Messages in this thread Patch in this message | | | From | Schspa Shi <> | Subject | [PATCH v3] sched/rt: fix bad task migration for rt tasks | Date | Fri, 8 Jul 2022 00:50:14 +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 and retry to retry to push the current running task on this CPU away.
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 --- kernel/sched/core.c | 6 ++++++ kernel/sched/rt.c | 11 +++++++++++ 2 files changed, 17 insertions(+)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c index da0bf6fe9ecdc..3003baa28d2d8 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -2512,6 +2512,11 @@ int push_cpu_stop(void *arg) if (!lowest_rq) goto out_unlock; + if (unlikely(is_migration_disabled(p))) { + p->migration_flags |= MDF_PUSH; + goto out_double_unlock; + } + // XXX validate p is still the highest prio task if (task_rq(p) == rq) { deactivate_task(rq, p, 0); @@ -2520,6 +2525,7 @@ int push_cpu_stop(void *arg) resched_curr(lowest_rq); } +out_double_unlock: double_unlock_balance(rq, lowest_rq); out_unlock: diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c index 8c9ed96648409..7948c9cfb472d 100644 --- a/kernel/sched/rt.c +++ b/kernel/sched/rt.c @@ -2153,6 +2153,17 @@ static int push_rt_task(struct rq *rq, bool pull) goto retry; } + /* + * It is possible the task has running for a while, we need to check + * task migration disable flag again. If task migration is disabled, + * the retry code will retry to push the current running task on this + * CPU away. + */ + if (unlikely(is_migration_disabled(next_task))) { + put_task_struct(next_task); + goto retry; + } + deactivate_task(rq, next_task, 0); set_task_cpu(next_task, lowest_rq->cpu); activate_task(lowest_rq, next_task, 0); -- 2.37.0
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