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

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Date:	Tue, 2 Apr 2013 11:02:36 +0900
From:	Joonsoo Kim <iamjoonsoo.kim@....com>
To:	Preeti U Murthy <preeti@...ux.vnet.ibm.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

Hello, Preeti.

On Mon, Apr 01, 2013 at 12:15:50PM +0530, Preeti U Murthy wrote:
> 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?

sysctl_sched_latency is a sched_slice when there is one task.
So, I think that this is proper as upper bound.

Thanks.

> Regards
> Preeti U Murthy
> 
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