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

Arnd Bergmann
2009-07-10T15:36:00Z

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.