linux-kernel - Re: [PATCH 1/2] sched: Drop the need_resched() loop from cond

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Date:	Fri, 10 Jul 2009 18:11:41 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Arnd Bergmann <arnd@...db.de>, LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 1/2] sched: Drop the need_resched() loop from
	cond_resched()


* Frederic Weisbecker <fweisbec@...il.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
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