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Mon, 27 Oct 2025 14:08:08 +0000 Message-ID: Date: Mon, 27 Oct 2025 22:07:52 +0800 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 02/19] sched/fair: Record per-LLC utilization to guide cache-aware scheduling decisions To: K Prateek Nayak CC: Vincent Guittot , Juri Lelli , Dietmar Eggemann , "Steven Rostedt" , Ben Segall , Mel Gorman , Valentin Schneider , "Madadi Vineeth Reddy" , Hillf Danton , "Shrikanth Hegde" , Jianyong Wu , "Yangyu Chen" , Tingyin Duan , Vern Hao , Len Brown , Aubrey Li , Zhao Liu , Chen Yu , Ingo Molnar , Adam Li , Tim Chen , , Tim Chen , "Peter Zijlstra" , "Gautham R . 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These values guide load balancing on aggregating tasks >> + * to a LLC. >> + */ >> +static void record_sg_llc_stats(struct lb_env *env, >> + struct sg_lb_stats *sgs, >> + struct sched_group *group) >> +{ >> + /* >> + * Find the child domain on env->dst_cpu. This domain >> + * is either the domain that spans this group(if the >> + * group is a local group), or the sibling domain of >> + * this group. >> + */ >> + struct sched_domain *sd = env->sd->child; > > Was this intentionally done to limit the update to sg_llc_stats to the > load balancing period of "sd_llc->parent"? > > Can't this be done with update_idle_cpu_scan()? I believe it is more > frequent, "sds->total_capacity" from caller gives you the equivalent of > "group_capacity", and "group_util" is already calculated as "sum_util". > > Checking "sd_llc->parent" there should be sufficient to check if there > are multiple LLC domains or not. Thoughts? > The original idea was to calculate the statistics for the CPUs within one LLC, and set the tag for that sched group as well as its sg_lb_stats (but not at the sched domain scope). With this flag set in that sched group, we can perform some comparisons in update_sd_pick_busiest() to determine if that sched group has any tasks that need to be moved to other LLC sched groups. If we do this in update_idle_cpu_scan(), might it be a bit late for update_sd_pick_busiest()? thanks, Chenyu >> + struct sched_domain_shared *sd_share; >> + >> + if (!sched_feat(SCHED_CACHE) || env->idle == CPU_NEWLY_IDLE) >> + return; >> + >> + /* only care about sched domains spanning a LLC */ >> + if (sd != rcu_dereference(per_cpu(sd_llc, env->dst_cpu))) >> + return; >> + >> + /* >> + * At this point we know this group spans a LLC domain. >> + * Record the statistic of this group in its corresponding >> + * shared LLC domain. >> + */ >> + sd_share = rcu_dereference(per_cpu(sd_llc_shared, >> + cpumask_first(sched_group_span(group)))); >> + if (!sd_share) >> + return; >> + >> + if (READ_ONCE(sd_share->util_avg) != sgs->group_util) >> + WRITE_ONCE(sd_share->util_avg, sgs->group_util); >> + >> + if (unlikely(READ_ONCE(sd_share->capacity) != sgs->group_capacity)) >> + WRITE_ONCE(sd_share->capacity, sgs->group_capacity); >> +} >