lkml.org 
[lkml]   [2022]   [Apr]   [14]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
SubjectRe: [RFC v2 2/2] sched/fair: introduce sched-idle balance
From
On 4/14/22 8:08 AM, Josh Don Wrote:
>>>> /*
>>>> * Use locality-friendly rq->overloaded to cache the status of the rq
>>>> * to minimize the heavy cost on LLC shared data.
>>>> @@ -7837,6 +7867,22 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env)
>>>> if (kthread_is_per_cpu(p))
>>>> return 0;
>>>>
>>>> + if (unlikely(task_h_idle(p))) {
>>>> + /*
>>>> + * Disregard hierarchically idle tasks during sched-idle
>>>> + * load balancing.
>>>> + */
>>>> + if (env->idle == CPU_SCHED_IDLE)
>>>> + return 0;
>>>> + } else if (!static_branch_unlikely(&sched_asym_cpucapacity)) {
>>>> + /*
>>>> + * It's not gonna help if stacking non-idle tasks on one
>>>> + * cpu while leaving some idle.
>>>> + */
>>>> + if (cfs_rq_busy(env->src_rq) && !need_pull_cfs_task(env->dst_rq))
>>>> + return 0;
>>>
>>> These checks don't involve the task at all, so this kind of check
>>> should be pushed into the more general load balance function. But, I'm
>>> not totally clear on the motivation here. If we have cpu A with 1
>>> non-idle task and 100 idle tasks, and cpu B with 1 non-idle task, we
>>> should definitely try to load balance some of the idle tasks from A to
>>> B. idle tasks _do_ get time to run (although little), and this can add
>>> up and cause antagonism to the non-idle task if there are a lot of
>>> idle threads.
>>
>> CPU_SCHED_IDLE means triggered by sched_idle_balance() in which pulls
>> a non-idle task for the unoccupied cpu from the overloaded ones, so
>> idle tasks are not the target and should be skipped.
>>
>> The second part is: if we have cpu A with 1 non-idle task and 100 idle
>> tasks, and B with >=1 non-idle task, we don't migrate the last non-idle
>> task on A to B.
>
> It could be possible that we do want to migrate the last non-idle task
> from A to B, if the weight sum of idle tasks on A is very high (easily
> possible with affinity restrictions). So I think we should leave
> regular load balance alone here if it really wants to move the
> non-idle task, and wrap this entire block in an if (env->idle ==
> CPU_SCHED_IDLE).

Makes sense. I will fix it in next version.

Thanks & BR,
Abel

\
 
 \ /
  Last update: 2022-04-14 18:08    [W:0.060 / U:0.592 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site