Messages in this thread | | | Date | Thu, 16 Nov 2023 17:34:01 +0100 | Subject | Re: [PATCH] sched/fair: Use all little CPUs for CPU-bound workload | From | Dietmar Eggemann <> |
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On 10/11/2023 13:59, Pierre Gondois wrote: > Running n CPU-bound tasks on an n CPUs platform with asymmetric CPU > capacity might result in a task placement where two tasks run on a > big CPU and none on a little CPU. This placement could be more optimal > by using all CPUs. > > Testing platform: > Juno-r2: > - 2 big CPUs (1-2), maximum capacity of 1024 > - 4 little CPUs (0,3-5), maximum capacity of 383 > > Testing workload ([1]): > Spawn 6 CPU-bound tasks. During the first 100ms (step 1), each tasks > is affine to a CPU, except for: > - one little CPU which is left idle. > - one big CPU which has 2 tasks affine. > After the 100ms (step 2), remove the cpumask affinity.
I used your workload on my Juno-r0 with LISA (rt-app overwrites the mainline CPU capacity values [446 1024 1024 446 446 466] to [675 1024 1024 675 675 675] to adapt to rt-app busy loop's instruction mix.
Here I can't see the issue you bring up here. The two tasks sharing a CPU have util_avg = ~512 and the load-balance from one task to the idle little CPU is happening,
I assume it's the diff in CPU capacity of the little CPUs: 383 < 512 < 675 ?
> > Before patch: > During step 2, the load balancer running from the idle CPU tags sched > domains as: > - little CPUs: 'group_has_spare'. Indeed, 3 CPU-bound tasks run on a > 4 CPUs sched-domain, and the idle CPU provides enough spare > capacity. > - big CPUs: 'group_overloaded'. Indeed, 3 tasks run on a 2 CPUs > sched-domain, so the following path is used: > group_is_overloaded() > \-if (sgs->sum_nr_running <= sgs->group_weight) return true; > > The following path which would change the migration type to > 'migrate_task' is not taken: > calculate_imbalance() > \-if (env->idle != CPU_NOT_IDLE && env->imbalance == 0) > as the local group has some spare capacity, so the imbalance > is not 0. > > The migration type requested is 'migrate_util' and the busiest > runqueue is the big CPU's runqueue having 2 tasks (each having a > utilization of 512). The idle little CPU cannot pull one of these > task as its capacity is too small for the task. The following path > is used:
Ah, here you're describing the issue I mentioned above.
> detach_tasks() > \-case migrate_util: > \-if (util > env->imbalance) goto next; > > After patch: > When the local group has spare capacity and the busiest group is at > least tagged as 'group_fully_busy', if the local group has more CPUs > than CFS tasks and the busiest group more CFS tasks than CPUs, > request a 'migrate_task' type migration. > > Improvement: > Running the testing workload [1] with the step 2 representing > a ~10s load for a big CPU: > Before patch: ~19.3s > After patch: ~18s (-6.7%) > > The issue only happens at the DIE level on platforms able to have > 'migrate_util' migration types, i.e. no DynamIQ systems where > SD_SHARE_PKG_RESOURCES is set.
Right, mainline Arm DynamIQ should only have 1 SD level (MC). Android might still be affected since they run MC and DIE.
> > Signed-off-by: Pierre Gondois <pierre.gondois@arm.com> > --- > kernel/sched/fair.c | 17 +++++++++++++++++ > 1 file changed, 17 insertions(+) > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index df348aa55d3c..5a215c96d420 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -10495,6 +10495,23 @@ static inline void calculate_imbalance(struct lb_env *env, struct sd_lb_stats *s > env->imbalance = max(local->group_capacity, local->group_util) - > local->group_util; > > + /* > + * On an asymmetric system with CPU-bound tasks, a > + * migrate_util balance might not be able to migrate a > + * task from a big to a little CPU, letting a little > + * CPU unused. > + * If local has an empty CPU and busiest is overloaded, > + * balance one task with a migrate_task migration type > + * instead. > + */ > + if (env->sd->flags & SD_ASYM_CPUCAPACITY && > + local->sum_nr_running < local->group_weight && > + busiest->sum_nr_running > busiest->group_weight) { > + env->migration_type = migrate_task; > + env->imbalance = 1; > + return; > + } > + > /* > * In some cases, the group's utilization is max or even > * higher than capacity because of migrations but the
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