linux-kernel - Re: [PATCH v4 04/10] sched/fair: Dynamically update cfs_overload

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Message-ID: <8e032913-a426-0f32-6c68-a183bfc7ed66@arm.com>
Date:   Sat, 8 Dec 2018 18:47:13 +0000
From:   Valentin Schneider <valentin.schneider@....com>
To:     Steven Sistare <steven.sistare@...cle.com>, mingo@...hat.com,
        peterz@...radead.org
Cc:     subhra.mazumdar@...cle.com, dhaval.giani@...cle.com,
        daniel.m.jordan@...cle.com, pavel.tatashin@...rosoft.com,
        matt@...eblueprint.co.uk, umgwanakikbuti@...il.com,
        riel@...hat.com, jbacik@...com, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, quentin.perret@....com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 04/10] sched/fair: Dynamically update cfs_overload_cpus

On 07/12/2018 22:35, Steven Sistare wrote:
[...]
>>> @@ -4468,8 +4495,12 @@ static void throttle_cfs_rq(struct cfs_rq *cfs_rq)
>>>  			dequeue = 0;
>>>  	}
>>>  
>>> -	if (!se)
>>> +	if (!se) {
>>>  		sub_nr_running(rq, task_delta);
>>> +		if (prev_nr >= 2 && prev_nr - task_delta < 2)
>>> +			overload_clear(rq);
>>> +
>>> +	}
>>
>> Eventually it'd be nice to squash those into {add, sub}_nr_running(), but
>> you already mentioned wanting to stick to CFS for now, so I don't think
>> it's *too* much of a big deal.
> 
> Maybe.  It depends on a design decision to be made if/when we add bitmap
> based stealing to other scheduling classes.  Do we maintain one bitmap
> for overloaded CPUs where the overload may be caused by any mix of different
> task classes?  If yes, then the bitmap search for one class such as RT
> will inspect and reject overloaded CPUs that only have CFS tasks, which 
> making the search less efficient.  I am leaning towards a separate bitmap 
> per class to avoid that.
> 

Didn't ponder too much about it, but a bitmap per class sounds sane. My
comment was mostly about saving ourselves the need to decorate every
{add, sub}_nr_running() call with overload_{set, clear}() calls.

> - Steve
>   
>