linux-kernel - Re: [PATCH v2] sched: let __sched_period() use rq's nr

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Date:	Mon, 13 Jul 2015 17:29:00 +0900
From:	Byungchul Park <byungchul.park@....com>
To:	Mike Galbraith <umgwanakikbuti@...il.com>
Cc:	Morten Rasmussen <morten.rasmussen@....com>, mingo@...nel.org,
	peterz@...radead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] sched: let __sched_period() use rq's nr_running

On Mon, Jul 13, 2015 at 09:07:01AM +0200, Mike Galbraith wrote:
> On Mon, 2015-07-13 at 09:56 +0900, Byungchul Park wrote:
> 
> > and i agree with that it makes latency increase for non-grouped tasks.
> 
> It's not only a latency hit for the root group, it's across the board.
> 
> I suspect an overloaded group foo/bar/baz would prefer small slices over
> a large wait as well. I certainly wouldn't want my root group taking the
> potentially huge hits that come with stretching period to accommodate an
> arbitrarily overloaded /foo/bar/baz.

hello, mike :)

ok, then, do you think that the period have to be stretched by the number of
rq's sched entity(e.i. rq->cfs.nr_running)? if it is done with rq->cfs.nr_running,
as you can guess, leaf sched entities(e.i. tasks) can have much smaller slice
than sysctl_sched_min_granularity. and some code using sysctl_sched_min_granularity
need to be fixed in addition.

anyway, current code looks broken since it stretching with local cfs's nr_running.
IMHO, it should be stretched with rq->*cfs.nr_running* though leaf tasks can have
very small slice, or it should be stretched with rq->*nr_running* to ensure that
any task can have a slice which can be comparable to sysctl_sched_min_granularity.

what do you think about this concern?

thank you,
byungchul

> 
> 	-Mike
> 
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