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

[Jan Kara]

On Tue 22-09-26 21:15:58, 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.
> 
> 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@xxxxxxxxxx>

With similar caveat as on patch 2/2 feel free to add:

Reviewed-by: Jan Kara <jack@xxxxxxx>

								Honza

> ---
>  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
> 
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR