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) <[email protected]>
Reported-and-tested-by: Lucian Grijincu <[email protected]>
Signed-off-by: Song Liu <[email protected]>
---
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 2d510ad750edc..4aa52f7a48c16 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 1cb1f9b8392e2..00c46d839ed90 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 @@ static int perf_mmap(struct file *file, struct vm_area_struct *vma)
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 {
--
2.30.2
The following commit has been merged into the perf/urgent branch of tip:
Commit-ID: f792565326825ed806626da50c6f9a928f1079c1
Gitweb: https://git.kernel.org/tip/f792565326825ed806626da50c6f9a928f1079c1
Author: Song Liu <[email protected]>
AuthorDate: Wed, 29 Sep 2021 12:43:13 -07:00
Committer: Peter Zijlstra <[email protected]>
CommitterDate: Fri, 01 Oct 2021 13:57:54 +02:00
perf/core: fix userpage->time_enabled of inactive events
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) <[email protected]>
Reported-and-tested-by: Lucian Grijincu <[email protected]>
Signed-off-by: Song Liu <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]
---
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 fe156a8..9b60bb8 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 0c000cb..f23ca26 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;
@@ -6324,6 +6348,8 @@ 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 {