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

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Date:	Fri, 10 Jul 2009 17:35:29 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Frederic Weisbecker <fweisbec@...il.com>
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 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.

	Arnd <><
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