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

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Date:	Mon, 13 Jul 2015 14:30:38 +0200
From:	Mike Galbraith <umgwanakikbuti@...il.com>
To:	Byungchul Park <byungchul.park@....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, 2015-07-13 at 20:07 +0900, Byungchul Park wrote:

> i still think stretching with local cfs's nr_running should be replaced with
> stretching with a top(=root) level one.

I think we just can't take 'slice' _too_ seriously.  Not only is it
annoying with cgroups, the scheduler simply doesn't deliver 'slices' in
the traditional sense, it equalizes vruntimes, planning to do that at
slice granularity.  FAIR_SLEEPERS doesn't make that planning any easier.
With a pure compute load and no HR_TICK, what you get is tick
granularity preemption checkpoints, but having just chewed up a 'slice'
means nothing if you're still leftmost.  It's all about vruntime, so
leftmost can have back to back 'slices'.  FAIR_SLEEPERS just increases
the odds that leftmost WILL take more than one 'slice'.

(we could perhaps decay deficit after a full slice or such to decrease
the spread growth that sleepers induce. annoying problem, especially so
with a gaggle of identical sleepers, as sleep time becomes meaningless,
there is no differential to equalize.. other than the ones we create..
but I'm digressing, a lot, time to stop thinking/typing, go do work;)

	-Mike

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