Re: [PATCH][RFC] sched: Stop searching for idle cpu if the LLC domain is overloaded

[Chen Yu]

On Mon, Feb 07, 2022 at 02:52:53PM +0100, Peter Zijlstra wrote:
> On Mon, Feb 07, 2022 at 11:40:13AM +0800, Chen Yu wrote:
> > It would be ideal to have a crystal ball to predict the success rate
> > of finding an idle cpu/core in the LLC. If it is doomed to fail,
> > there is no need to search in the LLC domain. There are many potential
> > metrics which could be used to predict the success rate. And the
> > metric should be carefully chosen that, it should help reduce the
> > unnecessary cpu runqueue scan, but meanwhile not give up the opportunity
> > to find an idle cpu.
> > 
> > Choose average cpu utilization as the candidate, since the util_avg is
> > a metric of accumulated historic activity, which seems to be more accurate
> > than instantaneous metrics(such as rq->nr_running) on calculating the probability
> > of find an idle cpu. Only when the average cpu utilization has reaches
> > 85% of the total cpu capacity, this domain is regarded as overloaded.
> > The reason to choose 85% is that, this is the threshold of an overloaded
> > LLC sched group(imbalance_pct = 117, threshold = 100 / 117 * 100%).
> 
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 5146163bfabb..1a58befe892d 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> 
> > @@ -6280,6 +6281,10 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
> >  	if (!this_sd)
> >  		return -1;
> >  
> > +	sd_share = rcu_dereference(per_cpu(sd_llc_shared, target));
> > +	if (sd_share && READ_ONCE(sd_share->overloaded))
> > +		return -1;
> > +
> >  	cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr);
> >  
> >  	if (sched_feat(SIS_PROP) && !has_idle_core) {
> 
> > @@ -9268,6 +9275,29 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd
> >  		WRITE_ONCE(rd->overutilized, SG_OVERUTILIZED);
> >  		trace_sched_overutilized_tp(rd, SG_OVERUTILIZED);
> >  	}
> > +
> > +	/*
> > +	 * Check if the LLC domain is overloaded. The overload hint
> > +	 * could be used to skip the LLC domain idle cpu search in
> > +	 * select_idle_cpu(). The update of this hint occurs during
> > +	 * periodic load balancing, rather than frequent newidle balance.
> > +	 */
> > +	if (env->idle != CPU_NEWLY_IDLE &&
> > +	    env->sd->span_weight == per_cpu(sd_llc_size, env->dst_cpu)) {
> > +		struct sched_domain_shared *sd_share =
> > +			rcu_dereference(per_cpu(sd_llc_shared, env->dst_cpu));
> > +
> > +		if (!sd_share)
> > +			return;
> > +
> > +		/*
> > +		 * Derived from group_is_overloaded(). The default imbalance_pct
> > +		 * is 117 on LLC domain, which means the threshold of average
> > +		 * utilization is 85%.
> > +		 */
> > +		WRITE_ONCE(sd_share->overloaded, (sds->total_capacity * 100) <
> > +			   (sum_util * env->sd->imbalance_pct));
> > +	}
> >  }
> 
> So the only problem I have with this is that this is somewhat of a
> binary toggle. The moment we hit that magical 85% we suddenly change
> behaviour.
> 
> Would it not be possible to replace the SIS_PROP logic with something
> based on this sum_util metric? Such that when sum_util is very low we
> scan more, while when sum_util hits 85% we naturally stop scanning
> entirely.
> 
> That way the behaviour is gradual.
Ok, let me do some evaluation on this, maybe something like nr_to_scan = f(sum_util)

thanks,
Chenyu