[PATCH 5.14 27/30] perf/core: fix userpage->time_enabled of inactive events

Greg Kroah-Hartman
2021-10-14T15:05:55Z

From: Song Liu <songliubraving@fb.com>

[ Upstream commit f792565326825ed806626da50c6f9a928f1079c1 ]

Users of rdpmc rely on the mmapped user page to calculate accurate
time_enabled. Currently, userpage->time_enabled is only updated when the
event is added to the pmu. As a result, inactive event (due to counter
multiplexing) does not have accurate userpage->time_enabled. This can
be reproduced with something like:

/* open 20 task perf_event "cycles", to create multiplexing */

fd = perf_event_open(); /* open task perf_event "cycles" */
userpage = mmap(fd); /* use mmap and rdmpc */

while (true) {
time_enabled_mmap = xxx; /* use logic in perf_event_mmap_page */
time_enabled_read = read(fd).time_enabled;
if (time_enabled_mmap > time_enabled_read)
BUG();
}

Fix this by updating userpage for inactive events in merge_sched_in.

Suggested-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reported-and-tested-by: Lucian Grijincu <lucian@fb.com>
Signed-off-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20210929194313.2398474-1-songliubraving@fb.com
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
include/linux/perf_event.h | 4 +++-
kernel/events/core.c | 34 ++++++++++++++++++++++++++++++----
2 files changed, 33 insertions(+), 5 deletions(-)

diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index 2d510ad750ed..4aa52f7a48c1 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -683,7 +683,9 @@ struct perf_event {
/*
* timestamp shadows the actual context timing but it can
* be safely used in NMI interrupt context. It reflects the
- * context time as it was when the event was last scheduled in.
+ * context time as it was when the event was last scheduled in,
+ * or when ctx_sched_in failed to schedule the event because we
+ * run out of PMC.
*
* ctx_time already accounts for ctx->timestamp. Therefore to
* compute ctx_time for a sample, simply add perf_clock().
diff --git a/kernel/events/core.c b/kernel/events/core.c
index e5c4aca620c5..22c5b1622c22 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -3707,6 +3707,29 @@ static noinline int visit_groups_merge(struct perf_cpu_context *cpuctx,
return 0;
}

+static inline bool event_update_userpage(struct perf_event *event)
+{
+ if (likely(!atomic_read(&event->mmap_count)))
+ return false;
+
+ perf_event_update_time(event);
+ perf_set_shadow_time(event, event->ctx);
+ perf_event_update_userpage(event);
+
+ return true;
+}
+
+static inline void group_update_userpage(struct perf_event *group_event)
+{
+ struct perf_event *event;
+
+ if (!event_update_userpage(group_event))
+ return;
+
+ for_each_sibling_event(event, group_event)
+ event_update_userpage(event);
+}
+
static int merge_sched_in(struct perf_event *event, void *data)
{
struct perf_event_context *ctx = event->ctx;
@@ -3725,14 +3748,15 @@ static int merge_sched_in(struct perf_event *event, void *data)
}

if (event->state == PERF_EVENT_STATE_INACTIVE) {
+ *can_add_hw = 0;
if (event->attr.pinned) {
perf_cgroup_event_disable(event, ctx);
perf_event_set_state(event, PERF_EVENT_STATE_ERROR);
+ } else {
+ ctx->rotate_necessary = 1;
+ perf_mux_hrtimer_restart(cpuctx);
+ group_update_userpage(event);
}
-
- *can_add_hw = 0;
- ctx->rotate_necessary = 1;
- perf_mux_hrtimer_restart(cpuctx);
}

return 0;
@@ -6311,6 +6335,8 @@ accounting:

ring_buffer_attach(event, rb);

+ perf_event_update_time(event);
+ perf_set_shadow_time(event, event->ctx);
perf_event_init_userpage(event);
perf_event_update_userpage(event);
} else {
--
2.33.0