Re: [PATCH 1/5] sched/deadline: Fix double accounting of rq/running bw in push_dl_task()

[luca abeni <luca.abeni@santannapisa.it>]

Hi Dietmar,

On Fri, 26 Jul 2019 09:27:52 +0100
Dietmar Eggemann <dietmar.eggemann@arm.com> wrote:

> push_dl_task() always calls deactivate_task() with flags=0 which sets
> p->on_rq=TASK_ON_RQ_MIGRATING.

Uhm... This is a recent change in the deactivate_task() behaviour,
right? Because I tested SCHED_DEADLINE a lot, but I've never seen this
issue :)


Anyway, looking at the current code the change looks OK. Thanks for
fixing this issue!


			Luca

> push_dl_task()->deactivate_task()->dequeue_task()->dequeue_task_dl()
> calls sub_[running/rq]_bw() since p->on_rq=TASK_ON_RQ_MIGRATING.
> So sub_[running/rq]_bw() in push_dl_task() is double-accounting for
> that task.
> 
> The same is true for add_[rq/running]_bw() and activate_task() on the
> destination (later) CPU.
> push_dl_task()->activate_task()->enqueue_task()->enqueue_task_dl()
> calls add_[rq/running]_bw() again since p->on_rq is still set to
> TASK_ON_RQ_MIGRATING.
> So the add_[rq/running]_bw() in enqueue_task_dl() is double-accounting
> for that task.
> 
> Fix this by removing the rq/running bw accounting in push_dl_task().
> 
> Trace (CONFIG_SCHED_DEBUG=y) before the fix on a 6 CPUs system with 6
> DL (12000, 100000, 100000) tasks showing the issue:
> 
> [   48.147868] dl_rq->running_bw > old
> [   48.147886] WARNING: CPU: 1 PID: 0 at kernel/sched/deadline.c:98
> ...
> [   48.274832]  inactive_task_timer+0x468/0x4e8
> [   48.279057]  __hrtimer_run_queues+0x10c/0x3b8
> [   48.283364]  hrtimer_interrupt+0xd4/0x250
> [   48.287330]  tick_handle_oneshot_broadcast+0x198/0x1d0
> ...
> [   48.360057] dl_rq->running_bw > dl_rq->this_bw
> [   48.360065] WARNING: CPU: 1 PID: 0 at kernel/sched/deadline.c:86
> ...
> [   48.488294]  task_contending+0x1a0/0x208
> [   48.492172]  enqueue_task_dl+0x3b8/0x970
> [   48.496050]  activate_task+0x70/0xd0
> [   48.499584]  ttwu_do_activate+0x50/0x78
> [   48.503375]  try_to_wake_up+0x270/0x7a0
> [   48.507167]  wake_up_process+0x14/0x20
> [   48.510873]  hrtimer_wakeup+0x1c/0x30
> ...
> [   50.062867] dl_rq->this_bw > old
> [   50.062885] WARNING: CPU: 1 PID: 2048 at
> kernel/sched/deadline.c:122 ...
> [   50.190520]  dequeue_task_dl+0x1e4/0x1f8
> [   50.194400]  __sched_setscheduler+0x1d0/0x860
> [   50.198707]  _sched_setscheduler+0x74/0x98
> [   50.202757]  do_sched_setscheduler+0xa8/0x110
> [   50.207065]  __arm64_sys_sched_setscheduler+0x1c/0x30
> 
> Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
> ---
>  kernel/sched/deadline.c | 4 ----
>  1 file changed, 4 deletions(-)
> 
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index de2bd006fe93..d1aeada374e1 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -2121,17 +2121,13 @@ static int push_dl_task(struct rq *rq)
>  	}
>  
>  	deactivate_task(rq, next_task, 0);
> -	sub_running_bw(&next_task->dl, &rq->dl);
> -	sub_rq_bw(&next_task->dl, &rq->dl);
>  	set_task_cpu(next_task, later_rq->cpu);
> -	add_rq_bw(&next_task->dl, &later_rq->dl);
>  
>  	/*
>  	 * Update the later_rq clock here, because the clock is used
>  	 * by the cpufreq_update_util() inside __add_running_bw().
>  	 */
>  	update_rq_clock(later_rq);
> -	add_running_bw(&next_task->dl, &later_rq->dl);
>  	activate_task(later_rq, next_task, ENQUEUE_NOCLOCK);
>  	ret = 1;
>