linux-kernel - Re: [PATCH 5/5] sched: limit sched_slice if it is more than sysctl_sched

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Date:	Mon, 01 Apr 2013 12:15:50 +0530
From:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
To:	Joonsoo Kim <iamjoonsoo.kim@....com>
CC:	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, Mike Galbraith <efault@....de>,
	Paul Turner <pjt@...gle.com>, Alex Shi <alex.shi@...el.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Morten Rasmussen <morten.rasmussen@....com>,
	Namhyung Kim <namhyung@...nel.org>
Subject: Re: [PATCH 5/5] sched: limit sched_slice if it is more than sysctl_sched_latency

Hi Joonsoo,

On 04/01/2013 10:39 AM, Joonsoo Kim wrote:
> Hello Preeti.
> So we should limit this possible weird situation.
>>>
>>> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@....com>
>>>
>>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>>> index e232421..6ceffbc 100644
>>> --- a/kernel/sched/fair.c
>>> +++ b/kernel/sched/fair.c
>>> @@ -645,6 +645,9 @@ static u64 sched_slice(struct cfs_rq *cfs_rq, struct sched_entity *se)
>>>  	}
>>>  	slice = calc_delta_mine(slice, se->load.weight, load);
>>>
>>> +	if (unlikely(slice > sysctl_sched_latency))
>>> +		slice = sysctl_sched_latency;
>>
>> Then in this case the highest priority thread would get
>> 20ms(sysctl_sched_latency), and the rest would get
>> sysctl_sched_min_granularity * 10 * (1024/97977) which would be 0.4ms.
>> Then all tasks would get scheduled ateast once within 20ms + (0.4*9) ms
>> = 23.7ms, while your scheduling latency period was extended to 40ms,just
>> so that each of these tasks don't have their sched_slices shrunk due to
>> large number of tasks.
> 
> I don't know I understand your question correctly.
> I will do my best to answer your comment. :)
> 
> With this patch, I just limit maximum slice at one time. Scheduling is
> controlled through the vruntime. So, in this case, the task with nice -20
> will be scheduled twice.
> 
> 20 + (0.4 * 9) + 20 = 43.9 ms
> 
> And after 43.9 ms, this process is repeated.
> 
> So I can tell you that scheduling period is preserved as before.
> 
> If we give a long period to a task at one go, it can cause
> a latency problem. So IMHO, limiting this is meaningful.

Thank you very much for the explanation. Just one question. What is the
reason behind you choosing sysctl_sched_latency as the upper bound here?

Regards
Preeti U Murthy

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