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).
