Messages in this thread Patch in this message | | | From | Ricardo Neri <> | Subject | [PATCH v5 6/6] sched/fair: Consider SMT in ASYM_PACKING load balance | Date | Fri, 10 Sep 2021 18:18:19 -0700 |
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When deciding to pull tasks in ASYM_PACKING, it is necessary not only to check for the idle state of the destination CPU, dst_cpu, but also of its SMT siblings.
If dst_cpu is idle but its SMT siblings are busy, performance suffers if it pulls tasks from a medium priority CPU that does not have SMT siblings.
Implement asym_smt_can_pull_tasks() to inspect the state of the SMT siblings of both dst_cpu and the CPUs in the candidate busiest group.
Cc: Aubrey Li <aubrey.li@intel.com> Cc: Ben Segall <bsegall@google.com> Cc: Daniel Bristot de Oliveira <bristot@redhat.com> Cc: Dietmar Eggemann <dietmar.eggemann@arm.com> Cc: Mel Gorman <mgorman@suse.de> Cc: Quentin Perret <qperret@google.com> Cc: Rafael J. Wysocki <rafael.j.wysocki@intel.com> Cc: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com> Cc: Steven Rostedt <rostedt@goodmis.org> Cc: Tim Chen <tim.c.chen@linux.intel.com> Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org> Reviewed-by: Len Brown <len.brown@intel.com> Signed-off-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com> --- Changes since v4: * Use sg_lb_stats::sum_nr_running the idle state of a scheduling group. (Vincent, Peter) * Do not even idle CPUs in asym_smt_can_pull_tasks(). (Vincent) * Updated function documentation and corrected a typo.
Changes since v3: * Removed the arch_asym_check_smt_siblings() hook. Discussions with the powerpc folks showed that this patch should not impact them. Also, more recent powerpc processor no longer use asym_packing. (PeterZ) * Removed unnecessary local variable in asym_can_pull_tasks(). (Dietmar) * Removed unnecessary check for local CPUs when the local group has zero utilization. (Joel) * Renamed asym_can_pull_tasks() as asym_smt_can_pull_tasks() to reflect the fact that it deals with SMT cases. * Made asym_smt_can_pull_tasks() return false for !CONFIG_SCHED_SMT so that callers can deal with non-SMT cases.
Changes since v2: * Reworded the commit message to reflect updates in code. * Corrected misrepresentation of dst_cpu as the CPU doing the load balancing. (PeterZ) * Removed call to arch_asym_check_smt_siblings() as it is now called in sched_asym().
Changes since v1: * Don't bailout in update_sd_pick_busiest() if dst_cpu cannot pull tasks. Instead, reclassify the candidate busiest group, as it may still be selected. (PeterZ) * Avoid an expensive and unnecessary call to cpumask_weight() when determining if a sched_group is comprised of SMT siblings. (PeterZ). --- kernel/sched/fair.c | 94 +++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 94 insertions(+)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 26db017c14a3..8d763dd0174b 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -8597,10 +8597,98 @@ group_type group_classify(unsigned int imbalance_pct, return group_has_spare; } +/** + * asym_smt_can_pull_tasks - Check whether the load balancing CPU can pull tasks + * @dst_cpu: Destination CPU of the load balancing + * @sds: Load-balancing data with statistics of the local group + * @sgs: Load-balancing statistics of the candidate busiest group + * @sg: The candidate busiest group + * + * Check the state of the SMT siblings of both @sds::local and @sg and decide + * if @dst_cpu can pull tasks. + * + * If @dst_cpu does not have SMT siblings, it can pull tasks if two or more of + * the SMT siblings of @sg are busy. If only one CPU in @sg is busy, pull tasks + * only if @dst_cpu has higher priority. + * + * If both @dst_cpu and @sg have SMT siblings, and @sg has exactly one more + * busy CPU than @sds::local, let @dst_cpu pull tasks if it has higher priority. + * Bigger imbalances in the number of busy CPUs will be dealt with in + * update_sd_pick_busiest(). + * + * If @sg does not have SMT siblings, only pull tasks if all of the SMT siblings + * of @dst_cpu are idle and @sg has lower priority. + */ +static bool asym_smt_can_pull_tasks(int dst_cpu, struct sd_lb_stats *sds, + struct sg_lb_stats *sgs, + struct sched_group *sg) +{ +#ifdef CONFIG_SCHED_SMT + bool local_is_smt, sg_is_smt; + int sg_busy_cpus; + + local_is_smt = sds->local->flags & SD_SHARE_CPUCAPACITY; + sg_is_smt = sg->flags & SD_SHARE_CPUCAPACITY; + + sg_busy_cpus = sgs->group_weight - sgs->idle_cpus; + + if (!local_is_smt) { + /* + * If we are here, @dst_cpu is idle and does not have SMT + * siblings. Pull tasks if candidate group has two or more + * busy CPUs. + */ + if (sg_is_smt && sg_busy_cpus >= 2) + return true; + + /* + * @dst_cpu does not have SMT siblings. @sg may have SMT + * siblings and only one is busy. In such case, @dst_cpu + * can help if it has higher priority and is idle (i.e., + * it has no running tasks). + */ + return !sds->local_stat.sum_nr_running && + sched_asym_prefer(dst_cpu, sg->asym_prefer_cpu); + } + + /* @dst_cpu has SMT siblings. */ + + if (sg_is_smt) { + int local_busy_cpus = sds->local->group_weight - + sds->local_stat.idle_cpus; + int busy_cpus_delta = sg_busy_cpus - local_busy_cpus; + + if (busy_cpus_delta == 1) + return sched_asym_prefer(dst_cpu, + sg->asym_prefer_cpu); + + return false; + } + + /* + * @sg does not have SMT siblings. Ensure that @sds::local does not end + * up with more than one busy SMT sibling and only pull tasks if there + * are not busy CPUs (i.e., no CPU has running tasks). + */ + if (!sds->local_stat.sum_nr_running) + return sched_asym_prefer(dst_cpu, sg->asym_prefer_cpu); + + return false; +#else + /* Always return false so that callers deal with non-SMT cases. */ + return false; +#endif +} + static inline bool sched_asym(struct lb_env *env, struct sd_lb_stats *sds, struct sg_lb_stats *sgs, struct sched_group *group) { + /* Only do SMT checks if either local or candidate have SMT siblings */ + if ((sds->local->flags & SD_SHARE_CPUCAPACITY) || + (group->flags & SD_SHARE_CPUCAPACITY)) + return asym_smt_can_pull_tasks(env->dst_cpu, sds, sgs, group); + return sched_asym_prefer(env->dst_cpu, group->asym_prefer_cpu); } @@ -9606,6 +9694,12 @@ static struct rq *find_busiest_queue(struct lb_env *env, nr_running == 1) continue; + /* Make sure we only pull tasks from a CPU of lower priority */ + if ((env->sd->flags & SD_ASYM_PACKING) && + sched_asym_prefer(i, env->dst_cpu) && + nr_running == 1) + continue; + switch (env->migration_type) { case migrate_load: /* -- 2.17.1
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