6.12-stable review patch. If anyone has any objections, please let me know.
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From: Yeoreum Yun yeoreum.yun@arm.com
[ Upstream commit a3c3c66670cee11eb13aa43905904bf29cb92d32 ]
The perf events code fails to account for total_time_enabled of inactive events.
Here is a failure case for accounting total_time_enabled for CPU PMU events:
sudo ./perf stat -vvv -e armv8_pmuv3_0/event=0x08/ -e armv8_pmuv3_1/event=0x08/ -- stress-ng --pthread=2 -t 2s ...
armv8_pmuv3_0/event=0x08/: 1138698008 2289429840 2174835740 armv8_pmuv3_1/event=0x08/: 1826791390 1950025700 847648440 ` ` ` ` ` > total_time_running with child ` > total_time_enabled with child > count with child
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%)
The two events above are opened on two different CPU PMUs, for example, each event is opened for a cluster in an Arm big.LITTLE system, they will never run on the same CPU. In theory, the total enabled time should be same for both events, as two events are opened and closed together.
As the result show, the two events' total enabled time including child event is different (2289429840 vs 1950025700).
This is because child events are not accounted properly if a event is INACTIVE state when the task exits:
perf_event_exit_event() `> perf_remove_from_context() `> __perf_remove_from_context() `> perf_child_detach() -> Accumulate child_total_time_enabled `> list_del_event() -> Update child event's time
The problem is the time accumulation happens prior to child event's time updating. Thus, it misses to account the last period's time when the event exits.
The perf core layer follows the rule that timekeeping is tied to state change. To address the issue, make __perf_remove_from_context() handle the task exit case by passing 'DETACH_EXIT' to it and invoke perf_event_state() for state alongside with accounting the time.
Then, perf_child_detach() populates the time into the parent's time metrics.
After this patch, the bug is fixed:
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%)
[ mingo: Clarified the changelog. ]
Fixes: ef54c1a476aef ("perf: Rework perf_event_exit_event()") Suggested-by: Peter Zijlstra peterz@infradead.org Signed-off-by: Yeoreum Yun yeoreum.yun@arm.com Signed-off-by: Ingo Molnar mingo@kernel.org Tested-by: Leo Yan leo.yan@arm.com Link: https://lore.kernel.org/r/20250326082003.1630986-1-yeoreum.yun@arm.com Signed-off-by: Sasha Levin sashal@kernel.org --- kernel/events/core.c | 18 +++++++++--------- 1 file changed, 9 insertions(+), 9 deletions(-)
diff --git a/kernel/events/core.c b/kernel/events/core.c index cf2ec0a1582fd..b5ccf52bb71ba 100644 --- a/kernel/events/core.c +++ b/kernel/events/core.c @@ -2407,6 +2407,7 @@ ctx_time_update_event(struct perf_event_context *ctx, struct perf_event *event) #define DETACH_GROUP 0x01UL #define DETACH_CHILD 0x02UL #define DETACH_DEAD 0x04UL +#define DETACH_EXIT 0x08UL
/* * Cross CPU call to remove a performance event @@ -2421,6 +2422,7 @@ __perf_remove_from_context(struct perf_event *event, void *info) { struct perf_event_pmu_context *pmu_ctx = event->pmu_ctx; + enum perf_event_state state = PERF_EVENT_STATE_OFF; unsigned long flags = (unsigned long)info;
ctx_time_update(cpuctx, ctx); @@ -2429,16 +2431,19 @@ __perf_remove_from_context(struct perf_event *event, * Ensure event_sched_out() switches to OFF, at the very least * this avoids raising perf_pending_task() at this time. */ - if (flags & DETACH_DEAD) + if (flags & DETACH_EXIT) + state = PERF_EVENT_STATE_EXIT; + if (flags & DETACH_DEAD) { event->pending_disable = 1; + state = PERF_EVENT_STATE_DEAD; + } event_sched_out(event, ctx); + perf_event_set_state(event, min(event->state, state)); if (flags & DETACH_GROUP) perf_group_detach(event); if (flags & DETACH_CHILD) perf_child_detach(event); list_del_event(event, ctx); - if (flags & DETACH_DEAD) - event->state = PERF_EVENT_STATE_DEAD;
if (!pmu_ctx->nr_events) { pmu_ctx->rotate_necessary = 0; @@ -13343,12 +13348,7 @@ perf_event_exit_event(struct perf_event *event, struct perf_event_context *ctx) mutex_lock(&parent_event->child_mutex); }
- perf_remove_from_context(event, detach_flags); - - raw_spin_lock_irq(&ctx->lock); - if (event->state > PERF_EVENT_STATE_EXIT) - perf_event_set_state(event, PERF_EVENT_STATE_EXIT); - raw_spin_unlock_irq(&ctx->lock); + perf_remove_from_context(event, detach_flags | DETACH_EXIT);
/* * Child events can be freed.