Re: [PATCH] sched/fair: Don't balance migration disabled tasks

[Yicong Yang <yangyicong@huawei.com>]

Hi Chenyu,

On 2023/3/16 14:43, Chen Yu wrote:
> On 2023-03-15 at 17:55:13 +0800, Yicong Yang wrote:
>> On 2023/3/14 11:08, Chen Yu wrote:
>>> On 2023-03-13 at 14:57:59 +0800, Yicong Yang wrote:
>>>> From: Yicong Yang <yangyicong@hisilicon.com>
>>>>
>>>> On load balance we didn't check whether the candidate task is migration
>>>> disabled or not, this may hit the WARN_ON in set_task_cpu() since the
>>>> migration disabled tasks are expected to run on their current CPU.
>>>> We've run into this case several times on our server:
>>>>
>>>>  ------------[ cut here ]------------
>>>>  WARNING: CPU: 7 PID: 0 at kernel/sched/core.c:3115 set_task_cpu+0x188/0x240
>>>>  Modules linked in: hclgevf xt_CHECKSUM ipt_REJECT nf_reject_ipv4 <...snip>
>>>>  CPU: 7 PID: 0 Comm: swapper/7 Kdump: loaded Tainted: G           O       6.1.0-rc4+ #1
>>>>  Hardware name: Huawei TaiShan 2280 V2/BC82AMDC, BIOS 2280-V2 CS V5.B221.01 12/09/2021
>>>>  pstate: 604000c9 (nZCv daIF +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
>>>>  pc : set_task_cpu+0x188/0x240
>>>>  lr : load_balance+0x5d0/0xc60
>>>>  sp : ffff80000803bc70
>>>>  x29: ffff80000803bc70 x28: ffff004089e190e8 x27: ffff004089e19040
>>>>  x26: ffff007effcabc38 x25: 0000000000000000 x24: 0000000000000001
>>>>  x23: ffff80000803be84 x22: 000000000000000c x21: ffffb093e79e2a78
>>>>  x20: 000000000000000c x19: ffff004089e19040 x18: 0000000000000000
>>>>  x17: 0000000000001fad x16: 0000000000000030 x15: 0000000000000000
>>>>  x14: 0000000000000003 x13: 0000000000000000 x12: 0000000000000000
>>>>  x11: 0000000000000001 x10: 0000000000000400 x9 : ffffb093e4cee530
>>>>  x8 : 00000000fffffffe x7 : 0000000000ce168a x6 : 000000000000013e
>>>>  x5 : 00000000ffffffe1 x4 : 0000000000000001 x3 : 0000000000000b2a
>>>>  x2 : 0000000000000b2a x1 : ffffb093e6d6c510 x0 : 0000000000000001
>>>>  Call trace:
>>>>   set_task_cpu+0x188/0x240
>>>>   load_balance+0x5d0/0xc60
>>>>   rebalance_domains+0x26c/0x380
>>>>   _nohz_idle_balance.isra.0+0x1e0/0x370
>>>>   run_rebalance_domains+0x6c/0x80
>>>>   __do_softirq+0x128/0x3d8
>>>>   ____do_softirq+0x18/0x24
>>>>   call_on_irq_stack+0x2c/0x38
>>>>   do_softirq_own_stack+0x24/0x3c
>>>>   __irq_exit_rcu+0xcc/0xf4
>>>>   irq_exit_rcu+0x18/0x24
>>>>   el1_interrupt+0x4c/0xe4
>>>>   el1h_64_irq_handler+0x18/0x2c
>>>>   el1h_64_irq+0x74/0x78
>>>>   arch_cpu_idle+0x18/0x4c
>>>>   default_idle_call+0x58/0x194
>>>>   do_idle+0x244/0x2b0
>>>>   cpu_startup_entry+0x30/0x3c
>>>>   secondary_start_kernel+0x14c/0x190
>>>>   __secondary_switched+0xb0/0xb4
>>>>  ---[ end trace 0000000000000000 ]---
>>>>
>>>> Signed-off-by: Yicong Yang <yangyicong@hisilicon.com>
>>>> ---
>>>>  kernel/sched/fair.c | 4 ++++
>>>>  1 file changed, 4 insertions(+)
>>>>
>>>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>>>> index 7a1b1f855b96..8fe767362d22 100644
>>>> --- a/kernel/sched/fair.c
>>>> +++ b/kernel/sched/fair.c
>>>> @@ -8433,6 +8433,10 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env)
>>>>  	if (kthread_is_per_cpu(p))
>>>>  		return 0;
>>>>  
>>>> +	/* Migration disabled tasks need to be kept on their running CPU. */
>>>> +	if (is_migration_disabled(p))
>>>> +		return 0;
>>>> +
>>>>  	if (!cpumask_test_cpu(env->dst_cpu, p->cpus_ptr)) {
>>>>  		int cpu;
>>>>  
>>>> -- 
>>>> 2.24.0
>>>>
>>> Looks reasonable to me. Would it be possible to also update the comments at the beginning of
>>> can_migrate_task() starts with: "We do not migrate tasks that are:"
>>>
>>
>> Thanks for the suggestion! It seems only uncommented conditions are summarized in that graph,
>> otherwise they're mentioned close to there branch like kthread_is_per_cpu(p) case. I can add
>> it in v2 if you think it'll be useful.
>>
> It seems that I overlooked migrate_disable(). It can only set current task rather than arbitrary task.
> As Valentin described in his reply, I'm also thinking of what type of race condition can trigger this.
> Are you refering to something like this:
> cpu1                                    cpu2
> load_balance
>   rq_lock(cpu2);
>   detach_task(cpu2, p)
>     can_migrate_task(p) returns true
> 					migrate_disable(current=p)
>     set_task_cpu(p, cpu1);
>       WARN(p can not migrate)
> But can_migrate_task(p) should return 0 because p is always the current one as
> long as the rq_lock been taken by cpu1.
> 

Yes it's right the current checks should avoid the issue. As I replied to Valentin there maybe
other reasons and needs to further check.

Thanks,
Yicong