[PATCH v2 1/2] fs/epoll: mark epoll_wait tasks TASK_FREEZABLE

[PATCH v2 1/2] fs/epoll: mark epoll_wait tasks TASK_FREEZABLE

[Sadrul Chowdhury]

During system suspend (freeze_processes) and resume (thaw_processes),
threads blocked in epoll_wait() sleep in TASK_INTERRUPTIBLE. When
try_to_freeze_tasks() executes, it must wake up every sleeping thread
with a synthetic signal so the thread can schedule, run try_to_freeze(),
enter TASK_FROZEN, and sleep again. Upon resume, the freezer wakes them
all up simultaneously, only for each thread to re-enter ep_poll(),
observe no ready I/O events, and call schedule() to go back to sleep.

On systems with numerous event-driven daemons (where hundreds of
threads idle in event loops), this causes a large wakeup stampede and
thousands of redundant context switches per suspend/resume cycle.

An earlier attempt to make epoll_wait() freezable in commit 1c441e921201
("epoll: use freezable blocking call") was reverted in commit
c511851de162 ("Revert "epoll: use freezable blocking call"") due to
reports of userspace corruption (Bugzilla #61781). Under the legacy
freezer design, freezable tasks could wake up prematurely while
hardware and peripheral drivers were still suspended.

With the 2022 core freezer rewrite (commit f5d39b020809 ("freezer,sched:
Rewrite core freezer logic")), the legacy fake-signal mechanism was
replaced with the TASK_FROZEN state. Tasks sleeping in TASK_FREEZABLE
are safely frozen in-place without waking them up, and the scheduler
guarantees they cannot be woken until thaw_processes() explicitly clears
the frozen state.

Mark sleeping tasks in ep_poll() as TASK_FREEZABLE. Benchmarks on an
ARM64 test device demonstrate that combining this with freezable
select/poll reduces thaw latency by 63% (from 20.64 ms down to 7.55 ms
mean at fixed CPU frequency) and reduces context switches during
freeze/thaw by 87%.

Reviewed-by: Jan Kara <jack@suse.cz>
Acked-by: Rafael J. Wysocki <rafael@kernel.org>
Signed-off-by: Sadrul Chowdhury <sadrul@google.com>
---
v1 -> v2:
- Rebased against current mainline (torvalds/linux.git master).
- Collected Reviewed-by from Jan Kara and Acked-by from Rafael J. Wysocki.

 fs/eventpoll.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index f48b829a710f..76906ced2b86 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -2327,7 +2327,7 @@ static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events,
 		 * the same lock on wakeup ep_poll_callback() side, so it
 		 * is safe to avoid an explicit barrier.
 		 */
-		__set_current_state(TASK_INTERRUPTIBLE);
+		__set_current_state(TASK_INTERRUPTIBLE | TASK_FREEZABLE);
 
 		/*
 		 * Do the final check under the lock. ep_start/done_scan()
-- 
2.56.0.212.g129ede20d9-goog

[PATCH v2 2/2] fs/select: mark select and poll tasks TASK_FREEZABLE

[Sadrul Chowdhury]

Similar to epoll_wait(), threads sleeping in do_select() and do_poll()
sleep in TASK_INTERRUPTIBLE via poll_schedule_timeout(). During system
suspend and resume, these threads are woken up by the freezer solely to
transition to TASK_FROZEN, and woken again upon thaw only to re-evaluate
file descriptors with no pending activity and sleep again.

A previous attempt to make select() and poll() freezable in commit
9745cdb36da8 ("select: use freezable blocking call") was reverted in
commit 59612d187912 ("Revert "select: use freezable blocking call"")
alongside the epoll revert due to issues caused by the legacy freezer
architecture.

Following the freezer overhaul in commit f5d39b020809 ("freezer,sched:
Rewrite core freezer logic"), tasks in TASK_FREEZABLE are safely frozen
in-place into TASK_FROZEN without running userspace code or experiencing
premature wakeups before resume completes.

Pass TASK_FREEZABLE to poll_schedule_timeout() in do_select() and
do_poll() (TASK_INTERRUPTIBLE | TASK_FREEZABLE) so that polling threads
remain asleep undisturbed across suspend/resume cycles.

Reviewed-by: Jan Kara <jack@suse.cz>
Acked-by: Rafael J. Wysocki <rafael@kernel.org>
Signed-off-by: Sadrul Chowdhury <sadrul@google.com>
---
v1 -> v2:
- Rebased against current mainline (torvalds/linux.git master).
- Collected Reviewed-by from Jan Kara and Acked-by from Rafael J. Wysocki.

 fs/select.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/select.c b/fs/select.c
index 95d76531015a..03318d678187 100644
--- a/fs/select.c
+++ b/fs/select.c
@@ -600,7 +600,7 @@ static noinline_for_stack int do_select(int n, fd_set_bits *fds, struct timespec
 			to = &expire;
 		}
 
-		if (!poll_schedule_timeout(&table, TASK_INTERRUPTIBLE,
+		if (!poll_schedule_timeout(&table, TASK_INTERRUPTIBLE | TASK_FREEZABLE,
 					   to, slack))
 			timed_out = 1;
 	}
@@ -962,7 +962,7 @@ static int do_poll(struct poll_list *list, struct poll_wqueues *wait,
 			to = &expire;
 		}
 
-		if (!poll_schedule_timeout(wait, TASK_INTERRUPTIBLE, to, slack))
+		if (!poll_schedule_timeout(wait, TASK_INTERRUPTIBLE | TASK_FREEZABLE, to, slack))
 			timed_out = 1;
 	}
 	return count;
-- 
2.56.0.212.g129ede20d9-goog

Re: [PATCH v2 1/2] fs/epoll: mark epoll_wait tasks TASK_FREEZABLE

[Christian Brauner]

On Wed, 21 Oct 2026 12:25:42 +0000, Sadrul Chowdhury wrote:
> During system suspend (freeze_processes) and resume (thaw_processes),
> threads blocked in epoll_wait() sleep in TASK_INTERRUPTIBLE. When
> try_to_freeze_tasks() executes, it must wake up every sleeping thread
> with a synthetic signal so the thread can schedule, run try_to_freeze(),
> enter TASK_FROZEN, and sleep again. Upon resume, the freezer wakes them
> all up simultaneously, only for each thread to re-enter ep_poll(),
> observe no ready I/O events, and call schedule() to go back to sleep.
> 
> [...]

Applied to the vfs-7.5.misc branch of the vfs/vfs.git tree.
Patches in the vfs-7.5.misc branch should appear in linux-next soon.

Please report any outstanding bugs that were missed during review in a
new review to the original patch series allowing us to drop it.

It's encouraged to provide Acked-bys and Reviewed-bys even though the
patch has now been applied. If possible patch trailers will be updated.

Note that commit hashes shown below are subject to change due to rebase,
trailer updates or similar. If in doubt, please check the listed branch.

tree:   https://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs.git
branch: vfs-7.5.misc

[1/2] fs/epoll: mark epoll_wait tasks TASK_FREEZABLE
      https://git.kernel.org/vfs/vfs/c/5f2199f2720c
[2/2] fs/select: mark select and poll tasks TASK_FREEZABLE
      https://git.kernel.org/vfs/vfs/c/33ef4cf97c32