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

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Message-ID: <20150702105359.GY19282@twins.programming.kicks-ass.net>
Date:	Thu, 2 Jul 2015 12:53:59 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Yuyang Du <yuyang.du@...el.com>
Cc:	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>,
	Morten Rasmussen <morten.rasmussen@....com>
Subject: Re: [PATCH?] Livelock in pick_next_task_fair() / idle_balance()

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.

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

> On Wed, Jul 01, 2015 at 10:44:04PM +0200, Peter Zijlstra wrote:
> > On Wed, Jul 01, 2015 at 04:55:51PM +0200, Rabin Vincent wrote:
> > >  PID: 413    TASK: 8edda408  CPU: 1   COMMAND: "rngd"
> > >   task_h_load():     0 [ = (load_avg_contrib {    0} * cfs_rq->h_load {    0}) / (cfs_rq->runnable_load_avg {    0} + 1) ]
> > >   SE: 8edda450 load_avg_contrib:     0 load.weight:  1024 PARENT: 8fffbd00 GROUPNAME: (null)
> > >   SE: 8fffbd00 load_avg_contrib:     0 load.weight:     2 PARENT: 8f531f80 GROUPNAME: rngd@...ng.service
> > >   SE: 8f531f80 load_avg_contrib:     0 load.weight:  1024 PARENT: 8f456e00 GROUPNAME: system-rngd.slice
> > >   SE: 8f456e00 load_avg_contrib:   118 load.weight:   911 PARENT: 00000000 GROUPNAME: system.slice
> > 
> > Firstly, a group (parent) load_avg_contrib should never be less than
> > that of its constituent parts, therefore the top 3 SEs should have at
> > least 118 too.
> 
> I think the downward is parent,

Ugh, I cannot read. Let me blame it on the heat.
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