Messages in this thread | | | Date | Tue, 8 Feb 2022 14:24:08 +0800 | From | Chen Yu <> | Subject | Re: [PATCH][RFC] sched: Stop searching for idle cpu if the LLC domain is overloaded |
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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
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