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Date: Mon, 27 Jun 2022 17:28:34 -0700 From: Josh Don <joshdon@...gle.com> To: Abel Wu <wuyun.abel@...edance.com> Cc: Peter Zijlstra <peterz@...radead.org>, Mel Gorman <mgorman@...e.de>, Vincent Guittot <vincent.guittot@...aro.org>, Chen Yu <yu.c.chen@...el.com>, Tim Chen <tim.c.chen@...ux.intel.com>, K Prateek Nayak <kprateek.nayak@....com>, "Gautham R . Shenoy" <gautham.shenoy@....com>, linux-kernel <linux-kernel@...r.kernel.org> Subject: Re: [PATCH v4 5/7] sched/fair: skip SIS domain search if fully busy On Sun, Jun 19, 2022 at 5:05 AM Abel Wu <wuyun.abel@...edance.com> wrote: > > If a full scan on SIS domain failed, then no unoccupied cpus available > and the LLC is fully busy. In this case we'd better spend the time on > something more useful, rather than wasting it trying to find an idle > cpu that probably not exist. > > The fully busy status will be re-evaluated when any core of this LLC > domain enters load balancing, and cleared once idle cpus found. > > Signed-off-by: Abel Wu <wuyun.abel@...edance.com> > --- > include/linux/sched/topology.h | 35 ++++++++++++++- > kernel/sched/fair.c | 82 +++++++++++++++++++++++++++++----- > 2 files changed, 104 insertions(+), 13 deletions(-) > > diff --git a/include/linux/sched/topology.h b/include/linux/sched/topology.h > index 56cffe42abbc..3e99ac98d766 100644 > --- a/include/linux/sched/topology.h > +++ b/include/linux/sched/topology.h > @@ -77,10 +77,43 @@ extern int sched_domain_level_max; > > struct sched_group; > > +/* > + * States of the sched-domain > + * > + * - sd_has_icores > + * This state is only used in LLC domains to indicate worthy > + * of a full scan in SIS due to idle cores available. > + * > + * - sd_has_icpus > + * This state indicates that unoccupied (sched-idle/idle) cpus > + * might exist in this domain. For the LLC domains it is the > + * default state since these cpus are the main targets of SIS > + * search, and is also used as a fallback state of the other > + * states. > + * > + * - sd_is_busy > + * This state indicates there are no unoccupied cpus in this > + * domain. So for LLC domains, it gives the hint on whether > + * we should put efforts on the SIS search or not. > + * > + * For LLC domains, sd_has_icores is set when the last non-idle cpu of > + * a core becomes idle. After a full SIS scan and if no idle cores found, > + * sd_has_icores must be cleared and the state will be set to sd_has_icpus > + * or sd_is_busy depending on whether there is any idle cpu. And during > + * load balancing on each SMT domain inside the LLC, the state will be > + * re-evaluated and switch from sd_is_busy to sd_has_icpus if idle cpus > + * exist. > + */ > +enum sd_state { > + sd_has_icores, > + sd_has_icpus, > + sd_is_busy > +}; > + > struct sched_domain_shared { > atomic_t ref; > atomic_t nr_busy_cpus; > - int has_idle_cores; > + int state; /* see enum sd_state */ nit: s/int/enum sd_state > }; > > struct sched_domain { > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index 1cc86e76e38e..2ca37fdc6c4d 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -5642,11 +5642,15 @@ static inline void update_overutilized_status(struct rq *rq) > static inline void update_overutilized_status(struct rq *rq) { } > #endif > > +static int unoccupied_rq(struct rq *rq) > +{ > + return rq->nr_running == rq->cfs.idle_h_nr_running; > +} nit: static inline int > + > /* Runqueue only has SCHED_IDLE tasks enqueued */ > static int sched_idle_rq(struct rq *rq) > { > - return unlikely(rq->nr_running == rq->cfs.idle_h_nr_running && > - rq->nr_running); > + return unlikely(rq->nr_running && unoccupied_rq(rq)); > } > > /* > @@ -6197,24 +6201,44 @@ static inline int __select_idle_cpu(int cpu, struct task_struct *p) > DEFINE_STATIC_KEY_FALSE(sched_smt_present); > EXPORT_SYMBOL_GPL(sched_smt_present); > > -static inline void set_idle_cores(int cpu, int val) > +static inline void sd_set_state(int cpu, enum sd_state state) > { > struct sched_domain_shared *sds; > > sds = rcu_dereference(per_cpu(sd_llc_shared, cpu)); > if (sds) > - WRITE_ONCE(sds->has_idle_cores, val); > + WRITE_ONCE(sds->state, state); > } > > -static inline bool test_idle_cores(int cpu) > +static inline enum sd_state sd_get_state(int cpu) > { > struct sched_domain_shared *sds; > > sds = rcu_dereference(per_cpu(sd_llc_shared, cpu)); > if (sds) > - return READ_ONCE(sds->has_idle_cores); > + return READ_ONCE(sds->state); > > - return false; > + return sd_has_icpus; > +} Why is default not sd_is_busy? > + > +static inline void set_idle_cores(int cpu, int idle) nit: Slightly confusing to call the param 'idle', since in the case it is false we still mark icpus. Consider possibly 'core_idle'. > +{ > + sd_set_state(cpu, idle ? sd_has_icores : sd_has_icpus); > +} > + > +static inline bool test_idle_cores(int cpu) > +{ > + return sd_get_state(cpu) == sd_has_icores; > +} > + > +static inline