[PATCH 6.18 086/752] perf/core: Fix slow perf_event_task_exit() with LBR callstacks

Sasha Levin
2026-02-28T17:49:21Z

From: Namhyung Kim <namhyung@kernel.org>

[ Upstream commit 4960626f956d63dce57f099016c2ecbe637a8229 ]

I got a report that a task is stuck in perf_event_exit_task() waiting
for global_ctx_data_rwsem. On large systems with lots threads, it'd
have performance issues when it grabs the lock to iterate all threads
in the system to allocate the context data.

And it'd block task exit path which is problematic especially under
memory pressure.

perf_event_open
perf_event_alloc
attach_perf_ctx_data
attach_global_ctx_data
percpu_down_write (global_ctx_data_rwsem)
for_each_process_thread
alloc_task_ctx_data
do_exit
perf_event_exit_task
percpu_down_read (global_ctx_data_rwsem)

It should not hold the global_ctx_data_rwsem on the exit path. Let's
skip allocation for exiting tasks and free the data carefully.

Reported-by: Rosalie Fang <rosaliefang@google.com>
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20260112165157.1919624-1-namhyung@kernel.org
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
kernel/events/core.c | 20 ++++++++++++++++++--
1 file changed, 18 insertions(+), 2 deletions(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 1d8ca8e34f5c4..c34b927e5ece3 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -5279,9 +5279,20 @@ attach_task_ctx_data(struct task_struct *task, struct kmem_cache *ctx_cache,
return -ENOMEM;

for (;;) {
- if (try_cmpxchg((struct perf_ctx_data **)&task->perf_ctx_data, &old, cd)) {
+ if (try_cmpxchg(&task->perf_ctx_data, &old, cd)) {
if (old)
perf_free_ctx_data_rcu(old);
+ /*
+ * Above try_cmpxchg() pairs with try_cmpxchg() from
+ * detach_task_ctx_data() such that
+ * if we race with perf_event_exit_task(), we must
+ * observe PF_EXITING.
+ */
+ if (task->flags & PF_EXITING) {
+ /* detach_task_ctx_data() may free it already */
+ if (try_cmpxchg(&task->perf_ctx_data, &cd, NULL))
+ perf_free_ctx_data_rcu(cd);
+ }
return 0;
}

@@ -5327,6 +5338,8 @@ attach_global_ctx_data(struct kmem_cache *ctx_cache)
/* Allocate everything */
scoped_guard (rcu) {
for_each_process_thread(g, p) {
+ if (p->flags & PF_EXITING)
+ continue;
cd = rcu_dereference(p->perf_ctx_data);
if (cd && !cd->global) {
cd->global = 1;
@@ -14223,8 +14236,11 @@ void perf_event_exit_task(struct task_struct *task)

/*
* Detach the perf_ctx_data for the system-wide event.
+ *
+ * Done without holding global_ctx_data_rwsem; typically
+ * attach_global_ctx_data() will skip over this task, but otherwise
+ * attach_task_ctx_data() will observe PF_EXITING.
*/
- guard(percpu_read)(&global_ctx_data_rwsem);
detach_task_ctx_data(task);
}

--
2.51.0