Messages in this thread Patch in this message | | | Subject | Re: [PATCH v3 04/10] sched/fair: rework load_balance | From | Dietmar Eggemann <> | Date | Mon, 30 Sep 2019 18:24:21 +0200 |
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Hi Vincent,
On 19/09/2019 09:33, Vincent Guittot wrote:
these are just some comments & questions based on a code study. Haven't run any tests with it yet.
[...]
> 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
s/group_asym_capacity/group_asym_packing
> group_imbalanced > group_overloaded > > Based on the type fo sched_group, load_balance now sets what it wants to
s/fo/for
> move in order to fix the imbalance. 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 asymmetrics system
s/asymmetrics/asymmetric
This stands for ASYM_CPUCAPACITY and ASYM_PACKING, right?
[...]
> #define LBF_ALL_PINNED 0x01 > @@ -7115,7 +7130,7 @@ struct lb_env { > unsigned int loop_max; > > enum fbq_type fbq_type; > - enum group_type src_grp_type; > + enum migration_type balance_type;
Minor thing:
Why not enum migration_type migration_type; or enum balance_type balance_type;
We do the same for other enums like fbq_type or group_type.
> struct list_head tasks; > }; >
The detach_tasks() comment still mentions only runnable load.
> @@ -7347,7 +7362,7 @@ static int detach_tasks(struct lb_env *env) > { > struct list_head *tasks = &env->src_rq->cfs_tasks; > struct task_struct *p; > - unsigned long load; > + unsigned long util, load;
Minor: Order by length or reduce scope to while loop ?
> int detached = 0; > > lockdep_assert_held(&env->src_rq->lock); > @@ -7380,19 +7395,53 @@ static int detach_tasks(struct lb_env *env) > if (!can_migrate_task(p, env)) > goto next; > > - load = task_h_load(p); > + switch (env->balance_type) { > + case migrate_load: > + load = task_h_load(p); > > - if (sched_feat(LB_MIN) && load < 16 && !env->sd->nr_balance_failed) > - goto next; > + if (sched_feat(LB_MIN) && > + load < 16 && !env->sd->nr_balance_failed) > + goto next; > > - if ((load / 2) > env->imbalance) > - goto next; > + if ((load / 2) > env->imbalance) > + goto next; > + > + env->imbalance -= load; > + break; > + > + case migrate_util: > + util = task_util_est(p); > + > + if (util > env->imbalance) > + goto next; > + > + env->imbalance -= util; > + break; > + > + case migrate_task: > + /* Migrate task */
Minor: IMHO, this comment is not necessary.
> + env->imbalance--; > + break; > + > + case migrate_misfit: > + load = task_h_load(p); > + > + /* > + * utilization of misfit task might decrease a bit
This patch still uses load. IMHO this comments becomes true only with 08/10.
[...]
> @@ -7707,13 +7755,11 @@ static inline void init_sd_lb_stats(struct sd_lb_stats *sds) > *sds = (struct sd_lb_stats){ > .busiest = NULL, > .local = NULL, > - .total_running = 0UL, > .total_load = 0UL, > .total_capacity = 0UL, > .busiest_stat = { > - .avg_load = 0UL,
There is a sentence in the comment above explaining why avg_load has to be cleared. And IMHO local group isn't cleared anymore but set/initialized.
> - .sum_h_nr_running = 0, > - .group_type = group_other, > + .idle_cpus = UINT_MAX, > + .group_type = group_has_spare, > }, > }; > }
[...]
> @@ -8042,14 +8104,24 @@ static inline void update_sg_lb_stats(struct lb_env *env, > } > } > > - /* Adjust by relative CPU capacity of the group */ > + /* Check if dst cpu is idle and preferred to this group */
s/preferred to/preferred by ? or the preferred CPU of this group ?
[...]
> @@ -8283,69 +8363,133 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd > */ > static inline void calculate_imbalance(struct lb_env *env, struct sd_lb_stats *sds) > { > - unsigned long max_pull, load_above_capacity = ~0UL; > struct sg_lb_stats *local, *busiest; > > local = &sds->local_stat; > busiest = &sds->busiest_stat; > > - if (busiest->group_asym_packing) { > + if (busiest->group_type == group_misfit_task) { > + /* Set imbalance to allow misfit task to be balanced. */ > + env->balance_type = migrate_misfit; > + env->imbalance = busiest->group_misfit_task_load; > + return; > + } > + > + if (busiest->group_type == group_asym_packing) { > + /* > + * In case of asym capacity, we will try to migrate all load to
Does asym capacity stands for asym packing or asym cpu capacity?
> + * the preferred CPU. > + */ > + env->balance_type = migrate_load; > env->imbalance = busiest->group_load; > return; > } > > + if (busiest->group_type == group_imbalanced) { > + /* > + * In the group_imb case we cannot rely on group-wide averages > + * to ensure CPU-load equilibrium, try to move any task to fix > + * the imbalance. The next load balance will take care of > + * balancing back the system.
balancing back ?
