Re: [Xen-devel] [PATCH] xen: Always ask the scheduler to re-place the vcpu when the affinity changes

[George Dunlap <george.dunlap@xxxxxxxxxxxxx>]

On 04/03/13 12:35, Jan Beulich wrote:
> > > > On 04.03.13 at 13:19, George Dunlap <george.dunlap@xxxxxxxxxxxxx> wrote:
> > It's probably a good idea to re-evaluate placement whenever the
> > affinity changes.
> >
> > This additionally has the benefit of removing scheduler-specific
> > exceptions introduced in git c/s e6a6fd63.
> >
> > Signed-off-by: George Dunlap <george.dunlap@xxxxxxxxxxxxx>
> > ---
> >   xen/common/schedule.c |    7 ++++---
> >   1 file changed, 4 insertions(+), 3 deletions(-)
> >
> > diff --git a/xen/common/schedule.c b/xen/common/schedule.c
> > index de11110..dbef5af 100644
> > --- a/xen/common/schedule.c
> > +++ b/xen/common/schedule.c
> > @@ -613,9 +613,10 @@ int vcpu_set_affinity(struct vcpu *v, const cpumask_t
> > *affinity)
> >       vcpu_schedule_lock_irq(v);
> >   
> >       cpumask_copy(v->cpu_affinity, affinity);
> > -    if ( VCPU2OP(v)->sched_id == XEN_SCHEDULER_SEDF ||
> > -         !cpumask_test_cpu(v->processor, v->cpu_affinity) )
> > -        set_bit(_VPF_migrating, &v->pause_flags);
> > +
> > +    /* Always ask the scheduler to re-evaluate placement
> > +     * when changing the affinity */
> > +    set_bit(_VPF_migrating, &v->pause_flags);
> >   
> >       vcpu_schedule_unlock_irq(v);
> >   
> The code right below the context visible here is
>
>      if ( test_bit(_VPF_migrating, &v->pause_flags) )
>      {
>          vcpu_sleep_nosync(v);
>          vcpu_migrate(v);
>      }
>
> and I think the conditional could (and should) now be removed.
Yeah, I wasn't sure what to make of that one -- it looked as though it 
was coded such that someone else might be able to clear _VPF_migrating 
between releasing the lock and this test.  But if that can happen, it's 
racy anyway.  vcpu_force_reschedule() has this "set, unlock, re-check" 
pattern.
It looks like there actually is a way that it could conceivably be 
cleared: if the vcpu is running on another pcpu, if after we release the 
lock the vcpu is de-scheduled and context_saved() is called, it will 
check for _VPF_migrating and call vcpu_migrate(), which can clear the flag.
But that's probably a rare enough occurrence that it's better overall to 
take the occasional double-migrate.
 -George

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