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

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Message-ID: <20090710155037.GC5318@nowhere>
Date:	Fri, 10 Jul 2009 17:50:38 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Ingo Molnar <mingo@...e.hu>, 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()

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? :-)

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