linux-kernel - Re: [PATCH v5] sched/deadline: support dl task migration during cpu hotplug

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Message-ID: <20141112230217.GB23249@kernel>
Date:	Thu, 13 Nov 2014 07:02:17 +0800
From:	Wanpeng Li <wanpeng.li@...ux.intel.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Juri Lelli <juri.lelli@....com>,
	Wanpeng Li <wanpeng.li@...ux.intel.com>,
	Ingo Molnar <mingo@...hat.com>,
	Kirill Tkhai <ktkhai@...allels.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5] sched/deadline: support dl task migration during cpu
 hotplug

Hi Peter,
On Wed, Nov 12, 2014 at 04:39:06PM +0100, Peter Zijlstra wrote:
>On Wed, Nov 12, 2014 at 03:08:44PM +0000, Juri Lelli wrote:
>> > @@ -1185,8 +1223,9 @@ static int find_later_rq(struct task_struct *task)
>> >  	 * We have to consider system topology and task affinity
>> >  	 * first, then we can look for a suitable cpu.
>> >  	 */
>> > -	cpumask_copy(later_mask, task_rq(task)->rd->span);
>> > -	cpumask_and(later_mask, later_mask, cpu_active_mask);
>> > +	cpumask_copy(later_mask, cpu_active_mask);
>> > +	if (likely(task_rq(task)->online))
>> > +		cpumask_and(later_mask, later_mask, task_rq(task)->rd->span);
>> 
>> So, here you consider the span only when the task_rq is online,
>> but there might be others cpus still online belonging to the same
>> rd->span. And you have to consider them when migrating. Actually,
>> migration must still be restricted to the online cpus of task's
>> original rd->span, or I fear you can break clustered scheduling.
>
>Ah, good point that, we must somehow find the right root domain to
>'restore' the task to. Now I'm not entirely sure we still have this.
>Lemme ponder that.

Any idea is a great appreciated. :-)

Regards,
Wanpeng Li 
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