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

[Schspa Shi]

Dietmar Eggemann <dietmar.eggemann@xxxxxxx> writes:

> 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?

This is the logs from 5.10.59-rt
Link: https://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-stable-rt.git
v5.10.59-rt52 (9007b684f615750b0ee4ec57b5e547a4bf4a223e).

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

But the master code have this BUG too.

>> 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?
>

It's because thie task is not migrated, it just get scheduled and
calling migrate_disable(); and then got preempted by it's CPU core
before enable migrate_enable(). the task_rq not changed in this
scenarios.

> [...]

-- 
BRs
Schspa Shi