linux-kernel - Re: [PATCH] perf/x86: fix event counter update issue

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Message-ID: <20161205102509.GH3124@twins.programming.kicks-ass.net>
Date:   Mon, 5 Dec 2016 11:25:09 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Odzioba, Lukasz" <lukasz.odzioba@...el.com>
Cc:     "Liang, Kan" <kan.liang@...el.com>,
        Stephane Eranian <eranian@...gle.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        "ak@...ux.intel.com" <ak@...ux.intel.com>
Subject: Re: [PATCH] perf/x86: fix event counter update issue

On Fri, Dec 02, 2016 at 12:58:17PM +0000, Odzioba, Lukasz wrote:
> On Tuesday, November 29, 2016 9:33 PM, Liang, Kan wrote:
> > Yes, the patch as below fixes the issue on my SLM.
> 
> It works for me as well. 
> Can we still have it in 4.9?

I'll certainly, try. I've queued it as per the below.

---
Subject: perf,x86: Fix full width counter, counter overflow
Date: Tue, 29 Nov 2016 20:33:28 +0000

Lukasz reported that perf stat counters overflow is broken on KNL/SLM.

Both these parts have full_width_write set, and that does indeed have
a problem. In order to deal with counter wrap, we must sample the
counter at at least half the counter period (see also the sampling
theorem) such that we can unambiguously reconstruct the count.

However commit:

  069e0c3c4058 ("perf/x86/intel: Support full width counting")

sets the sampling interval to the full period, not half.

Fixing that exposes another issue, in that we must not sign extend the
delta value when we shift it right; the counter cannot have
decremented after all.

With both these issues fixed, counter overflow functions correctly
again.

Cc: "mingo@...hat.com" <mingo@...hat.com>
Cc: "ak@...ux.intel.com" <ak@...ux.intel.com>
Cc: Stephane Eranian <eranian@...gle.com>
Cc: Alexander Shishkin <alexander.shishkin@...ux.intel.com>
Cc: stable@...r.kernel.org
Reported-by: Lukasz Odzioba <lukasz.odzioba@...el.com>
Tested-by: "Liang, Kan" <kan.liang@...el.com>
Tested-by: "Odzioba, Lukasz" <lukasz.odzioba@...el.com>
Fixes: 069e0c3c4058 ("perf/x86/intel: Support full width counting")
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 arch/x86/events/core.c       |    2 +-
 arch/x86/events/intel/core.c |    2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

--- a/arch/x86/events/core.c
+++ b/arch/x86/events/core.c
@@ -69,7 +69,7 @@ u64 x86_perf_event_update(struct perf_ev
 	int shift = 64 - x86_pmu.cntval_bits;
 	u64 prev_raw_count, new_raw_count;
 	int idx = hwc->idx;
-	s64 delta;
+	u64 delta;
 
 	if (idx == INTEL_PMC_IDX_FIXED_BTS)
 		return 0;
--- a/arch/x86/events/intel/core.c
+++ b/arch/x86/events/intel/core.c
@@ -4034,7 +4034,7 @@ __init int intel_pmu_init(void)
 
 	/* Support full width counters using alternative MSR range */
 	if (x86_pmu.intel_cap.full_width_write) {
-		x86_pmu.max_period = x86_pmu.cntval_mask;
+		x86_pmu.max_period = x86_pmu.cntval_mask >> 1;
 		x86_pmu.perfctr = MSR_IA32_PMC0;
 		pr_cont("full-width counters, ");
 	}