> + */ > + env->balance_type = migrate_task; > + env->imbalance = 1; > + return; > + } > + > /* > - * Avg load of busiest sg can be less and avg load of local sg can > - * be greater than avg load across all sgs of sd because avg load > - * factors in sg capacity and sgs with smaller group_type are > - * skipped when updating the busiest sg: > + * Try to use spare capacity of local group without overloading it or > + * emptying busiest > */ > - if (busiest->group_type != group_misfit_task && > - (busiest->avg_load <= sds->avg_load || > - local->avg_load >= sds->avg_load)) { > - env->imbalance = 0; > + if (local->group_type == group_has_spare) { > + if (busiest->group_type > group_fully_busy) {
So this could be 'busiest->group_type == group_overloaded' here to match the comment below? Since you handle group_misfit_task, group_asym_packing, group_imbalanced above and return.
> + /* > + * If busiest is overloaded, try to fill spare > + * capacity. This might end up creating spare capacity > + * in busiest or busiest still being overloaded but > + * there is no simple way to directly compute the > + * amount of load to migrate in order to balance the > + * system. > + */ > + env->balance_type = migrate_util; > + env->imbalance = max(local->group_capacity, local->group_util) - > + local->group_util; > + return; > + } > + > + if (busiest->group_weight == 1 || sds->prefer_sibling) { > + /* > + * When prefer sibling, evenly spread running tasks on > + * groups. > + */ > + env->balance_type = migrate_task; > + env->imbalance = (busiest->sum_h_nr_running - local->sum_h_nr_running) >> 1; > + return; > + } > + > + /* > + * If there is no overload, we just want to even the number of > + * idle cpus. > + */ > + env->balance_type = migrate_task; > + env->imbalance = max_t(long, 0, (local->idle_cpus - busiest->idle_cpus) >> 1);
Why do we need a max_t(long, 0, ...) here and not for the 'if (busiest->group_weight == 1 || sds->prefer_sibling)' case?
> return; > } > > /* > - * If there aren't any idle CPUs, avoid creating some. > + * Local is fully busy but have to take more load to relieve the
s/have/has
> + * busiest group > */
I thought that 'local->group_type == group_imbalanced' is allowed as well? So 'if (local->group_type < group_overloaded)' (further down) could include that?
> - if (busiest->group_type == group_overloaded && > - local->group_type == group_overloaded) { > - load_above_capacity = busiest->sum_h_nr_running * SCHED_CAPACITY_SCALE; > - if (load_above_capacity > busiest->group_capacity) { > - load_above_capacity -= busiest->group_capacity; > - load_above_capacity *= scale_load_down(NICE_0_LOAD); > - load_above_capacity /= busiest->group_capacity; > - } else > - load_above_capacity = ~0UL; > + if (local->group_type < group_overloaded) { > + /* > + * Local will become overloaded so the avg_load metrics are > + * finally needed. > + */
How does this relate to the decision_matrix[local, busiest] (dm[])? E.g. dm[overload, overload] == avg_load or dm[fully_busy, overload] == force. It would be nice to be able to match all allowed fields of dm to code sections.
[...]
> /******* find_busiest_group() helpers end here *********************/ > > +/* > + * Decision matrix according to the local and busiest group state
Minor s/state/type ?
> + * > + * busiest \ local has_spare fully_busy misfit asym imbalanced overloaded > + * has_spare nr_idle balanced N/A N/A balanced balanced > + * fully_busy nr_idle nr_idle N/A N/A balanced balanced > + * misfit_task force N/A N/A N/A force force > + * asym_capacity force force N/A N/A force force
s/asym_capacity/asym_packing
> + * imbalanced force force N/A N/A force force > + * overloaded force force N/A N/A force avg_load > + * > + * N/A : Not Applicable because already filtered while updating > + * statistics. > + * balanced : The system is balanced for these 2 groups. > + * force : Calculate the imbalance as load migration is probably needed. > + * avg_load : Only if imbalance is significant enough. > + * nr_idle : dst_cpu is not busy and the number of idle cpus is quite > + * different in groups. > + */ > + > /** > * find_busiest_group - Returns the busiest group within the sched_domain > * if there is an imbalance. > @@ -8380,17 +8524,17 @@ static struct sched_group *find_busiest_group(struct lb_env *env) > local = &sds.local_stat; > busiest = &sds.busiest_stat; > > - /* ASYM feature bypasses nice load balance check */ > - if (busiest->group_asym_packing) > - goto force_balance; > - > /* There is no busy sibling group to pull tasks from */ > - if (!sds.busiest || busiest->sum_h_nr_running == 0) > + if (!sds.busiest) > goto out_balanced; > > - /* XXX broken for overlapping NUMA groups */ > - sds.avg_load = (SCHED_CAPACITY_SCALE * sds.total_load) > - / sds.total_capacity; > + /* Misfit tasks should be dealt with regardless of the avg load */ > + if (busiest->group_type == group_misfit_task) > + goto force_balance; > + > + /* ASYM feature bypasses nice load balance check */
Minor: s/ASYM feature/ASYM_PACKING ... to distinguish clearly from ASYM_CPUCAPACITY.
> + if (busiest->group_type == group_asym_packing) > + goto force_balance;
[...]
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