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Message-Id: <20241220164628.1079271-1-yeoreum.yun@arm.com>
Date: Fri, 20 Dec 2024 16:46:28 +0000
From: Yeoreum Yun <yeoreum.yun@....com>
To: peterz@...radead.org,
mingo@...hat.com,
acme@...nel.org,
namhyung@...nel.org,
mark.rutland@....com,
alexander.shishkin@...ux.intel.com,
jolsa@...nel.org,
irogers@...gle.com,
adrian.hunter@...el.com,
kan.liang@...ux.intel.com
Cc: linux-perf-users@...r.kernel.org,
linux-kernel@...r.kernel.org,
Yeoreum Yun <yeoreum.yun@....com>
Subject: [PATCH v3] events/core: fix acoount failure for event's child_total_enable_time at task exit
The perf core code fails to account for total_enable_time of event
when its state is inactive.
Here is the error case for failure to account for total_enable_time for
core event:
sudo ./perf stat -vvv -e armv8_pmuv3_0/event=0x08/ -e armv8_pmuv3_1/event=0x08/ -- stress-ng --pthread=2 -t 2s
...
/*
* three number represetns each
* scaled count / total_enable_time / total_total_running_time
*/
armv8_pmuv3_0/event=0x08/: 1138698008 2289429840 2174835740
^^^^^^^^^^
armv8_pmuv3_1/event=0x08/: 1826791390 1950025700 847648440
^^^^^^^^^^
Performance counter stats for 'stress-ng --pthread=2 -t 2s':
1,138,698,008 armv8_pmuv3_0/event=0x08/ (94.99%)
1,826,791,390 armv8_pmuv3_1/event=0x08/ (43.47%)
Since above two events are belong to the same task context and
mutually-exclusive per cpu (they couldn't be active at the same time on the same cpu),
the total_enable_time should be the same (marked with ^^^^^^^^^^^)
and the summation of ratio should be 100%.
This account failure of total_enable_time because of
account failure of child_total_enable_time of child event when
child task exit.
Below table explains how the child_total_enable_time is failed to account at
exiting child task which switch cpus as time passes by (CPU0 -> CPU1 -> CPU0)
- in means sched_in.
- out means sched_out.
- exit means at the exit of child task.
NOTE: the value is before calling list_del_event(). which mean
the value at exit column will be added at parent event's
child_total_enable_time when child task exit.
- ctx is the child_task_ctx,
- e0 is the child_event which set with cpu == -1 and opened with pmu0 only
added in CPU0,
- e1 is the child_event which set with cpu == -1 and opened with pmu1 only
added in CPU1,
- e0 and e1 belongs to same child_task_ctx.
CPU0 (run t1) CPU1 (run t2) CPU0 (run t3)
| in | out | | in | out | | in | exit |
------------------------------------------------------------------------------
ctx->time | 0 | t1 | | t1 | t1 + t2 | | t1 + t2 | t1 + t2 + t3|
------------------------------------------------------------------------------
e0->ena | 0 | t1 | | t1 | t1 *| | t1 + t2 | t1 + t2 + t3|
------------------------------------------------------------------------------
e0->run | 0 | t1 | | t1 | t1 *| | t1 + t2 | t1 + t3 |
------------------------------------------------------------------------------
e1->ena | 0 | 0 *| | t1 | t1 + t2 | | t1 + t2 | t1 + t2 X|
------------------------------------------------------------------------------
e1->run | 0 | 0 *| | 0 | t2 | | t1 + t2 | t2 X|
------------------------------------------------------------------------------
The value marked with * means it doesn't updates since event->state was
INACTIVE.
Please see the last CPU0's column with exit (marked with X).
Since e1's state is INACTIVE its total_enable_time doesn't update
and it remains with former value without accounting t3 time.
In this situation, at __perf_remove_from_context() while exit child_task,
sync_child_event() where adds child_event's total_enable_time to
parent event's child_total_enable_time is called before
list_del_event() in which event time is updated by setting the event state
as OFF. that means child_total_enable_time is added with missing
amount of last enable time -- t3.
In case of parent event's total_enable_time is updated properly in
list_del_event() when the task exit,
However the child_total_enable_time is missed when child_task exited,
the perf prints error amount of enable_time (which is summation of
total_enable_time + child_total_enable_time).
To address this, before updating child_total_enable_time of parent's
event. update event time.
After this patch, this problem is gone like:
sudo ./perf stat -vvv -e armv8_pmuv3_0/event=0x08/ -e armv8_pmuv3_1/event=0x08/ -- stress-ng --pthread=2 -t 10s
...
armv8_pmuv3_0/event=0x08/: 15396770398 32157963940 21898169000
armv8_pmuv3_1/event=0x08/: 22428964974 32157963940 10259794940
Performance counter stats for 'stress-ng --pthread=2 -t 10s':
15,396,770,398 armv8_pmuv3_0/event=0x08/ (68.10%)
22,428,964,974 armv8_pmuv3_1/event=0x08/ (31.90%)
Signed-off-by: Yeoreum Yun <yeoreum.yun@....com>
---
kernel/events/core.c | 1 +
1 file changed, 1 insertion(+)
diff --git a/kernel/events/core.c b/kernel/events/core.c
index 065f9188b44a..d27717c44924 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -13337,6 +13337,7 @@ static void sync_child_event(struct perf_event *child_event)
}
child_val = perf_event_count(child_event, false);
+ perf_event_update_time(child_event);
/*
* Add back the child's count to the parent's count:
--
LEVI:{C3F47F37-75D8-414A-A8BA-3980EC8A46D7}
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