linux-kernel - Re: sys_epoll_wait high CPU load in 2.6.37

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Message-ID: <1296067537.2899.153.camel@edumazet-laptop>
Date:	Wed, 26 Jan 2011 19:45:37 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Davide Libenzi <davidel@...ilserver.org>
Cc:	Shawn Bohrer <shawn.bohrer@...il.com>,
	Simon Kirby <sim@...tway.ca>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: sys_epoll_wait high CPU load in 2.6.37

Le mercredi 26 janvier 2011 à 19:18 +0100, Eric Dumazet a écrit :
> Le mercredi 26 janvier 2011 à 10:06 -0800, Davide Libenzi a écrit :
> 
> > Eric, if you look at fs/select.c (~line 925), poll does exactly the same 
> > thing as epoll do.
> > It too, ignores the eventual return value of poll_select_set_timeout(), so 
> > maybe a little bit more optimized ktime_get_ts+timespec_add_ms could make 
> > sense.
> > 
> 
> OK, I'll post a V3 ;)

Well in the poll() case we handle a zero timeout, not in epoll().

So the helper function cannot be shared and can be static to epoll.


[PATCH v3] epoll: epoll_wait() should not use timespec_add_ns()

commit 95aac7b1cd224f (epoll: make epoll_wait() use the hrtimer range
feature) added a performance regression because it uses
timespec_add_ns() with potential very large 'ns' values.

Reported-by: Simon Kirby <sim@...tway.ca>
Signed-off-by: Eric Dumazet <eric.dumazet@...il.com>
CC: Shawn Bohrer <shawn.bohrer@...il.com>
CC: Davide Libenzi <davidel@...ilserver.org>
CC: Thomas Gleixner <tglx@...utronix.de>
CC: Andrew Morton <akpm@...ux-foundation.org>
---
 fs/eventpoll.c |   16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index cc8a9b7..d517aa3 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -1114,6 +1114,17 @@ static int ep_send_events(struct eventpoll *ep,
 	return ep_scan_ready_list(ep, ep_send_events_proc, &esed);
 }
 
+static inline struct timespec epoll_set_mstimeout(long ms)
+{
+	struct timespec now, ts = {
+		.tv_sec = ms / MSEC_PER_SEC,
+		.tv_nsec = NSEC_PER_MSEC * (ms % MSEC_PER_SEC),
+	};
+
+	ktime_get_ts(&now);
+	return timespec_add_safe(now, ts);
+}
+
 static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events,
 		   int maxevents, long timeout)
 {
@@ -1121,12 +1132,11 @@ static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events,
 	unsigned long flags;
 	long slack;
 	wait_queue_t wait;
-	struct timespec end_time;
 	ktime_t expires, *to = NULL;
 
 	if (timeout > 0) {
-		ktime_get_ts(&end_time);
-		timespec_add_ns(&end_time, (u64)timeout * NSEC_PER_MSEC);
+		struct timespec end_time = epoll_set_mstimeout(timeout);
+
 		slack = select_estimate_accuracy(&end_time);
 		to = &expires;
 		*to = timespec_to_ktime(end_time);


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