linux-kernel - Re: [PATCH 2/2] fs/epoll: deal with wait

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Date:   Wed, 14 Nov 2018 14:51:19 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Davidlohr Bueso <dave@...olabs.net>
Cc:     jbaron@...mai.com, viro@...iv.linux.org.uk,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Davidlohr Bueso <dbueso@...e.de>
Subject: Re: [PATCH 2/2] fs/epoll: deal with wait_queue only once

On Wed, 14 Nov 2018 10:25:32 -0800 Davidlohr Bueso <dave@...olabs.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 (;;) {
 		/*
_