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Subject[PATCH 1/5] sched/deadline: Fix double accounting of rq/running bw in push_dl_task()
Date
push_dl_task() always calls deactivate_task() with flags=0 which sets
p->on_rq=TASK_ON_RQ_MIGRATING.
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;

--
2.17.1
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