[PATCH v2] sched/rt: fix bad task migration for rt tasks
From: Schspa Shi
Date: Mon Jun 27 2022 - 11:41:15 EST
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@xxxxxxxxx>
--
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.
---
kernel/sched/core.c | 6 ++++++
kernel/sched/rt.c | 9 +++++++++
2 files changed, 15 insertions(+)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index da0bf6fe9ecd..3003baa28d2d 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 8c9ed9664840..6a1efccdc122 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -2115,6 +2115,15 @@ static int push_rt_task(struct rq *rq, bool pull)
if (WARN_ON(next_task == rq->curr))
return 0;
+ /*
+ * 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)))
+ goto retry;
+
/* We might release rq lock */
get_task_struct(next_task);
--
2.24.3 (Apple Git-128)