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Date: Tue, 21 Nov 2023 09:58:19 +0100
From: Pierre Gondois <pierre.gondois@....com>
To: Vincent Guittot <vincent.guittot@...aro.org>
Cc: linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Juri Lelli <juri.lelli@...hat.com>,
Dietmar Eggemann <dietmar.eggemann@....com>,
Steven Rostedt <rostedt@...dmis.org>,
Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
Daniel Bristot de Oliveira <bristot@...hat.com>,
Valentin Schneider <vschneid@...hat.com>
Subject: Re: [PATCH] sched/fair: Use all little CPUs for CPU-bound workload
Hello Vincent,
On 11/17/23 15:17, Vincent Guittot wrote:
> Hi Pierre,
>
> On Fri, 10 Nov 2023 at 13:59, Pierre Gondois <pierre.gondois@....com> 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.
>>
>> 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;
>
> This remembers me a similar discussion with Qais:
> https://lore.kernel.org/lkml/20230716014125.139577-1-qyousef@layalina.io/
Yes indeed, this is exactly the same case and same backstory actually.
>
>>
>> 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:
>> 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
>
> the busiest group is more than 'group_fully_busy'
>
>> 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.
>>
>> Signed-off-by: Pierre Gondois <pierre.gondois@....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,
>
> group_has_spare doesn't mean that the local has an empty cpu but that
> one or more cpu might be idle some time which could not be the case
> when the load balance happen
>
>> + * 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;
>
> I wonder if this is too aggressive. We can have cases where
> (local->sum_nr_running < local->group_weight) at the time of the load
> balance because one cpu can be shortly idle and you will migrate the
> task that will then compete with another one on a little core. So
Ok right.
> maybe you should do something similar to the migrate_load in
> detach_tasks like:
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index fc8e9ced6aa8..3a04fa0f1eae 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -8977,7 +8977,7 @@ static int detach_tasks(struct lb_env *env)
> case migrate_util:
> util = task_util_est(p);
>
> - if (util > env->imbalance)
> + if (shr_bound(util,
> env->sd->nr_balance_failed) > env->imbalance)
> goto next;
>
> env->imbalance -= util;
> --
>
> This should cover more intermediate cases and would benefit to more
> topology and cases
Your change also solves the issue. I'll try to see if this might
raise other issues,
Thanks for the suggestion,
Regards,
Pierre
>
>> + return;
>> + }
>> +
>> /*
>> * In some cases, the group's utilization is max or even
>> * higher than capacity because of migrations but the
>> --
>> 2.25.1
>>
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