Messages in this thread Patch in this message | | | Subject | Re: [Patch v3 1/6] sched/fair: Determine active load balance for SMT sched groups | From | Tim Chen <> | Date | Fri, 14 Jul 2023 16:05:53 -0700 |
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On Fri, 2023-07-14 at 18:36 +0530, Shrikanth Hegde wrote:
> > > If we consider symmetric platforms which have SMT4 such as power10. > we have a topology like below. multiple such MC will form DIE(PKG) > > > [0 2 4 6][1 3 5 7][8 10 12 14][9 11 13 15] > [--SMT--][--SMT--][----SMT---][---SMT----] > [--sg1--][--sg1--][---sg1----][---sg1----] > [--------------MC------------------------] > > In case of SMT4, if there is any group which has 2 or more tasks, that > group will be marked as group_smt_balance. previously, if that group had 2 > or 3 tasks, it would have been marked as group_has_spare. Since all the groups have > SMT that means behavior would be same fully busy right? That can cause some > corner cases. No?
You raised a good point. I was looking from SMT2 perspective so group_smt_balance implies group_fully_busy. That is no longer true for SMT4.
I am thinking of the following fix on the current patch to take care of SMT4. Do you think this addresses concerns from you and Tobias?
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 294a662c9410..3fc8d3a3bd22 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -9588,6 +9588,17 @@ static bool update_sd_pick_busiest(struct lb_env *env, break; case group_smt_balance: + /* no idle cpus on both groups handled by group_fully_busy below */ + if (sgs->idle_cpus != 0 || busiest->idle_cpus != 0) { + if (sgs->idle_cpus > busiest->idle_cpus) + return false; + if (sgs->idle_cpus < busiest->idle_cpus) + return true; + if (sgs->sum_nr_running <= busiest_sum_nr_running) + return false; + else + return true; + }
I will be on vacation next three weeks so my response will be slow.
Tim > > One example is Lets say sg1 has 4 tasks. and sg2 has 0 tasks and is trying to do > load balance. Previously imbalance would have been 2, instead now imbalance would be 1. > But in subsequent lb it would be balanced. > > > > > + return false; > > +} > > + > > static inline bool > > sched_reduced_capacity(struct rq *rq, struct sched_domain *sd) > > { > > @@ -9425,6 +9464,10 @@ static inline void update_sg_lb_stats(struct lb_env *env, > > sgs->group_asym_packing = 1; > > } > > > > + /* Check for loaded SMT group to be balanced to dst CPU */ > > + if (!local_group && smt_balance(env, sgs, group)) > > + sgs->group_smt_balance = 1; > > + > > sgs->group_type = group_classify(env->sd->imbalance_pct, group, sgs); > > > > /* Computing avg_load makes sense only when group is overloaded */ > > @@ -9509,6 +9552,7 @@ static bool update_sd_pick_busiest(struct lb_env *env, > > return false; > > break; > > > > + case group_smt_balance: > > case group_fully_busy: > > /* > > * Select the fully busy group with highest avg_load. In > > @@ -9537,6 +9581,18 @@ static bool update_sd_pick_busiest(struct lb_env *env, > > break; > > > > case group_has_spare: > > + /* > > + * Do not pick sg with SMT CPUs over sg with pure CPUs, > > + * as we do not want to pull task off SMT core with one task > > + * and make the core idle. > > + */ > > + if (smt_vs_nonsmt_groups(sds->busiest, sg)) { > > + if (sg->flags & SD_SHARE_CPUCAPACITY && sgs->sum_h_nr_running <= 1) > > + return false; > > + else > > + return true;> + } > > + > > /* > > * Select not overloaded group with lowest number of idle cpus > > * and highest number of running tasks. We could also compare > > @@ -9733,6 +9789,7 @@ static bool update_pick_idlest(struct sched_group *idlest, > > > > case group_imbalanced: > > case group_asym_packing: > > + case group_smt_balance: > > /* Those types are not used in the slow wakeup path */ > > return false; > > > > @@ -9864,6 +9921,7 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p, int this_cpu) > > > > case group_imbalanced: > > case group_asym_packing: > > + case group_smt_balance: > > /* Those type are not used in the slow wakeup path */ > > return NULL; > > > > @@ -10118,6 +10176,13 @@ static inline void calculate_imbalance(struct lb_env *env, struct sd_lb_stats *s > > return; > > } > > > > + if (busiest->group_type == group_smt_balance) { > > + /* Reduce number of tasks sharing CPU capacity */ > > + env->migration_type = migrate_task; > > + env->imbalance = 1; > > + return; > > + } > > + > > if (busiest->group_type == group_imbalanced) { > > /* > > * In the group_imb case we cannot rely on group-wide averages > > @@ -10363,16 +10428,23 @@ static struct sched_group *find_busiest_group(struct lb_env *env) > > goto force_balance; > > > > if (busiest->group_type != group_overloaded) { > > - if (env->idle == CPU_NOT_IDLE) > > + if (env->idle == CPU_NOT_IDLE) { > > /* > > * If the busiest group is not overloaded (and as a > > * result the local one too) but this CPU is already > > * busy, let another idle CPU try to pull task. > > */ > > goto out_balanced; > > + } > > + > > + if (busiest->group_type == group_smt_balance && > > + smt_vs_nonsmt_groups(sds.local, sds.busiest)) { > > + /* Let non SMT CPU pull from SMT CPU sharing with sibling */ > > + goto force_balance; > > + } > > > > if (busiest->group_weight > 1 && > > - local->idle_cpus <= (busiest->idle_cpus + 1)) > > + local->idle_cpus <= (busiest->idle_cpus + 1)) { > > /* > > * If the busiest group is not overloaded > > * and there is no imbalance between this and busiest > > @@ -10383,12 +10455,14 @@ static struct sched_group *find_busiest_group(struct lb_env *env) > > * there is more than 1 CPU per group. > > */ > > goto out_balanced; > > + } > > > > - if (busiest->sum_h_nr_running == 1) > > + if (busiest->sum_h_nr_running == 1) { > > /* > > * busiest doesn't have any tasks waiting to run > > */ > > goto out_balanced; > > + } > > } > > > > force_balance:
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