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    SubjectRe: [PATCH v3] sched/fair: Use all little CPUs for CPU-bound workload
    On 12/06/23 10:00, Pierre Gondois wrote:
    > Running n CPU-bound tasks on an n CPUs platform:
    > - with asymmetric CPU capacity
    > - not being a DynamIq system (i.e. having a PKG level sched domain
    > without the SD_SHARE_PKG_RESOURCES flag set)
    > 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.
    >
    > Before patch:
    > During step 2, the load balancer running from the idle CPU tags sched
    > domains as:
    > - little CPUs: 'group_has_spare'. Cf. group_has_capacity() and
    > group_is_overloaded(), 3 CPU-bound tasks run on a 4 CPUs
    > sched-domain, and the idle CPU provides enough spare capacity
    > regarding the imbalance_pct
    > - 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:
    > detach_tasks()
    > \-case migrate_util:
    > \-if (util > env->imbalance) goto next;
    >
    > After patch:
    > As the number of failed balancing attempts grows (with
    > 'nr_balance_failed'), progressively make it easier to migrate
    > a big task to the idling little CPU. A similar mechanism is
    > used for the 'migrate_load' migration type.
    >
    > 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%)
    >
    > Similar issue reported at:
    > https://lore.kernel.org/lkml/20230716014125.139577-1-qyousef@layalina.io/
    >
    > v1:
    > https://lore.kernel.org/all/20231110125902.2152380-1-pierre.gondois@arm.com/
    > v2:
    > https://lore.kernel.org/all/20231124153323.3202444-1-pierre.gondois@arm.com/
    >
    > Suggested-by: Vincent Guittot <vincent.guittot@linaro.org>
    > Signed-off-by: Pierre Gondois <pierre.gondois@arm.com>
    > Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
    > Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
    > ---

    Thanks Pierre! I think this is a good candidate to stable. It is likely to help
    some folks shipping with phantom domains on impacted stable kernels.


    Cheers

    --
    Qais Yousef

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