Messages in this thread Patch in this message | | | From | Schspa Shi <> | Subject | [PATCH v4 1/2] sched/rt: fix bad task migration for rt tasks | Date | Sat, 9 Jul 2022 05:17:54 +0800 |
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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 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
To fix it, we need to check migration_disabled flag again to avoid bad migration.
Fixes: 95158a89dd50 ("sched,rt: Use the full cpumask for balancing")
Signed-off-by: Schspa Shi <schspa@gmail.com>
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Changelog: v1 -> v2: - Modify commit message to add fixed commit information. - Going to retry to push the current running task on this CPU away, instead doing nothing for this migrate disabled task. v2 -> v3: - Change migration disabled check to the correct position v3 -> v4: - Check migrate disabled in find_lock_lowest_rq to avoid not necessary check when task rq is not released as Steven adviced. --- kernel/sched/deadline.c | 1 + kernel/sched/rt.c | 3 +++ 2 files changed, 4 insertions(+)
diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c index b5152961b7432..cb3b886a081c3 100644 --- a/kernel/sched/deadline.c +++ b/kernel/sched/deadline.c @@ -2238,6 +2238,7 @@ static struct rq *find_lock_later_rq(struct task_struct *task, struct rq *rq) !cpumask_test_cpu(later_rq->cpu, &task->cpus_mask) || task_running(rq, task) || !dl_task(task) || + is_migration_disabled(task) || !task_on_rq_queued(task))) { double_unlock_balance(rq, later_rq); later_rq = NULL; diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c index 8c9ed96648409..0e0bc9aeaa394 100644 --- a/kernel/sched/rt.c +++ b/kernel/sched/rt.c @@ -1998,11 +1998,14 @@ 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 has running for a while, + * And we check task migration disable flag here. */ 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) || !task_on_rq_queued(task))) { double_unlock_balance(rq, lowest_rq); -- 2.37.0
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