Re: [Xen-devel] [PATCH v2 08/48] xen/sched: switch vcpu_schedule_lock to unit_schedule_lock

[Juergen Gross <jgross@xxxxxxxx>]

On 04.09.19 16:02, Jan Beulich wrote:
> On 09.08.2019 16:57, Juergen Gross wrote:
> > --- a/xen/common/schedule.c
> > +++ b/xen/common/schedule.c
> > @@ -250,7 +250,8 @@ static inline void vcpu_runstate_change(
> >   
> >   void vcpu_runstate_get(struct vcpu *v, struct vcpu_runstate_info *runstate)
> >   {
> > -    spinlock_t *lock = likely(v == current) ? NULL : vcpu_schedule_lock_irq(v);
> > +    spinlock_t *lock = likely(v == current)
> > +                       ? NULL : unit_schedule_lock_irq(v->sched_unit);
> >       s_time_t delta;
> >   
> >       memcpy(runstate, &v->runstate, sizeof(*runstate));
> > @@ -259,7 +260,7 @@ void vcpu_runstate_get(struct vcpu *v, struct 
> > vcpu_runstate_info *runstate)
> >           runstate->time[runstate->state] += delta;
> >   
> >       if ( unlikely(lock != NULL) )
> > -        vcpu_schedule_unlock_irq(lock, v);
> > +        unit_schedule_unlock_irq(lock, v->sched_unit);
> >   }
> At the example of this: The more coarse granularity of the lock
> means that no two vCPU-s within a unit can obtain their runstate
> in parallel. While this may be acceptable for core scheduling,
> I'm afraid it's too restrictive for sockets or nodes as units.
> Therefore I think this lock needs to either be split (I'm not
> sure that's feasible) or become an r/w lock.
You are aware that even today with credit2 all cpus of a socket share
the same lock (if not modified via boot parameter)?


Juergen


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