void set_idle_cpus(int cpu, int idle) > +{ > + sd_set_state(cpu, idle ? sd_has_icpus : sd_is_busy); > +} > + > +static inline bool test_idle_cpus(int cpu) > +{ > + return sd_get_state(cpu) != sd_is_busy; > } > > /* > @@ -6298,7 +6322,7 @@ static int select_idle_smt(struct task_struct *p, int target) > > #else /* CONFIG_SCHED_SMT */ > > -static inline void set_idle_cores(int cpu, int val) > +static inline void set_idle_cores(int cpu, int idle) > { > } > > @@ -6307,6 +6331,15 @@ static inline bool test_idle_cores(int cpu) > return false; > } > > +static inline void set_idle_cpus(int cpu, int idle) > +{ > +} > + > +static inline bool test_idle_cpus(int cpu) > +{ > + return true; > +} > + > static inline int select_idle_core(struct task_struct *p, int core, struct cpumask *cpus, int *idle_cpu) > { > return __select_idle_cpu(core, p); > @@ -6382,7 +6415,9 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool > } > } > > - if (has_idle_core) > + if (idle_cpu == -1) > + set_idle_cpus(target, false); > + else if (has_idle_core) > set_idle_cores(target, false); > > if (sched_feat(SIS_PROP) && !has_idle_core) { > @@ -6538,6 +6573,9 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target) > if ((unsigned int)i < nr_cpumask_bits) > return i; > } > + > + if (!has_idle_core && !test_idle_cpus(target)) > + return target; > } > > i = select_idle_cpu(p, sd, has_idle_core, target); > @@ -8303,6 +8341,8 @@ struct sd_lb_stats { > unsigned long avg_load; /* Average load across all groups in sd */ > unsigned int prefer_sibling; /* tasks should go to sibling first */ > > + int sd_state; > + > struct sg_lb_stats busiest_stat;/* Statistics of the busiest group */ > struct sg_lb_stats local_stat; /* Statistics of the local group */ > }; > @@ -8321,6 +8361,7 @@ static inline void init_sd_lb_stats(struct sd_lb_stats *sds) > .local = NULL, > .total_load = 0UL, > .total_capacity = 0UL, > + .sd_state = sd_is_busy, > .busiest_stat = { > .idle_cpus = UINT_MAX, > .group_type = group_has_spare, > @@ -8661,6 +8702,12 @@ sched_asym(struct lb_env *env, struct sd_lb_stats *sds, struct sg_lb_stats *sgs > return sched_asym_prefer(env->dst_cpu, group->asym_prefer_cpu); > } > > +static inline void sd_classify(struct sd_lb_stats *sds, struct rq *rq) > +{ > + if (sds->sd_state != sd_has_icpus && unoccupied_rq(rq)) > + sds->sd_state = sd_has_icpus; > +} > + > /** > * update_sg_lb_stats - Update sched_group's statistics for load balancing. > * @env: The load balancing environment. > @@ -8675,11 +8722,12 @@ static inline void update_sg_lb_stats(struct lb_env *env, > struct sg_lb_stats *sgs, > int *sg_status) > { > - int i, nr_running, local_group; > + int i, nr_running, local_group, update_core; > > memset(sgs, 0, sizeof(*sgs)); > > local_group = group == sds->local; > + update_core = env->sd->flags & SD_SHARE_CPUCAPACITY; Nothing special about SD_SHARE_CPUCAPACITY here other than you want to do the update early on at the lowest domain level during balancing right? > for_each_cpu_and(i, sched_group_span(group), env->cpus) { > struct rq *rq = cpu_rq(i); > @@ -8692,6 +8740,9 @@ 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) > + sd_classify(sds, rq); > + > if (nr_running > 1) > *sg_status |= SG_OVERLOAD; > > @@ -9220,6 +9271,12 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p, int this_cpu) > return idlest; > } > > +static void sd_update_state(struct lb_env *env, struct sd_lb_stats *sds) > +{ > + if (sds->sd_state == sd_has_icpus && !test_idle_cpus(env->dst_cpu)) > + set_idle_cpus(env->dst_cpu, true); > +} We're only setting state to has_icpus here in sd_update_state. That doesn't feel good enough, since we're only updating state for env->dst_cpu; all the other per-cpu state will remain stale (ie. falsely sd_is_busy). I think you also want a case in __update_idle_core() to call set_idle_cores(core, 0) in the case where we have a non-idle sibling, since we want to at least mark has_icpus even if the entire core isn't idle. Still, that doesn't feel quite good enough, since we're only updating the per_cpu sd state for the given cpu. That seems like it will frequently leave us with idle cpus, and select_idle_sibling() skipping select_idle_cpu due to a false negative from test_idle_cpus(). Am I missing something there? > + > /** > * update_sd_lb_stats - Update sched_domain's statistics for load balancing. > * @env: The load balancing environment. > @@ -9270,8 +9327,9 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd > /* Tag domain that child domain prefers tasks go to siblings first */ > sds->prefer_sibling = child && child->flags & SD_PREFER_SIBLING; > > - > - if (env->sd->flags & SD_NUMA) > + if (env->sd->flags & SD_SHARE_CPUCAPACITY) > + sd_update_state(env, sds); > + else if (env->sd->flags & SD_NUMA) > env->fbq_type = fbq_classify_group(&sds->busiest_stat); > > if (!env->sd->parent) { > -- > 2.31.1 >
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