perf/x86: Fix full width counter, counter overflow

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
again.

Reported-by: Lukasz Odzioba <lukasz.odzioba@intel.com>
Tested-by: Liang, Kan <kan.liang@intel.com>
Tested-by: Odzioba, Lukasz <lukasz.odzioba@intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vince Weaver <vincent.weaver@maine.edu>
Fixes: 069e0c3c4058 ("perf/x86/intel: Support full width counting")
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
index a4e070a515843ece7ee08b893613f5e65b47d0a4..8c925ecaf5341cb1b8fa19153718976337124efb 100644 (file)
--- a/arch/x86/events/core.c
+++ b/arch/x86/events/core.c
@@ -68,7 +68,7 @@ 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;
 
        if (idx == INTEL_PMC_IDX_FIXED_BTS)
                return 0;

diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c
index 4c9a79b9cd691ad8b44650ca8ea1b3b1295c0d3d..3ef34c6e63ca21f491688f412d598062c76cee1f 100644 (file)
--- a/arch/x86/events/intel/core.c
+++ b/arch/x86/events/intel/core.c
@@ -4024,7 +4024,7 @@ __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, ");
        }