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

[Peter Zijlstra]

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.

Ah, we should be able to find this by looking at the cpuset cgroup
information. The cpuset cgroup knows the available cpumask of this task,
which we can translate to the correct root domain in two separate ways
(either run up the cpuset cgroup hierarchy and find the highest domain
with balancing enabled, or look at whatever the rq->rd is for any one of
the allowed CPUs of the immediate cgroup this task belongs to).
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