Re: [BUG] perf/core: Task stuck on global_ctx_data_rwsem

[Namhyung Kim]

Hello,

On Mon, Dec 22, 2025 at 03:36:53PM -0800, Namhyung Kim wrote:
> On Mon, Dec 22, 2025 at 03:34:23PM -0800, Namhyung Kim wrote:
> > Hello,
> > 
> > I got a report that a task is stuck in perf_event_exit_task() waiting
> > for global_ctx_data_rwsem.  On large systems, 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)
> > 
> > I think attach_global_ctx_data() should skip tasks with PF_EXITING and
> > it'd be nice if perf_event_exit_task() could release the ctx_data
> > unconditionally.  But I'm not sure how to synchronize them properly.
> > 
> > Any thoughts?

I'm curious if this makes any sense..  I feel like it needs to check the
flag again before allocation.

Thanks,
Namhyung


diff --git a/kernel/events/core.c b/kernel/events/core.c
index 376fb07d869b8b50..2a8847e95d7eb698 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -5469,6 +5469,8 @@ 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;
@@ -14563,7 +14565,6 @@ void perf_event_exit_task(struct task_struct *task)
 	/*
 	 * Detach the perf_ctx_data for the system-wide event.
 	 */
-	guard(percpu_read)(&global_ctx_data_rwsem);
 	detach_task_ctx_data(task);
 }