linux-kernel - Re: [PATCH 4/5] sched: don't consider upper se in sched

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Message-ID: <515B163B.8050509@linux.vnet.ibm.com>
Date:	Tue, 02 Apr 2013 23:02:43 +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 4/5] sched: don't consider upper se in sched_slice()

Hi Joonsoo,


>>> I think that it is real problem that sysctl_sched_min_granularity is not
>>> guaranteed for each task.
>>> Instead of this patch, how about considering low bound?
>>>
>>> if (slice < sysctl_sched_min_granularity)
>>> 	slice = sysctl_sched_min_granularity;
>>
>> Consider the below scenario.
>>
>> A runqueue has two task groups,each with 10 tasks.
>>
>> With the current implementation,each of these tasks get a sched_slice of
>> 2ms.Hence in a matter of (10*2) + (10*2) = 40 ms, all tasks( all tasks
>> of both the task groups) will get the chance to run.
>>
>> But what is the scheduling period in this scenario? Is it 40ms (extended
>> sysctl_sched_latency), which is the scheduling period for each of the
>> runqueues with 10 tasks in it?
>> Or is it 80ms which is the total of the scheduling periods of each of
>> the run queues with 10 tasks.Either way all tasks seem to get scheduled
>> atleast once within the scheduling period.So we appear to be safe.
>> Although the sched_slice < sched_min_granularity.
>>
>> With your above lower bound of sysctl_sched_min_granularity, each task
>> of each tg gets 4ms as its sched_slice.So in a matter of
>> (10*4) + (10*4) = 80ms,all tasks get to run. With the above question
>> being put forth here as well, we don't appear to be safe if the
>> scheduling_period is considered to be 40ms, otherwise it appears fine to
>> me, because it ensures the sched_slice is atleast sched_min_granularity
>> no matter what.
> 
> So, you mean that we should guarantee to schedule each task atleast once
> in sysctl_sched_latency? 

I would rather say all tasks get to run atleast once in a sched_period,
which could be just sysctl_sched_latency or more depending on the number
of tasks in the current implementation.

But this is not guaranteed in current code,
> this is why I made this patch. Please refer a patch description.

Ok,take the example of a runqueue with 2 task groups,each with 10
tasks.Same as your previous example. Can you explain how your patch
ensures that all 20 tasks get to run atleast once in a sched_period?

Regards
Preeti U Murthy

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