linux-kernel - Re: [PATCH 5/5] sched/fair: Add exception for hints in load balancing path

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Date:   Mon, 12 Sep 2022 11:59:50 +0530
From:   K Prateek Nayak <kprateek.nayak@....com>
To:     Hillf Danton <hdanton@...a.com>
Cc:     linux-kernel@...r.kernel.org, mgorman@...hsingularity.net,
        peterz@...radead.org, linux-mm@...ck.org,
        vincent.guittot@...aro.org
Subject: Re: [PATCH 5/5] sched/fair: Add exception for hints in load balancing
 path

Hello Hillf,

Thank you for looking into this patch.

On 9/11/2022 1:35 PM, Hillf Danton wrote:
> On 10 Sep 2022 16:23:26 +0530 K Prateek Nayak <kprateek.nayak@....com> wrote:
>> - Load balancing considerations
>>
>> If we have more tasks than the CPUs in the MC Domain, ignore the hint
>> set by the user. This prevents losing the consolidation done at the
>> wakeup time.
> 
> It is waste of time to cure ten pains with a pill in five days a week.

This patch mainly tries to stop tasks with a wakeup hint being
pulled apart by the load-balancer and then end up moving back again
to the same LLC during subsequent wakeup leading to ping-ponging and
lot of wasted migrations.

This is not a complete solution in any form and was a stop-gap for
this experiment. I bet there are better alternatives to handle hints
in the load-balancing path.
I'm all ears any suggestions from the community :)

> 
>> @@ -7977,6 +7980,21 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env)
>>  		return 0;
>>  	}
>>  
>> +	/*
>> +	 * Hints are followed only if the MC Domain is still ideal
>> +	 * for the task.
>> +	 */
>> +	if (!env->ignore_hint) {
>> +		/*
>> +		 * Only consider the hints from the wakeup path to maintain
>> +		 * data locality.
>> +		 */
>> +		if (READ_ONCE(p->hint) &
>> +		    (PR_SCHED_HINT_WAKE_AFFINE | PR_SCHED_HINT_WAKE_HOLD))
>> +			return 0;
>> +	}
> 
> The wake hints are not honored during lb without PR_SCHED_HINT_IGNORE_LB set
> then the scheduler works as you hint.

Are you suggesting we leave it to the user, to control whether the
load-balancer can spread the task apart, even if hints are set,
via another userspace hint "PR_SCHED_HINT_IGNORE_LB"?

I had not considered it before but it may benefit some workloads.
Again, this API is not the final API in any form but we can have
a knob as you suggested that can be set for a class of workloads
which may benefit from this behavior.

> 
> Hillf

--
Thanks and Regards,
Prateek