Re: [PATCH] sched/fair: vruntime should normalize when switching from fair

[Dietmar Eggemann <dietmar.eggemann@arm.com>]

On 08/27/2018 12:14 PM, Peter Zijlstra wrote:
> On Fri, Aug 24, 2018 at 02:24:48PM -0700, Steve Muckle wrote:
>> On 08/24/2018 02:47 AM, Peter Zijlstra wrote:
>>>>> On 08/17/2018 11:27 AM, Steve Muckle wrote:
>>>
>>>>>> When rt_mutex_setprio changes a task's scheduling class to RT,
>>>>>> we're seeing cases where the task's vruntime is not updated
>>>>>> correctly upon return to the fair class.
>>>
>>>>>> Specifically, the following is being observed:
>>>>>> - task is deactivated while still in the fair class
>>>>>> - task is boosted to RT via rt_mutex_setprio, which changes
>>>>>>      the task to RT and calls check_class_changed.
>>>>>> - check_class_changed leads to detach_task_cfs_rq, at which point
>>>>>>      the vruntime_normalized check sees that the task's state is TASK_WAKING,
>>>>>>      which results in skipping the subtraction of the rq's min_vruntime
>>>>>>      from the task's vruntime
>>>>>> - later, when the prio is deboosted and the task is moved back
>>>>>>      to the fair class, the fair rq's min_vruntime is added to
>>>>>>      the task's vruntime, even though it wasn't subtracted earlier.
>>>
>>> I'm thinking that is an incomplete scenario; where do we get to
>>> TASK_WAKING.
>>
>> Yes there's a missing bit of context here at the beginning that the task to
>> be boosted had already been put into TASK_WAKING.
> 
> See, I'm confused...
> 
> The only time TASK_WAKING is visible, is if we've done a remote wakeup
> and it's 'stuck' on the remote wake_list. And in that case we've done
> migrate_task_rq_fair() on it.
> 
> So by the time either rt_mutex_setprio() or __sched_setscheduler() get
> to calling check_class_changed(), under both pi_lock and rq->lock, the
> vruntime_normalized() thing should be right.
> 
> So please detail the exact scenario. Because I'm not seeing it.

Using Steve's test program (https://lkml.org/lkml/2018/8/24/686) I see the
issue but only if the two tasks (rt_task, fair_task) run on 2 cpus which
don't share LLC (e.g. CPU0 and CPU4 on hikey960).

So the wakeup goes the TTWU_QUEUE && !share_cache (ttwu_queue_remote) path.

...
rt_task-3579  [000] 35.391573: sched_waking:     comm=fair_task pid=3580 prio=120 target_cpu=004
rt_task-3579  [000] 35.391580: bprint:           try_to_wake_up: try_to_wake_up: task=fair_task pid=3580 task_cpu(p)=4 p->on_rq=0
rt_task-3579  [000] 35.391584: bprint:           try_to_wake_up: ttwu_queue: task=fair_task pid=3580
rt_task-3579  [000] 35.391588: bprint:           try_to_wake_up: ttwu_queue_remote: task=fair_task pid=3580
rt_task-3579  [000] 35.391591: bprint:           try_to_wake_up: ttwu_queue_remote: cpu=4 smp_send_reschedule
rt_task-3579  [000] 35.391627: sched_pi_setprio: comm=fair_task pid=3580 oldprio=120 newprio=19
rt_task-3579  [000] 35.391635: bprint:           rt_mutex_setprio: task=fair_task pid=3580 prio=120->19 queued=0 running=0 state=0x200 vruntime=46922871 cpu=4 cfs_rq->min_vruntime=7807420844
rt_task-3579  [000] 35.391641: bprint:           rt_mutex_setprio: p->prio set: task=fair_task pid=3580 prio=19 queued=0 running=0 state=0x200 vruntime=46922871
rt_task-3579  [000] 35.391646: bprint:           rt_mutex_setprio: queued checked: task=fair_task pid=3580 prio=19 queued=0 running=0 state=0x200 vruntime=46922871
rt_task-3579  [000] 35.391652: bprint:           rt_mutex_setprio: running checked: task=fair_task pid=3580 prio=19 queued=0 running=0 state=0x200 vruntime=46922871
rt_task-3579  [000] 35.391657: bprint:           rt_mutex_setprio: fair_class=0xffff000008da2c80 rt_class=0xffff000008da2d70 prev_class=0xffff000008da2c80 p->sched_class=0xffff000008da2d70 oldprio=120 p->prio=19
rt_task-3579  [000] 35.391661: bprint:           detach_task_cfs_rq: task=fair_task pid=3580 cpu=4 vruntime_normalized=1
rt_task-3579  [000] 35.391706: sched_switch:     rt_task:3579 [19] D ==> swapper/0:0 [120] 
 <idle>-0     [004] 35.391828: bprint:           ttwu_do_activate: ttwu_do_activate: task=fair_task pid=3580
 <idle>-0     [004] 35.391832: bprint:           ttwu_do_activate: ttwu_activate: task=fair_task pid=3580
 <idle>-0     [004] 35.391833: bprint:           ttwu_do_wakeup: ttwu_do_wakeup: task=fair_task pid=3580
 <idle>-0     [004] 35.391834: sched_wakeup:     fair_task:3580 [19] success=1 CPU:004

It doesn't happen on hikey960 when I use two cpus of the same LLC or on my
laptop (i7-4750HQ).