linux-kernel - Re: [PATCH?] Livelock in pick_next_task_fair() / idle

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Message-ID: <20150702114454.GB7598@e105550-lin.cambridge.arm.com>
Date:	Thu, 2 Jul 2015 12:44:55 +0100
From:	Morten Rasmussen <morten.rasmussen@....com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Yuyang Du <yuyang.du@...el.com>,
	Rabin Vincent <rabin.vincent@...s.com>,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Paul Turner <pjt@...gle.com>, Ben Segall <bsegall@...gle.com>
Subject: Re: [PATCH?] Livelock in pick_next_task_fair() / idle_balance()

On Thu, Jul 02, 2015 at 12:53:59PM +0200, Peter Zijlstra wrote:
> On Thu, Jul 02, 2015 at 07:25:11AM +0800, Yuyang Du wrote:
> > And obviously, the idle balancing livelock SHOULD happen: one CPU pulls
> > tasks from the other, makes the other idle, and this iterates...
> > 
> > That being said, it is also obvious to prevent the livelock from happening:
> > idle pulling until the source rq's nr_running is 1, becuase otherwise we
> > just avoid idleness by making another idleness.
> 
> Well, ideally the imbalance calculation would be so that it would avoid
> this from happening in the first place. Its a 'balance' operation, not a
> 'steal everything'.
> 
> We want to take work -- as we have none -- but we want to ensure that
> afterwards we have equal work, ie we're balanced.

Agreed, I think this is the true problem. See my other reply.

> 
> So clearly that all is hosed. Now Morten was looking into simplifying
> calculate_imbalance() recently.

Yes. I'm held up doing other stuff at the moment, but I think
calculate_imbalance() needs some attention and I'm planning on looking at
that next.
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