Messages in this thread | | | Date | Tue, 12 Jul 2022 11:16:16 -0400 | From | Steven Rostedt <> | Subject | Re: [PATCH v6 1/2] sched/rt: fix bad task migration for rt tasks |
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On Tue, 12 Jul 2022 23:05:05 +0800 Schspa Shi <schspa@gmail.com> 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 > 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>
Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>
-- Steve
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