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

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Date:	Wed, 26 Jan 2011 10:06:28 -0800 (PST)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	Eric Dumazet <eric.dumazet@...il.com>
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

On Wed, 26 Jan 2011, Eric Dumazet wrote:

> > > --- 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);
> > 
> > poll_select_set_timeout() jumps through some extra hoops that
> > aren't necessary in the epoll case so I actually like your previous
> > patch better.
> 
> Well, I dont care, I let Davide decide, he is the boss ;)
> 
> This is a stable candidate, so adding timespec_add_ms() sounds overkill.

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.


- Davide


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