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

[Sadrul Chowdhury <sadrul@google.com>]

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%.

Signed-off-by: Sadrul Chowdhury <sadrul@google.com>
---
 fs/eventpoll.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index 6cbef8b4c2e0..35992f6457bf 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -39,6 +39,7 @@
 #include <linux/rculist.h>
 #include <linux/capability.h>
 #include <net/busy_poll.h>
+#include <linux/freezer.h>
 
 #include <trace/hooks/fs.h>
 
@@ -1987,7 +1988,7 @@ 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()
-- 
2.55.0.1082.g2b9226bbc0-goog