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

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Date:	Fri, 10 Jul 2015 06:34:02 +0800
From:	Yuyang Du <yuyang.du@...el.com>
To:	Morten Rasmussen <morten.rasmussen@....com>
Cc:	Mike Galbraith <umgwanakikbuti@...il.com>,
	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>
Subject: Re: [PATCH?] Livelock in pick_next_task_fair() / idle_balance()

On Thu, Jul 09, 2015 at 02:53:14PM +0100, Morten Rasmussen wrote:
> On Mon, Jul 06, 2015 at 04:12:41AM +0800, Yuyang Du wrote:
> > Hi Morten,
> > 
> > On Fri, Jul 03, 2015 at 10:34:41AM +0100, Morten Rasmussen wrote:
> > > > > IOW, since task groups include blocked load in the load_avg_contrib (see
> > > > > __update_group_entity_contrib() and __update_cfs_rq_tg_load_contrib()) the
> > > > > imbalance includes blocked load and hence env->imbalance >=
> > > > > sum(task_h_load(p)) for all tasks p on the rq. Which leads to
> > > > > detach_tasks() emptying the rq completely in the reported scenario where
> > > > > blocked load > runnable load.
> > > > 
> > > > Whenever I want to know the load avg concerning task group, I need to
> > > > walk through the complete codes again, I prefer not to do it this time.
> > > > But it should not be that simply to say "the 118 comes from the blocked load".
> > > 
> > > But the whole hierarchy of group entities is updated each time we enqueue
> > > or dequeue a task. I don't see how the group entity load_avg_contrib is
> > > not up to date? Why do you need to update it again?
> > > 
> > > In any case, we have one task in the group hierarchy which has a
> > > load_avg_contrib of 0 and the grand-grand parent group entity has a
> > > load_avg_contrib of 118 and no additional tasks. That load contribution
> > > must be from tasks which are no longer around on the rq? No?
> > 
> > load_avg_contrib has WEIGHT inside, so the most I can say is:
> > SE: 8f456e00's load_avg_contrib 118 = (its cfs_rq's runnable + blocked) / (tg->load_avg + 1) * tg->shares
> > 
> > The tg->shares is probably 1024 (at least 911). So we are just left with:
> > 
> > cfs_rq / tg = 11.5%
> 
> Yes, we also know that there is only one runnable task in the task group
> hierarchy and its contribution is 0. Hence the rest must be from
> non-runnable tasks belonging to some child group.

Agreed.
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