Messages in this thread Patch in this message | | | Date | Tue, 6 Aug 2019 17:56:20 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH v2 4/8] sched/fair: rework load_balance |
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On Thu, Aug 01, 2019 at 04:40:20PM +0200, Vincent Guittot wrote: > The load_balance algorithm contains some heuristics which have becomes > meaningless since the rework of metrics and the introduction of PELT. > > Furthermore, it's sometimes difficult to fix wrong scheduling decisions > because everything is based on load whereas some imbalances are not > related to the load. The current algorithm ends up to create virtual and > meaningless value like the avg_load_per_task or tweaks the state of a > group to make it overloaded whereas it's not, in order to try to migrate > tasks. > > load_balance should better qualify the imbalance of the group and define > cleary what has to be moved to fix this imbalance. > > The type of sched_group has been extended to better reflect the type of > imbalance. We now have : > group_has_spare > group_fully_busy > group_misfit_task > group_asym_capacity > group_imbalanced > group_overloaded > > Based on the type of sched_group, load_balance now sets what it wants to > move in order to fix the imnbalance. It can be some load as before but > also some utilization, a number of task or a type of task: > migrate_task > migrate_util > migrate_load > migrate_misfit > > This new load_balance algorithm fixes several pending wrong tasks > placement: > - the 1 task per CPU case with asymetrics system > - the case of cfs task preempted by other class > - the case of tasks not evenly spread on groups with spare capacity > > The load balance decisions have been gathered in 3 functions: > - update_sd_pick_busiest() select the busiest sched_group. > - find_busiest_group() checks if there is an imabalance between local and > busiest group. > - calculate_imbalance() decides what have to be moved.
I like this lots, very good stuff.
It is weird how misfit is still load, but istr later patches cure that.
Here's a whole pile of nits, some overlap with what Valentin already noted. His suggestions for the changelog also make sense to me.
--- --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -8205,10 +8205,10 @@ static bool update_sd_pick_busiest(struc { struct sg_lb_stats *busiest = &sds->busiest_stat; - /* Make sure that there is at least one task to pull */ if (!sgs->sum_h_nr_running) return false; + /* * Don't try to pull misfit tasks we can't help. * We can use max_capacity here as reduction in capacity on some @@ -8239,11 +8239,11 @@ static bool update_sd_pick_busiest(struc break; case group_imbalanced: - /* Select the 1st imbalanced group as we don't have - * any way to choose one more than another + /* + * Select the 1st imbalanced group as we don't have any way to + * choose one more than another */ return false; - break; case group_asym_capacity: /* Prefer to move from lowest priority CPU's work */ @@ -8253,8 +8253,8 @@ static bool update_sd_pick_busiest(struc case group_misfit_task: /* - * If we have more than one misfit sg go with the - * biggest misfit. + * If we have more than one misfit sg go with the biggest + * misfit. */ if (sgs->group_misfit_task_load < busiest->group_misfit_task_load) return false; @@ -8262,14 +8262,14 @@ static bool update_sd_pick_busiest(struc case group_fully_busy: /* - * Select the fully busy group with highest avg_load. - * In theory, there is no need to pull task from such - * kind of group because tasks have all compute - * capacity that they need but we can still improve the - * overall throughput by reducing contention when - * accessing shared HW resources. - * XXX for now avg_load is not computed and always 0 so - * we select the 1st one. + * Select the fully busy group with highest avg_load. In + * theory, there is no need to pull task from such kind of + * group because tasks have all compute capacity that they need + * but we can still improve the overall throughput by reducing + * contention when accessing shared HW resources. + * + * XXX for now avg_load is not computed and always 0 so we + * select the 1st one. */ if (sgs->avg_load <= busiest->avg_load) return false; @@ -8277,11 +8277,11 @@ static bool update_sd_pick_busiest(struc case group_has_spare: /* - * Select not overloaded group with lowest number of - * idle cpus. We could also compare the spare capacity - * which is more stable but it can end up that the - * group has less spare capacity but finally more idle - * cpus which means less opportunity to pull tasks. + * Select not overloaded group with lowest number of idle cpus. + * We could also compare the spare capacity which is more + * stable but it can end up that the group has less spare + * capacity but finally more idle cpus which means less + * opportunity to pull tasks. */ if (sgs->idle_cpus >= busiest->idle_cpus) return false; @@ -8289,10 +8289,10 @@ static bool update_sd_pick_busiest(struc } /* - * Candidate sg has no more than one task per CPU and - * has higher per-CPU capacity. Migrating tasks to less - * capable CPUs may harm throughput. Maximize throughput, - * power/energy consequences are not considered. + * Candidate sg has no more than one task per CPU and has higher + * per-CPU capacity. Migrating tasks to less capable CPUs may harm + * throughput. Maximize throughput, power/energy consequences are not + * considered. */ if ((env->sd->flags & SD_ASYM_CPUCAPACITY) && (sgs->group_type <= group_fully_busy) && @@ -8428,6 +8428,7 @@ static inline void calculate_imbalance(s local = &sds->local_stat; busiest = &sds->busiest_stat; + if (busiest->group_type == group_imbalanced) { /* * In the group_imb case we cannot rely on group-wide averages @@ -8442,8 +8443,8 @@ static inline void calculate_imbalance(s if (busiest->group_type == group_asym_capacity) { /* - * In case of asym capacity, we will try to migrate all load - * to the preferred CPU + * In case of asym capacity, we will try to migrate all load to + * the preferred CPU */ env->balance_type = migrate_load; env->imbalance = busiest->group_load; @@ -8483,7 +8484,8 @@ static inline void calculate_imbalance(s * groups. */ env->balance_type = migrate_task; - env->imbalance = (busiest->sum_h_nr_running - local->sum_h_nr_running) >> 1; + env->imbalance = (busiest->sum_h_nr_running - + local->sum_h_nr_running) >> 1; return; } @@ -8502,7 +8504,7 @@ static inline void calculate_imbalance(s */ if (local->group_type < group_overloaded) { /* - * Local will become overvloaded so the avg_load metrics are + * Local will become overloaded so the avg_load metrics are * finally needed */ @@ -8514,12 +8516,12 @@ static inline void calculate_imbalance(s } /* - * Both group are or will become overloaded and we're trying to get - * all the CPUs to the average_load, so we don't want to push - * ourselves above the average load, nor do we wish to reduce the - * max loaded CPU below the average load. At the same time, we also - * don't want to reduce the group load below the group capacity. - * Thus we look for the minimum possible imbalance. + * Both group are or will become overloaded and we're trying to get all + * the CPUs to the average_load, so we don't want to push ourselves + * above the average load, nor do we wish to reduce the max loaded CPU + * below the average load. At the same time, we also don't want to + * reduce the group load below the group capacity. Thus we look for + * the minimum possible imbalance. */ env->balance_type = migrate_load; env->imbalance = min( @@ -8641,7 +8643,6 @@ static struct sched_group *find_busiest_ if (100 * busiest->avg_load <= env->sd->imbalance_pct * local->avg_load) goto out_balanced; - } /* Try to move all excess tasks to child's sibling domain */ @@ -8651,7 +8652,7 @@ static struct sched_group *find_busiest_ if (busiest->group_type != group_overloaded && (env->idle == CPU_NOT_IDLE || - 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 group @@ -8661,15 +8662,14 @@ static struct sched_group *find_busiest_ * group. */ goto out_balanced; + } force_balance: /* Looks like there is an imbalance. Compute it */ calculate_imbalance(env, &sds); - return env->imbalance ? sds.busiest : NULL; out_balanced: - env->imbalance = 0; return NULL; }
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