Re: [PATCH v6 1/2] sched/rt: fix bad task migration for rt tasks

Dietmar Eggemann
2022-07-12T15:27:08Z

On 12/07/2022 17:05, Schspa Shi wrote:
> Commit 95158a89dd50 ("sched,rt: Use the full cpumask for balancing")
> allow find_lock_lowest_rq to pick a task with migration disabled.
> This commit is intended to push the current running task on this CPU
> away.
>
> There is a race scenarios, which allows a migration disabled task to
> be migrated to another CPU.
>
> When there is a RT task with higher priority, rt sched class was
> intended to migrate higher priority task to lowest rq via push_rt_tasks,
> this BUG will happen here.
>
> With the system running on PREEMPT_RT, rt_spin_lock will disable
> migration, this will make the problem easier to reproduce.
>
> I have seen this crash on PREEMPT_RT, from the logs, there is a race
> when trying to migrate higher priority tasks to the lowest rq.
>
> Please refer to the following scenarios.
>
> CPU0 CPU1
> ------------------------------------------------------------------
> push_rt_task
> check is_migration_disabled(next_task)
> task not running and
> migration_disabled == 0
> find_lock_lowest_rq(next_task, rq);
> _double_lock_balance(this_rq, busiest);
> raw_spin_rq_unlock(this_rq);
> double_rq_lock(this_rq, busiest);
> <<wait for busiest rq>>
> <wakeup>
> task become running
> migrate_disable();
> <context out>
> deactivate_task(rq, next_task, 0);
> set_task_cpu(next_task, lowest_rq->cpu);
> WARN_ON_ONCE(is_migration_disabled(p));
> ---------OOPS-------------
>
> Crash logs as fellowing:
> [123671.996430] WARNING: CPU: 2 PID: 13470 at kernel/sched/core.c:2485

What code-base is this?

IMHO, currently this `WARN_ON_ONCE(is_migration_disabled(p))` in
set_task_cpu() is at > line 3000.

> set_task_cpu+0x8c/0x108
> [123671.996800] pstate: 20400009 (nzCv daif +PAN -UAO -TCO BTYPE=--)
> [123671.996811] pc : set_task_cpu+0x8c/0x108
> [123671.996820] lr : set_task_cpu+0x7c/0x108
> [123671.996828] sp : ffff80001268bd30
> [123671.996832] pmr_save: 00000060
> [123671.996835] x29: ffff80001268bd30 x28: ffff0001a3d68e80
> [123671.996844] x27: ffff80001225f4a8 x26: ffff800010ab62cb
> [123671.996854] x25: ffff80026d95e000 x24: 0000000000000005
> [123671.996864] x23: ffff00019746c1b0 x22: 0000000000000000
> [123671.996873] x21: ffff00027ee33a80 x20: 0000000000000000
> [123671.996882] x19: ffff00019746ba00 x18: 0000000000000000
> [123671.996890] x17: 0000000000000000 x16: 0000000000000000
> [123671.996899] x15: 000000000000000a x14: 000000000000349e
> [123671.996908] x13: ffff800012f4503d x12: 0000000000000001
> [123671.996916] x11: 0000000000000000 x10: 0000000000000000
> [123671.996925] x9 : 00000000000c0000 x8 : ffff00027ee58700
> [123671.996933] x7 : ffff00027ee8da80 x6 : ffff00027ee8e580
> [123671.996942] x5 : ffff00027ee8dcc0 x4 : 0000000000000005
> [123671.996951] x3 : ffff00027ee8e338 x2 : 0000000000000000
> [123671.996959] x1 : 00000000000000ff x0 : 0000000000000002
> [123671.996969] Call trace:
> [123671.996975] set_task_cpu+0x8c/0x108
> [123671.996984] push_rt_task.part.0+0x144/0x184
> [123671.996995] push_rt_tasks+0x28/0x3c
> [123671.997002] task_woken_rt+0x58/0x68
> [123671.997009] ttwu_do_wakeup+0x5c/0xd0
> [123671.997019] ttwu_do_activate+0xc0/0xd4
> [123671.997028] try_to_wake_up+0x244/0x288
> [123671.997036] wake_up_process+0x18/0x24
> [123671.997045] __irq_wake_thread+0x64/0x80
> [123671.997056] __handle_irq_event_percpu+0x110/0x124
> [123671.997064] handle_irq_event_percpu+0x50/0xac
> [123671.997072] handle_irq_event+0x84/0xfc
>

[...]

> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> index 8c9ed96648409..7bd3e6ecbe45e 100644
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -1998,11 +1998,15 @@ static struct rq *find_lock_lowest_rq(struct task_struct *task, struct rq *rq)
> * the mean time, task could have
> * migrated already or had its affinity changed.
> * Also make sure that it wasn't scheduled on its rq.
> + * It is possible the task was scheduled, set
> + * "migrate_disabled" and then got preempted, so we must
> + * check the task migration disable flag here too.
> */
> if (unlikely(task_rq(task) != rq ||
> !cpumask_test_cpu(lowest_rq->cpu, &task->cpus_mask) ||
> task_running(rq, task) ||
> !rt_task(task) ||
> + is_migration_disabled(task) ||

I wonder why this isn't covered by `task_rq(task) != rq` in this condition? …