RE: [PATCH] Prevent immediate process rescheduling

[Langsdorf, Mark]

> -----Original Message-----
> From: Ingo Molnar [mailto:mingo@xxxxxxx] 
> Sent: Friday, September 18, 2009 2:55 PM
> To: Langsdorf, Mark; Peter Zijlstra; Mike Galbraith
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Subject: Re: [PATCH] Prevent immediate process rescheduling
> 
> 
> (fixed the Cc: lines)
> 
> * Mark Langsdorf <mark.langsdorf@xxxxxxx> wrote:
> 
> > Prevent the scheduler from immediately rescheduling a process
> > that just yielded if there is another process available.
> > 
> > Originally suggested by Mike Galbraith (efault@xxxxxx).
> > 
> > Signed-off-by: Mark Langsdorf <mark.langsdorf@xxxxxxx>
> > ---
> >  kernel/sched_fair.c |   16 +++++++++++++++-
> >  1 files changed, 15 insertions(+), 1 deletions(-)
> > 
> > diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
> > index 652e8bd..4fad08f 100644
> > --- a/kernel/sched_fair.c
> > +++ b/kernel/sched_fair.c
> > @@ -353,11 +353,25 @@ static void __dequeue_entity(struct 
> cfs_rq *cfs_rq, struct sched_entity *se)
> >  static struct sched_entity *__pick_next_entity(struct 
> cfs_rq *cfs_rq)
> >  {
> >  	struct rb_node *left = cfs_rq->rb_leftmost;
> > +	struct sched_entity *se, *curr;
> >  
> >  	if (!left)
> >  		return NULL;
> >  
> > -	return rb_entry(left, struct sched_entity, run_node);
> > +	se = rb_entry(left, struct sched_entity, run_node);
> > +	curr = &current->se;
> > +
> > +	/* 
> > +	 * Don't select the entity who just tried to schedule away
> > +	 * if there's another entity available.
> > +	 */
> > +	if (unlikely(se == curr && cfs_rq->nr_running > 1)) {
> > +		struct rb_node *next_node = rb_next(&curr->run_node);
> > +		if (next_node)
> > +			se = rb_entry(next_node, struct 
> sched_entity, run_node);
> > +	}
> > +
> > +	return se;
> >  }
> 
> I suspect some real workload is the motivation of this - what is that 
> workload?

Thanks for fixing the cc lines.

The next patch I submitted enables the Pause Filter feature
from the most recent AMD Operton processors.  Pause Filter
lets us detect when a KVM guest VCPU is spinning useless on
a contended lock because the lockholder VCPU isn't scheduled.
Ideally, we'd like to switch to that VCPU thread, but at a
minimum, we'd like to meaningful yield the contending VCPU.
This patch prevents schedule() from just rescheduling the
thread that is trying to yield, and didn't have any
performance regressions in my tests.

-Mark Langsdorf
Operating System Research Center
AMD
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