utronix.de, linux-tip-commits@...r.kernel.org
Subject: Re: [tip:perf/core] perf/x86/intel: Clean-up/reduce PEBS code

On Mon, Sep 16, 2013 at 08:07:36AM +0200, Peter Zijlstra wrote:
> There already was an implicit division there, and
> sizeof(pebs_record_hsw) = 176, can it really optimize that constant
> division?
> 
> I suppose we could go and introduce CONFIG_PERF_DEBUG and stuff sanity
> checks under that.. :/

Or we could write it like so:

---
--- a/arch/x86/kernel/cpu/perf_event_intel_ds.c
+++ b/arch/x86/kernel/cpu/perf_event_intel_ds.c
@@ -954,16 +954,16 @@ static void intel_pmu_drain_pebs_nhm(str
 
 	ds->pebs_index = ds->pebs_buffer_base;
 
-	n = (top - at) / x86_pmu.pebs_record_size;
-	if (n <= 0)
+	if (unlikely(at > top))
 		return;
 
 	/*
 	 * Should not happen, we program the threshold at 1 and do not
 	 * set a reset value.
 	 */
-	WARN_ONCE(n > x86_pmu.max_pebs_events,
-		  "Unexpected number of pebs records %d\n", n);
+	WARN_ONCE(top - at > x86_pmu.max_pebs_events * x86_pmu.pebs_record_size,
+		  "Unexpected number of pebs records %d\n", 
+		  (top - at) / x86_pmu.pebs_record_size);
 
 	for (; at < top; at += x86_pmu.pebs_record_size) {
 		struct pebs_record_nhm *p = at;
--
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