Re: [PATCH v4 5/7] sched/fair: skip SIS domain search if fully busy

[Abel Wu]

Hi Gautham, thanks for your reviewing and sorry for my late reply..

On 7/20/22 11:34 PM, Gautham R. Shenoy Wrote:
> [..snip..]
>
> > @@ -6197,24 +6201,44 @@ static inline int __select_idle_cpu(int cpu, struct task_struct *p)
> >   DEFINE_STATIC_KEY_FALSE(sched_smt_present);
> >   EXPORT_SYMBOL_GPL(sched_smt_present);
> >   
> > -static inline void set_idle_cores(int cpu, int val)
> > +static inline void sd_set_state(int cpu, enum sd_state state)
>
> Nit: We are setting the state of only the LLC domain and not any other
> domain via this function. So should we name it as
> set_llc_state()/get_llc_state() for better readability ?

Makes sense, will rename in next version.

> >   {
> >   	struct sched_domain_shared *sds;
> >   
> >   	sds = rcu_dereference(per_cpu(sd_llc_shared, cpu));
> >   	if (sds)
> > -		WRITE_ONCE(sds->has_idle_cores, val);
> > +		WRITE_ONCE(sds->state, state);
> >   }
> >   
> > -static inline bool test_idle_cores(int cpu)
> > +static inline enum sd_state sd_get_state(int cpu)
> >   {
> >   	struct sched_domain_shared *sds;
> >   
> >   	sds = rcu_dereference(per_cpu(sd_llc_shared, cpu));
> >   	if (sds)
> > -		return READ_ONCE(sds->has_idle_cores);
> > +		return READ_ONCE(sds->state);
> >   
> > -	return false;
> > +	return sd_has_icpus;
> > +}
> > +
> > +static inline void set_idle_cores(int cpu, int idle)
>                                                    ^^^^^
> I agree with Josh. We can use core_idle instead of idle here.

OK, I will make the param more verbose...

> > +{
> > +	sd_set_state(cpu, idle ? sd_has_icores : sd_has_icpus);
> > +}
> > +
> > +static inline bool test_idle_cores(int cpu)
> > +{
> > +	return sd_get_state(cpu) == sd_has_icores;
> > +}
> > +
> > +static inline void set_idle_cpus(int cpu, int idle)

and this one too.

> > +{
> > +	sd_set_state(cpu, idle ? sd_has_icpus : sd_is_busy);
> > +}
> > +
> > +static inline bool test_idle_cpus(int cpu)
> > +{
> > +	return sd_get_state(cpu) != sd_is_busy;
> >   }
> >   
> >   /*
>
> [...]
>
>
> > @@ -8661,6 +8702,12 @@ sched_asym(struct lb_env *env, struct sd_lb_stats *sds,  struct sg_lb_stats *sgs
> >   	return sched_asym_prefer(env->dst_cpu, group->asym_prefer_cpu);
> >   }
> >   
> > +static inline void sd_classify(struct sd_lb_stats *sds, struct rq *rq)
> > +{
> > +	if (sds->sd_state != sd_has_icpus && unoccupied_rq(rq))
>
> Nit: sds->sd_state can either be sd_has_icpus or sd_is_busy. So for
> better readability, we can just use the positive check

For now, yes. But sd_state can be expanded and once that happens, the
positive check could be error prone.

>          if (sds->sd_state == sd_is_busy && unoccupied_rq(rq))
> 	       sds->sd_state  = sd_has_icpus;
>
>
> > +		sds->sd_state = sd_has_icpus;
> > +}
> > +
> >   /**
> >    * update_sg_lb_stats - Update sched_group's statistics for load balancing.
> >    * @env: The load balancing environment.
> > @@ -8675,11 +8722,12 @@ static inline void update_sg_lb_stats(struct lb_env *env,
> >   				      struct sg_lb_stats *sgs,
> >   				      int *sg_status)
> >   {
> > -	int i, nr_running, local_group;
> > +	int i, nr_running, local_group, update_core;
> >   
> >   	memset(sgs, 0, sizeof(*sgs));
> >   
> >   	local_group = group == sds->local;
> > +	update_core = env->sd->flags & SD_SHARE_CPUCAPACITY;
> >   
> >   	for_each_cpu_and(i, sched_group_span(group), env->cpus) {
> >   		struct rq *rq = cpu_rq(i);
> > @@ -8692,6 +8740,9 @@ static inline void update_sg_lb_stats(struct lb_env *env,
> >   		nr_running = rq->nr_running;
> >   		sgs->sum_nr_running += nr_running;
> >   
> > +		if (update_core)
> > +			sd_classify(sds, rq);
> > +
> >   		if (nr_running > 1)
> >   			*sg_status |= SG_OVERLOAD;
> >   
> > @@ -9220,6 +9271,12 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p, int this_cpu)
> >   	return idlest;
> >   }
> >   
> > +static void sd_update_state(struct lb_env *env, struct sd_lb_stats *sds)
> > +{
> > +	if (sds->sd_state == sd_has_icpus && !test_idle_cpus(env->dst_cpu))
> > +		set_idle_cpus(env->dst_cpu, true);
>
> We could enter this if condition when env->dst_cpu is the only idle
> CPU in the SMT domain (which is likely to be the case every time we do
> a NEW_IDLE balance). By the end of this load-balancing round, the
> env->dst_cpu can pull a task from some other CPU and thereby no longer
> remain idle but the LLC state would still be sd_has_icpus.
>
> That would mean that some CPU on this LLC would do a full scan during
> the wakeup only to find no idle CPU and reset the state to
> sd_is_busy. Have you seen instances where this false-positive pattern
> can result in wasteful scan thereby cause a performance degradation?
> Ideally it should not be worse that what we currently have.

Yes, indeed. We will talk about this later in the 7th patch.

> Apart from this, patch looks good to me.

Thanks!

> I would be worth the while to explore if the LLC state can be used
> early on in select_task_rq_fair() to determine if we need to do a
> wake-affine or allow the task to stick to its previous LLC depending
> on which among the previous LLC and the waker's LLC have an idle CPU.

Sounds like a good idea!

Best Regards,
Abel