From: Kan Liang <kan.li...@linux.intel.com>
There is bug when mmap read event->count with large PEBS enabled.
Here is an example.
#./read_count
0x71f0
0x122c0
0x1000000001c54
0x100000001257d
0x200000000bdc5
There is auto-reload mechanism enabled for PEBS events in fixed period
mode. But the calculation of event->count does not take the auto-reload
values into account. Anyone who read the event->count will get wrong
result, e.g x86_pmu_read. Also, the calculation of hwc->period_left is
wrong either. It impacts the accuracy of period for the first record in
PEBS multiple records.
The issue is introduced with the auto-reload mechanism enabled by
commit 851559e35fd5 ("perf/x86/intel: Use the PEBS auto reload mechanism
when possible")
For the auto-reload before the last time, it went through the whole
period (reload value) every time. So period * times should be added into
the event->count.
For the last load, the elapsed delta (event-)time need to be corrected
by adding the period (reload value). Because the start point is -period.
Signed-off-by: Kan Liang <kan.li...@linux.intel.com>
---
arch/x86/events/core.c | 14 ++++++++++++++
arch/x86/events/intel/ds.c | 8 +++++++-
2 files changed, 21 insertions(+), 1 deletion(-)
diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
index 35552ea..f74e21d 100644
--- a/arch/x86/events/core.c
+++ b/arch/x86/events/core.c
@@ -100,6 +100,20 @@ u64 x86_perf_event_update(struct perf_event *event,
* of the count.
*/
delta = (new_raw_count << shift) - (prev_raw_count << shift);
+
+ /*
+ * Take auto-reload into account
+ * For the auto-reload before the last time, it went through the
+ * whole period (reload_val) every time.
+ * Just simply add period * times to the event.
+ *
+ * For the last load, the elapsed delta (event-)time need to be
+ * corrected by adding the period. Because the start point is -period.
+ */
+ if (reload_times > 0) {
+ delta += (reload_val << shift);
+ local64_add(reload_val * (reload_times - 1), &event->count);
+ }
delta >>= shift;
local64_add(delta, &event->count);
diff --git a/arch/x86/events/intel/ds.c b/arch/x86/events/intel/ds.c
index 0b693b7..f0f6026 100644
--- a/arch/x86/events/intel/ds.c
+++ b/arch/x86/events/intel/ds.c
@@ -1256,11 +1256,17 @@ static void __intel_pmu_pebs_event(struct perf_event
*event,
void *base, void *top,
int bit, int count)
{
+ struct hw_perf_event *hwc = &event->hw;
struct perf_sample_data data;
struct pt_regs regs;
void *at = get_next_pebs_record_by_bit(base, top, bit);
- if (!intel_pmu_save_and_restart(event, 0, 0) &&
+ /*
+ * Now, auto-reload is only enabled in fixed period mode.
+ * The reload value is always hwc->sample_period.
+ * May need to change it, if auto-reload is enabled in freq mode later.
+ */
+ if (!intel_pmu_save_and_restart(event, hwc->sample_period, count - 1) &&
!(event->hw.flags & PERF_X86_EVENT_AUTO_RELOAD))
return;
--
2.7.4
