Re: [PATCH 1/2] sched: Drop the need_resched() loop from cond_resched()

Frederic Weisbecker
2009-07-10T16:26:33Z

On Fri, Jul 10, 2009 at 06:11:41PM +0200, Ingo Molnar wrote:
>
> * Frederic Weisbecker <fweisbec@gmail.com> wrote:
>
> > On Fri, Jul 10, 2009 at 05:35:29PM +0200, Arnd Bergmann wrote:
> > > On Friday 10 July 2009, Frederic Weisbecker wrote:
> > > > On Fri, Jul 10, 2009 at 05:17:38PM +0200, Arnd Bergmann wrote:
> > > > > On Friday 10 July 2009, Frederic Weisbecker wrote:
> > > > > > --- a/kernel/sched.c
> > > > > > +++ b/kernel/sched.c
> > > > > > @@ -6613,11 +6613,9 @@ static void __cond_resched(void)
> > > > > > * PREEMPT_ACTIVE, which could trigger a second
> > > > > > * cond_resched() call.
> > > > > > */
> > > > > > - do {
> > > > > > - add_preempt_count(PREEMPT_ACTIVE);
> > > > > > - schedule();
> > > > > > - sub_preempt_count(PREEMPT_ACTIVE);
> > > > > > - } while (need_resched());
> > > > > > + add_preempt_count(PREEMPT_ACTIVE);
> > > > > > + schedule();
> > > > > > + sub_preempt_count(PREEMPT_ACTIVE);
> > > > > > }
> > > > > >
> > > > >
> > > > > If you drop the loop, then you should also remove the comment that
> > > > > explains why it was put there.
> > > > >
> > > >
> > > > Hmm, these comments seem to actually explain why we do the PREEMPT_ACTIVE
> > > > trick, which is to prevent from cond_resched() recursion, right?
> > > >
> > >
> > > I think we both misinterpreted the comment, which seemed to refer
> > > to older code added by Ingo in 5bbcfd900 "cond_resched(): fix bogus
> > > might_sleep() warning" and removed by Andrew in e7b384043e2
> > > "cond_resched() fix".
> > >
> > > The original code in Ingos version looked like
> > >
> > > static inline void __cond_resched(void)
> > > {
> > > /*
> > > * The BKS might be reacquired before we have dropped
> > > * PREEMPT_ACTIVE, which could trigger a second
> > > * cond_resched() call.
> > > */
> > > if (unlikely(preempt_count()))
> > > return;
> > > do {
> > > add_preempt_count(PREEMPT_ACTIVE);
> > > schedule();
> > > ...
> > >
> > >
> > > So, it's got nothing to do with the loop, but should still be removed
> > > because the 'if (unlikely(preempt_count()))' is no longer there.
> >
> >
> > Yeah, but the comment still fits the code after this patch, don't
> > you think? :-)
>
> ... except that there's no Big Kernel Semaphore anymore ;-)
>
> Ingo

Ah, I lack some backgrounds about Linux heroic ages :)
I thought it was a mispell of BKL.

I guess the comment should be removed anyway, while reading it
more, it doesn't explain the code that follows it.