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

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Date:	Thu, 2 Jul 2015 09:05:39 +0800
From:	Yuyang Du <yuyang.du@...el.com>
To:	Mike Galbraith <umgwanakikbuti@...il.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Rabin Vincent <rabin.vincent@...s.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>,
	Morten Rasmussen <morten.rasmussen@....com>
Subject: Re: [PATCH?] Livelock in pick_next_task_fair() / idle_balance()

Hi Mike,

On Thu, Jul 02, 2015 at 10:05:47AM +0200, Mike Galbraith wrote:
> On Thu, 2015-07-02 at 07:25 +0800, Yuyang Du wrote:
> 
> > 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.
> 
> Yeah, but that's just the symptom, not the disease.  Better for the idle
> balance symptom may actually be to only pull one when idle balancing.
> After all, the immediate goal is to find something better to do than
> idle, not to achieve continual perfect (is the enemy of good) balance.
> 
Symptom? :)

You mean "pull one and stop, can't be greedy"? Right, but still need to
assure you don't make another idle CPU (meaning until nr_running == 1), which
is the cure to disease.

I am ok with at most "pull one", but probably we stick to the load_balance()
by pulling an fair amount, assuming load_balance() magically computes the
right imbalance, otherwise you may have to do multiple "pull one"s.

Thanks,
Yuyang
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