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authorKan Liang <kan.liang@linux.intel.com>2018-02-12 14:20:31 -0800
committerIngo Molnar <mingo@kernel.org>2018-03-09 08:22:19 +0100
commitd31fc13fdcb20e1c317f9a7dd6273c18fbd58308 (patch)
treea1d55689e973047c82a3df23a2196445dbcb83de /arch/x86/events/core.c
parent82d71ed0277efc45360828af8c4e4d40e1b45352 (diff)
perf/x86/intel: Fix event update for auto-reload
There is a bug when reading event->count with large PEBS enabled. Here is an example: # ./read_count 0x71f0 0x122c0 0x1000000001c54 0x100000001257d 0x200000000bdc5 In fixed period mode, the auto-reload mechanism could be enabled for PEBS events, but the calculation of event->count does not take the auto-reload values into account. Anyone who reads event->count will get the wrong result, e.g x86_pmu_read(). This bug was introduced with the auto-reload mechanism enabled since commit: 851559e35fd5 ("perf/x86/intel: Use the PEBS auto reload mechanism when possible") Introduce intel_pmu_save_and_restart_reload() to calculate the event->count only for auto-reload. Since the counter increments a negative counter value and overflows on the sign switch, giving the interval: [-period, 0] the difference between two consequtive reads is: A) value2 - value1; when no overflows have happened in between, B) (0 - value1) + (value2 - (-period)); when one overflow happened in between, C) (0 - value1) + (n - 1) * (period) + (value2 - (-period)); when @n overflows happened in between. Here A) is the obvious difference, B) is the extension to the discrete interval, where the first term is to the top of the interval and the second term is from the bottom of the next interval and C) the extension to multiple intervals, where the middle term is the whole intervals covered. The equation for all cases is: value2 - value1 + n * period Previously the event->count is updated right before the sample output. But for case A, there is no PEBS record ready. It needs to be specially handled. Remove the auto-reload code from x86_perf_event_set_period() since we'll not longer call that function in this case. Based-on-code-from: Peter Zijlstra (Intel) <peterz@infradead.org> Signed-off-by: Kan Liang <kan.liang@linux.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> Cc: acme@kernel.org Fixes: 851559e35fd5 ("perf/x86/intel: Use the PEBS auto reload mechanism when possible") Link: http://lkml.kernel.org/r/1518474035-21006-2-git-send-email-kan.liang@linux.intel.com Signed-off-by: Ingo Molnar <mingo@kernel.org>
Diffstat (limited to 'arch/x86/events/core.c')
-rw-r--r--arch/x86/events/core.c15
1 files changed, 6 insertions, 9 deletions
diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
index 140d33288e78..5a3ccd1715e2 100644
--- a/arch/x86/events/core.c
+++ b/arch/x86/events/core.c
@@ -1156,16 +1156,13 @@ int x86_perf_event_set_period(struct perf_event *event)
per_cpu(pmc_prev_left[idx], smp_processor_id()) = left;
- if (!(hwc->flags & PERF_X86_EVENT_AUTO_RELOAD) ||
- local64_read(&hwc->prev_count) != (u64)-left) {
- /*
- * The hw event starts counting from this event offset,
- * mark it to be able to extra future deltas:
- */
- local64_set(&hwc->prev_count, (u64)-left);
+ /*
+ * The hw event starts counting from this event offset,
+ * mark it to be able to extra future deltas:
+ */
+ local64_set(&hwc->prev_count, (u64)-left);
- wrmsrl(hwc->event_base, (u64)(-left) & x86_pmu.cntval_mask);
- }
+ wrmsrl(hwc->event_base, (u64)(-left) & x86_pmu.cntval_mask);
/*
* Due to erratum on certan cpu we need