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Shenoy" , linux-kernel@vger.kernel.org References: <20220619120451.95251-1-wuyun.abel@bytedance.com> <20220619120451.95251-8-wuyun.abel@bytedance.com> From: Abel Wu In-Reply-To: <20220619120451.95251-8-wuyun.abel@bytedance.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 6/19/22 8:04 PM, Abel Wu Wrote: > Now when updating core state, there are two main problems that could > pollute the SIS filter: > > - The updating is before task migration, so if dst_cpu is > selected to be propagated which might be fed with tasks > soon, the efforts we paid is no more than setting a busy > cpu into the SIS filter. While on the other hand it is > important that we update as early as possible to keep the > filter fresh, so it's not wise to delay the update to the > end of load balancing. > > - False negative propagation hurts performance since some > idle cpus could be out of reach. So in general we will > aggressively propagate idle cpus but allow false positive > continue to exist for a while, which may lead to filter > being fully polluted. > > Pains can be relieved by a force correction when false positive > continuously detected. > > Signed-off-by: Abel Wu > --- > include/linux/sched/topology.h | 7 +++++ > kernel/sched/fair.c | 51 ++++++++++++++++++++++++++++++++-- > 2 files changed, 55 insertions(+), 3 deletions(-) > > diff --git a/include/linux/sched/topology.h b/include/linux/sched/topology.h > index b93edf587d84..e3552ce192a9 100644 > --- a/include/linux/sched/topology.h > +++ b/include/linux/sched/topology.h > @@ -91,6 +91,12 @@ struct sched_group; > * search, and is also used as a fallback state of the other > * states. > * > + * - sd_may_idle > + * This state implies the unstableness of the SIS filter, and > + * some bits of it may out of date. This state is only used in > + * SMT domains as an intermediate state between sd_has_icpus > + * and sd_is_busy. > + * > * - sd_is_busy > * This state indicates there are no unoccupied cpus in this > * domain. So for LLC domains, it gives the hint on whether > @@ -111,6 +117,7 @@ struct sched_group; > enum sd_state { > sd_has_icores, > sd_has_icpus, > + sd_may_idle, > sd_is_busy > }; > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index d55fdcedf2c0..9713d183d35e 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -8768,6 +8768,7 @@ static inline void update_sg_lb_stats(struct lb_env *env, > > for_each_cpu_and(i, sched_group_span(group), env->cpus) { > struct rq *rq = cpu_rq(i); > + bool update = update_core && (env->dst_cpu != i); > > sgs->group_load += cpu_load(rq); > sgs->group_util += cpu_util_cfs(i); > @@ -8777,7 +8778,11 @@ 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) > + /* > + * The dst_cpu is not preferred since it might > + * be fed with tasks soon. > + */ > + if (update) > sd_classify(sds, rq, i); > > if (nr_running > 1) > @@ -8801,7 +8806,8 @@ static inline void update_sg_lb_stats(struct lb_env *env, > * and fed with tasks, so we'd better choose > * a candidate in an opposite way. > */ > - sds->idle_cpu = i; > + if (update) > + sds->idle_cpu = i; > sgs->idle_cpus++; > > /* Idle cpu can't have misfit task */ > @@ -9321,7 +9327,7 @@ static void sd_update_state(struct lb_env *env, struct sd_lb_stats *sds) > { > struct sched_domain_shared *sd_smt_shared = env->sd->shared; > enum sd_state new = sds->sd_state; > - int this = env->dst_cpu; > + int icpu = sds->idle_cpu, this = env->dst_cpu; > > /* > * Parallel updating can hardly contribute accuracy to > @@ -9331,6 +9337,22 @@ static void sd_update_state(struct lb_env *env, struct sd_lb_stats *sds) > if (cmpxchg(&sd_smt_shared->updating, 0, 1)) > return; > > + /* > + * The dst_cpu is likely to be fed with tasks soon. > + * If it is the only unoccupied cpu in this domain, > + * we still handle it the same way as as_has_icpus > + * but turn the SMT into the unstable state, rather > + * than waiting to the end of load balancing since > + * it's also important that update the filter as > + * early as possible to keep it fresh. > + */ > + if (new == sd_is_busy) { > + if (idle_cpu(this) || sched_idle_cpu(this)) { > + new = sd_may_idle; > + icpu = this; > + } > + } > + > /* > * There is at least one unoccupied cpu available, so > * propagate it to the filter to avoid false negative > @@ -9338,6 +9360,12 @@ static void sd_update_state(struct lb_env *env, struct sd_lb_stats *sds) > * idle cpus thus throughupt downgraded. > */ > if (new != sd_is_busy) { > + /* > + * The sd_may_idle state is taken into > + * consideration as well because from > + * here we couldn't actually know task > + * migrations would happen or not. > + */ > if (!test_idle_cpus(this)) > set_idle_cpus(this, true); > } else { > @@ -9347,9 +9375,26 @@ static void sd_update_state(struct lb_env *env, struct sd_lb_stats *sds) > */ > if (sd_smt_shared->state == sd_is_busy) > goto out; > + > + /* > + * Allow false positive to exist for some time > + * to make a tradeoff of accuracy of the filter > + * for relieving cache traffic. > + */ > + if (sd_smt_shared->state == sd_has_icpus) { > + new = sd_may_idle; > + goto update; > + } > + > + /* > + * If the false positive issue has already been > + * there for a while, a correction of the filter > + * is needed. > + */ > } > > sd_update_icpus(this, sds->idle_cpu); The @icpu should be used here rather than 'sds->idle_cpu'.. Will be fixed in next version. > +update: > sd_smt_shared->state = new; > out: > xchg(&sd_smt_shared->updating, 0);