Re: [Xen-devel] [PATCH v3 35/47] xen/sched: make vcpu_wake() and vcpu_sleep() core scheduling aware

[Jürgen Groß <jgross@xxxxxxxx>]

On 24.09.19 13:55, Jan Beulich wrote:
> On 14.09.2019 10:52, Juergen Gross wrote:
> > --- a/xen/common/schedule.c
> > +++ b/xen/common/schedule.c
> > @@ -724,8 +724,10 @@ void sched_destroy_domain(struct domain *d)
> >       }
> >   }
> >   
> > -void vcpu_sleep_nosync_locked(struct vcpu *v)
> > +static void vcpu_sleep_nosync_locked(struct vcpu *v)
> >   {
> > +    struct sched_unit *unit = v->sched_unit;
> > +
> >       ASSERT(spin_is_locked(get_sched_res(v->processor)->schedule_lock));
> >   
> >       if ( likely(!vcpu_runnable(v)) )
> > @@ -733,7 +735,14 @@ void vcpu_sleep_nosync_locked(struct vcpu *v)
> >           if ( v->runstate.state == RUNSTATE_runnable )
> >               vcpu_runstate_change(v, RUNSTATE_offline, NOW());
> >   
> > -        sched_sleep(vcpu_scheduler(v), v->sched_unit);
> > +        if ( likely(!unit_runnable(unit)) )
> > +            sched_sleep(vcpu_scheduler(v), unit);
> unit_scheduler(unit) (also elsewhere)?
Yes.

> > @@ -765,16 +774,22 @@ void vcpu_wake(struct vcpu *v)
> >   {
> >       unsigned long flags;
> >       spinlock_t *lock;
> > +    struct sched_unit *unit = v->sched_unit;
> >   
> >       TRACE_2D(TRC_SCHED_WAKE, v->domain->domain_id, v->vcpu_id);
> >   
> > -    lock = unit_schedule_lock_irqsave(v->sched_unit, &flags);
> > +    lock = unit_schedule_lock_irqsave(unit, &flags);
> >   
> >       if ( likely(vcpu_runnable(v)) )
> >       {
> >           if ( v->runstate.state >= RUNSTATE_blocked )
> >               vcpu_runstate_change(v, RUNSTATE_runnable, NOW());
> > -        sched_wake(vcpu_scheduler(v), v->sched_unit);
> > +        sched_wake(vcpu_scheduler(v), unit);
> Is this correct / necessary when the unit is not asleep as a whole?
> After all the corresponding sched_sleep() further up is called
> conditionally only.
Oh, indeed. Will change that.

> > @@ -1998,6 +2013,62 @@ static void sched_context_switch(struct vcpu *vprev, 
> > struct vcpu *vnext,
> >       context_switch(vprev, vnext);
> >   }
> >   
> > +/*
> > + * Force a context switch of a single vcpu of an unit.
> > + * Might be called either if a vcpu of an already running unit is woken up
> > + * or if a vcpu of a running unit is put asleep with other vcpus of the same
> > + * unit still running.
> > + */
> > +static struct vcpu *sched_force_context_switch(struct vcpu *vprev,
> > +                                               struct vcpu *v,
> > +                                               int cpu, s_time_t now)
> unsigned int cpu? (Aiui it's suppose to equal smp_processor_id()
> anyway.)
Yes and yes.

> > +{
> > +    v->force_context_switch = false;
> > +
> > +    if ( vcpu_runnable(v) == v->is_running )
> > +        return NULL;
> This and other NULL returns suggest that the comment ahead of the
> function might better state what the return value here is / means.
Okay.

> > +    if ( vcpu_runnable(v) )
> > +    {
> > +        if ( is_idle_vcpu(vprev) )
> > +        {
> > +            vcpu_runstate_change(vprev, RUNSTATE_runnable, now);
> > +            vprev->sched_unit = get_sched_res(cpu)->sched_unit_idle;
> > +        }
> > +        vcpu_runstate_change(v, RUNSTATE_running, now);
> > +    }
> > +    else
> > +    {
> > +        /* Make sure not to switch last vcpu of an unit away. */
> > +        if ( unit_running(v->sched_unit) == 1 )
> > +            return NULL;
> > +
> > +        v->new_state = vcpu_runstate_blocked(v);
> > +        vcpu_runstate_change(v, v->new_state, now);
> > +        v = sched_unit2vcpu_cpu(vprev->sched_unit, cpu);
> > +        if ( v != vprev )
> > +        {
> > +            if ( is_idle_vcpu(vprev) )
> > +            {
> > +                vcpu_runstate_change(vprev, RUNSTATE_runnable, now);
> > +                vprev->sched_unit = get_sched_res(cpu)->sched_unit_idle;
> > +            }
> > +            else
> > +            {
> > +                v->sched_unit = vprev->sched_unit;
> > +                vcpu_runstate_change(v, RUNSTATE_running, now);
> > +            }
> > +        }
> > +    }
> > +
> > +    v->is_running = 1;
> Besides this wanting to use "true", how come this is unconditional
> despite the function here being used for both waking and putting to
> sleep of a vCPU?
At that time v is the vcpu which will be running next, so either the
just woken up one, or the idle vcpu. I can add a comment.

> > @@ -2067,9 +2160,29 @@ static void sched_slave(void)
> >   
> >       now = NOW();
> >   
> > +    v = unit2vcpu_cpu(prev, cpu);
> > +    if ( v && v->force_context_switch )
> > +    {
> > +        v = sched_force_context_switch(vprev, v, cpu, now);
> > +
> > +        if ( v )
> > +        {
> > +            pcpu_schedule_unlock_irq(lock, cpu);
> I can't figure what it is that guarantees that this unlock isn't
> going to be followed ...
>
> > +            sched_context_switch(vprev, v, false, now);
> > +        }
> > +
> > +        do_softirq = true;
> > +    }
> > +
> >       if ( !prev->rendezvous_in_cnt )
> >       {
> >           pcpu_schedule_unlock_irq(lock, cpu);
> ... by another unlock here. Or wait - is sched_context_switch()
> (and perhaps other functions involved there) lacking a "noreturn"
> annotation?
Indeed it is. Like context_switch() today. :-)

I'll annotate the functions.

> > --- a/xen/include/xen/sched-if.h
> > +++ b/xen/include/xen/sched-if.h
> > @@ -100,6 +100,11 @@ static inline bool unit_runnable(const struct sched_unit 
> > *unit)
> >       return false;
> >   }
> >   
> > +static inline int vcpu_runstate_blocked(struct vcpu *v)
> const?
Yes.


Juergen

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