[PATCH v5 2/2] perf/core: Fix incorrect time diff in tick adjust period

[Luo Gengkun <luogengkun@huaweicloud.com>]

Perf events has the notion of sampling frequency which is implemented in
software by dynamically adjusting the counter period so that samples occur
at approximately the target frequency.  Period adjustment is done in 2
places:
 - when the counter overflows (and a sample is recorded)
 - each timer tick, when the event is active
The later case is slightly flawed because it assumes that the time since
the last timer-tick period adjustment is 1 tick, whereas the event may not
have been active (e.g. for a task that is sleeping).

Fix by using jiffies to determine the elapsed time in that case.

Signed-off-by: Luo Gengkun <luogengkun@huaweicloud.com>
Reviewed-by: Adrian Hunter <adrian.hunter@intel.com>
---
 include/linux/perf_event.h |  1 +
 kernel/events/core.c       | 12 +++++++++---
 2 files changed, 10 insertions(+), 3 deletions(-)

diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index 1a8942277dda..d29b7cf971a1 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -265,6 +265,7 @@ struct hw_perf_event {
 	 * State for freq target events, see __perf_event_overflow() and
 	 * perf_adjust_freq_unthr_context().
 	 */
+	u64				freq_tick_stamp;
 	u64				freq_time_stamp;
 	u64				freq_count_stamp;
 #endif
diff --git a/kernel/events/core.c b/kernel/events/core.c
index a9395bbfd4aa..183291e0d070 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -55,6 +55,7 @@
 #include <linux/pgtable.h>
 #include <linux/buildid.h>
 #include <linux/task_work.h>
+#include <linux/jiffies.h>
 
 #include "internal.h"
 
@@ -4120,9 +4121,11 @@ static void perf_adjust_freq_unthr_events(struct list_head *event_list)
 {
 	struct perf_event *event;
 	struct hw_perf_event *hwc;
-	u64 now, period = TICK_NSEC;
+	u64 now, period, tick_stamp;
 	s64 delta;
 
+	tick_stamp = jiffies64_to_nsecs(get_jiffies_64());
+
 	list_for_each_entry(event, event_list, active_list) {
 		if (event->state != PERF_EVENT_STATE_ACTIVE)
 			continue;
@@ -4148,6 +4151,9 @@ static void perf_adjust_freq_unthr_events(struct list_head *event_list)
 		 */
 		event->pmu->stop(event, PERF_EF_UPDATE);
 
+		period = tick_stamp - hwc->freq_tick_stamp;
+		hwc->freq_tick_stamp = tick_stamp;
+
 		now = local64_read(&event->count);
 		delta = now - hwc->freq_count_stamp;
 		hwc->freq_count_stamp = now;
@@ -4157,9 +4163,9 @@ static void perf_adjust_freq_unthr_events(struct list_head *event_list)
 		 * reload only if value has changed
 		 * we have stopped the event so tell that
 		 * to perf_adjust_period() to avoid stopping it
-		 * twice.
+		 * twice. And skip if it is the first tick adjust period.
 		 */
-		if (delta > 0)
+		if (delta > 0 && likely(period != tick_stamp))
 			perf_adjust_period(event, period, delta, false);
 
 		event->pmu->start(event, delta > 0 ? PERF_EF_RELOAD : 0);
-- 
2.34.1