Re: [PATCH] sched: fix the intention to re-evalute tick dependency for offline cpu

[Juri Lelli <juri.lelli@arm.com>]

On 11/08/16 16:45, Peter Zijlstra wrote:
> On Thu, Aug 04, 2016 at 05:51:20PM +0800, Wanpeng Li wrote:
> > From: Wanpeng Li <wanpeng.li@hotmail.com>
> > 
> > The dl task will be replenished after dl task timer fire and start a new 
> > period. It will be enqueued and to re-evaluate its dependency on the tick 
> > in order to restart it. However, if cpu is hot-unplug, irq_work_queue will 
> > splash since the target cpu is offline.
> > 
> > As a result:
> > 
> >     WARNING: CPU: 2 PID: 0 at kernel/irq_work.c:69 irq_work_queue_on+0xad/0xe0
> >     Call Trace:
> >      dump_stack+0x99/0xd0
> >      __warn+0xd1/0xf0
> >      warn_slowpath_null+0x1d/0x20
> >      irq_work_queue_on+0xad/0xe0
> >      tick_nohz_full_kick_cpu+0x44/0x50
> >      tick_nohz_dep_set_cpu+0x74/0xb0
> >      enqueue_task_dl+0x226/0x480
> >      activate_task+0x5c/0xa0
> >      dl_task_timer+0x19b/0x2c0
> >      ? push_dl_task.part.31+0x190/0x190
> 
> Hurm, so this is after hot-unplug succeeded. We get a timer (which is
> also already migrated), but we enqueue the dl task on the offline CPU,
> because we need to do replenish because start_dl_timer() -- see the
> comment in dl_task_timer() at #ifdef CONFIG_SMP.
> 
> Then, once we've enqueued the task on the offline cpu, do we migrate it.
> 
> Bit icky that, but I don't immediately see a better way.
> 

[...]

> Juri, any opinions?
> 

So, we would need to do something like calling replenish_dl_entity()
directly, instead of enqueue_task_dl(). pi_se shouldn't be a problem as
the task shouldn't be boosted if it was throttled.

dl_task_offline_migration() will still need to deactivate_task(), but
that should be fine as we check RB_EMPTY_NODE() in __dequeue_dl_entity()
and dequeue_pushable_dl_task().

I'm pretty sure that there is something I'm not considering that will
make everything explode, though. :)