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

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Message-ID: <alpine.DEB.2.00.1101260747320.1889@localhost6.localdomain6>
Date:	Wed, 26 Jan 2011 07:52:23 -0800 (PST)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	Eric Dumazet <eric.dumazet@...il.com>
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>
Subject: Re: sys_epoll_wait high CPU load in 2.6.37

On Wed, 26 Jan 2011, Eric Dumazet wrote:

> Le mercredi 26 janvier 2011 à 07:31 -0800, Davide Libenzi a écrit :
> > On Wed, 26 Jan 2011, Eric Dumazet wrote:
> > 
> 
> > > diff --git a/fs/eventpoll.c b/fs/eventpoll.c
> > > index cc8a9b7..7ec0890 100644
> > > --- a/fs/eventpoll.c
> > > +++ b/fs/eventpoll.c
> > > @@ -1126,7 +1126,9 @@ static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events,
> > >  
> > >  	if (timeout > 0) {
> > >  		ktime_get_ts(&end_time);
> > > -		timespec_add_ns(&end_time, (u64)timeout * NSEC_PER_MSEC);
> > > +		end_time.tv_sec += timeout / MSEC_PER_SEC;
> > > +		timeout %= MSEC_PER_SEC;
> > > +		timespec_add_ns(&end_time, timeout * NSEC_PER_MSEC);
> > >  		slack = select_estimate_accuracy(&end_time);
> > >  		to = &expires;
> > >  		*to = timespec_to_ktime(end_time);
> > 
> > Yep, we can overflow the timeout, with the calculation above.
> > A timespec_add_ms()?
> 
> Well, given timeout after modulo contains a number between 0 and 999,
> multiply by 1.000.000 (NSEC_PER_MSEC) cant overflow.

For "above", I meant the current epoll expire time calculation, which was 
described above in the message ;)
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)?