linux-kernel - Re: [patch] CFS scheduler, -v19

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Message-Id: <1184779891.4178.56.camel@raven.themaw.net>
Date:	Thu, 19 Jul 2007 01:31:31 +0800
From:	Ian Kent <raven@...maw.net>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Ingo Molnar <mingo@...e.hu>, Chuck Ebbert <cebbert@...hat.com>,
	Bill Davidsen <davidsen@....com>, linux-kernel@...r.kernel.org
Subject: Re: [patch] CFS scheduler, -v19

On Wed, 2007-07-18 at 09:03 -0700, Linus Torvalds wrote:
> 
> On Tue, 17 Jul 2007, Ingo Molnar wrote:
> > 
> > * Ian Kent <raven@...maw.net> wrote:
> > > 
> > > In several places I have code similar to:
> > > 
> > > wait.tv_sec = time(NULL) + 1;
> > > wait.tv_nsec = 0;
> 
> Ok, that definitely should work.
> 
> Does the patch below help?
> 
> > ah! It passes in a low-res time source into a high-res time interface 
> > (pthread_cond_timedwait()). Could you change the time(NULL) + 1 to 
> > time(NULL) + 2, or change it to:
> > 
> > 	gettimeofday(&wait, NULL);
> > 	wait.tv_sec++;
> 
> This is wrong. It's wrong for two reasons:
> 
>  - it really shouldn't be needed. I don't think "time()" has to be 
>    *exactly* in sync, but I don't think it can be off by a third of a 
>    second or whatever (as the "30% CPU load" would seem to imply)
> 
>  - gettimeofday works on a timeval, pthread_cond_timedwait() works on a 
>    timespec.
> 
> So if it actually makes a difference, it makes a difference for the 
> *wrong* reason: the time is still totally nonsensical in the tv_nsec field 
> (because it actually got filled in with msecs!), but now the tv_sec field 
> is in sync, so it hides the bug.

Oh ya .. I thought it wouldn't hurt to add the fraction of the current
second for correctness and actually put things like:

gettimeofday(&now, NULL);
wait.tv_sec = now.tv_sec + 1;
wait.tv_nsec = now.tv_usec * 1000;

in autofs.

Ian


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