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

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Message-ID: <1296058409.2899.81.camel@edumazet-laptop>
Date:	Wed, 26 Jan 2011 17:13:29 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Davide Libenzi <davidel@...ilserver.org>
Cc:	Simon Kirby <sim@...tway.ca>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Shawn Bohrer <shawn.bohrer@...il.com>,
	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 16:59 +0100, Eric Dumazet a écrit :
> Le mercredi 26 janvier 2011 à 07:52 -0800, Davide Libenzi a écrit :
> 
> > For "above", I meant the current epoll expire time calculation, which was 
> > described above in the message ;)
> 
> Well, problem was not an overflow, but doing a loop 2.000.000 times ;)
> 
> > The hint for a timespec_add_ms() was because we must be doing something 
> > similar in poll, don't we (/me got no code in front ATM)?
> 
> Apparently its done differently in poll(), using
> poll_select_set_timeout() helper.
> 
> 
> Give me some minutes I'll try to cook an alternate patch
> 

Here is the alternate patch, using poll_select_set_timeout() helper

Thanks

[PATCH v2] 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.

Use poll_select_set_timeout() helper like poll()/select()

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 |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index cc8a9b7..94d887b 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -1125,8 +1125,8 @@ static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events,
 	ktime_t expires, *to = NULL;
 
 	if (timeout > 0) {
-		ktime_get_ts(&end_time);
-		timespec_add_ns(&end_time, (u64)timeout * NSEC_PER_MSEC);
+		poll_select_set_timeout(&end_time, timeout / MSEC_PER_SEC,
+				NSEC_PER_MSEC * (timeout % MSEC_PER_SEC));
 		slack = select_estimate_accuracy(&end_time);
 		to = &expires;
 		*to = timespec_to_ktime(end_time);


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