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SubjectRe: [PATCH 2/2] fs/epoll: deal with wait_queue only once
On Wed, 14 Nov 2018 10:25:32 -0800 Davidlohr Bueso <dave@stgolabs.net> wrote:

> There is no reason why we rearm the waitiqueue upon every
> fetch_events retry (for when events are found yet send_events()
> fails). If nothing else, this saves four lock operations per
> retry, and furthermore reduces the scope of the lock even
> further.
>
> ..
>
> --- a/fs/eventpoll.c
> +++ b/fs/eventpoll.c
> @@ -1749,6 +1749,7 @@ static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events,
> {
> int res = 0, eavail, timed_out = 0;
> u64 slack = 0;
> + bool waiter = false;
> wait_queue_entry_t wait;
> ktime_t expires, *to = NULL;
>
> @@ -1786,6 +1787,15 @@ static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events,
> if (eavail)
> goto send_events;
>
> + if (!waiter) {
> + waiter = true;
> + init_waitqueue_entry(&wait, current);
> +
> + spin_lock_irq(&ep->wq.lock);
> + __add_wait_queue_exclusive(&ep->wq, &wait);
> + spin_unlock_irq(&ep->wq.lock);
> + }
> +
> /*
> * Busy poll timed out. Drop NAPI ID for now, we can add
> * it back in when we have moved a socket with a valid NAPI
> @@ -1798,10 +1808,6 @@ static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events,
> * We need to sleep here, and we will be wake up by
> * ep_poll_callback() when events will become available.
> */
> - init_waitqueue_entry(&wait, current);
> - spin_lock_irq(&ep->wq.lock);
> - __add_wait_queue_exclusive(&ep->wq, &wait);
> - spin_unlock_irq(&ep->wq.lock);

Why was this moved to before the ep_reset_busy_poll_napi_id() call?
That movement placed the code ahead of the block comment which serves
to explain its function.


This? Which also fixes that comment and reflows it to use 80 cols.

--- a/fs/eventpoll.c~fs-epoll-deal-with-wait_queue-only-once-fix
+++ a/fs/eventpoll.c
@@ -1787,15 +1787,6 @@ fetch_events:
if (eavail)
goto send_events;

- if (!waiter) {
- waiter = true;
- init_waitqueue_entry(&wait, current);
-
- spin_lock_irq(&ep->wq.lock);
- __add_wait_queue_exclusive(&ep->wq, &wait);
- spin_unlock_irq(&ep->wq.lock);
- }
-
/*
* Busy poll timed out. Drop NAPI ID for now, we can add
* it back in when we have moved a socket with a valid NAPI
@@ -1804,10 +1795,18 @@ fetch_events:
ep_reset_busy_poll_napi_id(ep);

/*
- * We don't have any available event to return to the caller.
- * We need to sleep here, and we will be wake up by
- * ep_poll_callback() when events will become available.
+ * We don't have any available event to return to the caller. We need
+ * to sleep here, and we will be woken by ep_poll_callback() when events
+ * become available.
*/
+ if (!waiter) {
+ waiter = true;
+ init_waitqueue_entry(&wait, current);
+
+ spin_lock_irq(&ep->wq.lock);
+ __add_wait_queue_exclusive(&ep->wq, &wait);
+ spin_unlock_irq(&ep->wq.lock);
+ }

for (;;) {
/*
_
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