[tip: perf/core] perf: Avoid the read if the count is already updated

tip-bot2 for Peter Zijlstra (Intel)
2025-02-03T12:48:19Z

Commit-ID: 186dbd9cd489e7f895f909d336c58e4ba2f660a1
Gitweb: https://git.kernel.org/tip/186dbd9cd489e7f895f909d336c58e4ba2f660a1
Author: Peter Zijlstra (Intel) <peterz@infradead.org>
AuthorDate: Tue, 21 Jan 2025 07:23:02 -08:00
Committer: Peter Zijlstra <peterz@infradead.org>
CommitterDate: Mon, 03 Feb 2025 11:46:07 +01:00

perf: Avoid the read if the count is already updated

The event may have been updated in the PMU-specific implementation,
e.g., Intel PEBS counters snapshotting. The common code should not
read and overwrite the value.

The PERF_SAMPLE_READ in the data->sample_type can be used to detect
whether the PMU-specific value is available. If yes, avoid the
pmu->read() in the common code. Add a new flag, skip_read, to track the
case.

Factor out a perf_pmu_read() to clean up the code.

Signed-off-by: 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>
Link: https://lkml.kernel.org/r/20250121152303.3128733-3-kan.liang@linux.intel.com
---
include/linux/perf_event.h | 8 +++++++-
kernel/events/core.c | 33 ++++++++++++++++-----------------
kernel/events/ring_buffer.c | 1 +
3 files changed, 24 insertions(+), 18 deletions(-)

diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index 8333f13..2d07bc1 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -1062,7 +1062,13 @@ struct perf_output_handle {
struct perf_buffer *rb;
unsigned long wakeup;
unsigned long size;
- u64 aux_flags;
+ union {
+ u64 flags; /* perf_output*() */
+ u64 aux_flags; /* perf_aux_output*() */
+ struct {
+ u64 skip_read : 1;
+ };
+ };
union {
void *addr;
unsigned long head;
diff --git a/kernel/events/core.c b/kernel/events/core.c
index bcb09e0..0f8c559 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -1191,6 +1191,12 @@ static void perf_assert_pmu_disabled(struct pmu *pmu)
WARN_ON_ONCE(*this_cpu_ptr(pmu->pmu_disable_count) == 0);
}

+static inline void perf_pmu_read(struct perf_event *event)
+{
+ if (event->state == PERF_EVENT_STATE_ACTIVE)
+ event->pmu->read(event);
+}
+
static void get_ctx(struct perf_event_context *ctx)
{
refcount_inc(&ctx->refcount);
@@ -3473,8 +3479,7 @@ static void __perf_event_sync_stat(struct perf_event *event,
* we know the event must be on the current CPU, therefore we
* don't need to use it.
*/
- if (event->state == PERF_EVENT_STATE_ACTIVE)
- event->pmu->read(event);
+ perf_pmu_read(event);

perf_event_update_time(event);

@@ -4618,15 +4623,8 @@ static void __perf_event_read(void *info)

pmu->read(event);

- for_each_sibling_event(sub, event) {
- if (sub->state == PERF_EVENT_STATE_ACTIVE) {
- /*
- * Use sibling's PMU rather than @event's since
- * sibling could be on different (eg: software) PMU.
- */
- sub->pmu->read(sub);
- }
- }
+ for_each_sibling_event(sub, event)
+ perf_pmu_read(sub);

data->ret = pmu->commit_txn(pmu);

@@ -7444,9 +7442,8 @@ static void perf_output_read_group(struct perf_output_handle *handle,
if (read_format & PERF_FORMAT_TOTAL_TIME_RUNNING)
values[n++] = running;

- if ((leader != event) &&
- (leader->state == PERF_EVENT_STATE_ACTIVE))
- leader->pmu->read(leader);
+ if ((leader != event) && !handle->skip_read)
+ perf_pmu_read(leader);

values[n++] = perf_event_count(leader, self);
if (read_format & PERF_FORMAT_ID)
@@ -7459,9 +7456,8 @@ static void perf_output_read_group(struct perf_output_handle *handle,
for_each_sibling_event(sub, leader) {
n = 0;

- if ((sub != event) &&
- (sub->state == PERF_EVENT_STATE_ACTIVE))
- sub->pmu->read(sub);
+ if ((sub != event) && !handle->skip_read)
+ perf_pmu_read(sub);

values[n++] = perf_event_count(sub, self);
if (read_format & PERF_FORMAT_ID)
@@ -7520,6 +7516,9 @@ void perf_output_sample(struct perf_output_handle *handle,
{
u64 sample_type = data->type;

+ if (data->sample_flags & PERF_SAMPLE_READ)
+ handle->skip_read = 1;
+
perf_output_put(handle, *header);

if (sample_type & PERF_SAMPLE_IDENTIFIER)
diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c
index 1805091..59a52b1 100644
--- a/kernel/events/ring_buffer.c
+++ b/kernel/events/ring_buffer.c
@@ -185,6 +185,7 @@ __perf_output_begin(struct perf_output_handle *handle,

handle->rb = rb;
handle->event = event;
+ handle->flags = 0;

have_lost = local_read(&rb->lost);
if (unlikely(have_lost)) {