[PATCH 3.12 006/235] perf/x86: Fix full width counter, counter overflow

From: Jiri Slaby
Date: Fri Jan 27 2017 - 07:06:54 EST

From: "Peter Zijlstra (Intel)" <peterz@xxxxxxxxxxxxx>

3.12-stable review patch. If anyone has any objections, please let me know.


commit 7f612a7f0bc13a2361a152862435b7941156b6af upstream.

Lukasz reported that perf stat counters overflow handling 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

Reported-by: Lukasz Odzioba <lukasz.odzioba@xxxxxxxxx>
Tested-by: Liang, Kan <kan.liang@xxxxxxxxx>
Tested-by: Odzioba, Lukasz <lukasz.odzioba@xxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Cc: Alexander Shishkin <alexander.shishkin@xxxxxxxxxxxxxxx>
Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
Cc: Jiri Olsa <jolsa@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Stephane Eranian <eranian@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Vince Weaver <vincent.weaver@xxxxxxxxx>
Fixes: 069e0c3c4058 ("perf/x86/intel: Support full width counting")
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Signed-off-by: Jiri Slaby <jslaby@xxxxxxx>
arch/x86/kernel/cpu/perf_event.c | 2 +-
arch/x86/kernel/cpu/perf_event_intel.c | 2 +-
2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/cpu/perf_event.c b/arch/x86/kernel/cpu/perf_event.c
index 0271272d55d0..050784bcd71f 100644
--- a/arch/x86/kernel/cpu/perf_event.c
+++ b/arch/x86/kernel/cpu/perf_event.c
@@ -64,7 +64,7 @@ u64 x86_perf_event_update(struct perf_event *event)
int shift = 64 - x86_pmu.cntval_bits;
u64 prev_raw_count, new_raw_count;
int idx = hwc->idx;
- s64 delta;
+ u64 delta;

return 0;
diff --git a/arch/x86/kernel/cpu/perf_event_intel.c b/arch/x86/kernel/cpu/perf_event_intel.c
index 04e7df068f0e..0c6527a168f0 100644
--- a/arch/x86/kernel/cpu/perf_event_intel.c
+++ b/arch/x86/kernel/cpu/perf_event_intel.c
@@ -2578,7 +2578,7 @@ __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, ");