linux-kernel - Re: 4.3 group scheduling regression

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Date:	Mon, 12 Oct 2015 11:12:06 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Yuyang Du <yuyang.du@...el.com>
Cc:	Mike Galbraith <umgwanakikbuti@...il.com>,
	linux-kernel@...r.kernel.org
Subject: Re: 4.3 group scheduling regression

On Mon, Oct 12, 2015 at 08:53:51AM +0800, Yuyang Du wrote:
> Good morning, Peter.
> 
> On Mon, Oct 12, 2015 at 10:04:07AM +0200, Peter Zijlstra wrote:
> > On Mon, Oct 12, 2015 at 09:44:57AM +0200, Mike Galbraith wrote:
> > 
> > > It's odd to me that things look pretty much the same good/bad tree with
> > > hogs vs hogs or hogs vs tbench (with top anyway, just adding up times).
> > > Seems Xorg+mplayer more or less playing cross group ping-pong must be
> > > the BadThing trigger.
> >
> > Ohh, wait, Xorg and mplayer are _not_ in the same group? I was assuming
> > you had your entire user session in 1 (auto) group and was competing
> > against 8 manual cgroups.
> > 
> > So how exactly are things configured?
>  
> Hmm... my impression is the naughty boy mplayer (+Xorg) isn't favored, due 
> to the per CPU group entity share distribution. Let me dig more.

So in the old code we had 'magic' to deal with the case where a cgroup
was consuming less than 1 cpu's worth of runtime. For example, a single
task running in the group.

In that scenario it might be possible that the group entity weight:

	se->weight = (tg->shares * cfs_rq->weight) / tg->weight;

Strongly deviates from the tg->shares; you want the single task reflect
the full group shares to the next level; due to the whole distributed
approximation stuff.

I see you've deleted all that code; see the former
__update_group_entity_contrib().

It could be that we need to bring that back. But let me think a little
bit more on this.. I'm having a hard time waking :/
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