Re: [patch] CFS scheduler, -v19

Ian Kent
2007-07-18T17:31:51Z

On Wed, 2007-07-18 at 09:03 -0700, Linus Torvalds wrote:
>
> On Tue, 17 Jul 2007, Ingo Molnar wrote:
> >
> > * Ian Kent <raven@themaw.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