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

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Date:	Fri, 3 Jul 2015 02:42:34 +0800
From:	Yuyang Du <yuyang.du@...el.com>
To:	Morten Rasmussen <morten.rasmussen@....com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	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 12:44:55PM +0100, Morten Rasmussen wrote:
> 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.

Yes, this is agreed at all time. Like I said load_balance() (for idle balancing)
should compute the right imbalance and move a fair amount, to achieve we are
balanced. Whatever is wrong in how much computed and moved "right imbalance" is
should be fixed anyway.

But still, I think, even with the above, in idle balancing, pulling until the source
rq's nr_running == 1 is not just "a short term fix", but should be there permanently
acting like a last guard with no overhead, why not.
 
> 
> > 
> > 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.

Thanks,
Yuyang
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