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

[Yicong Yang]

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@xxxxxxxxxxxxx>
>>
>> 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@xxxxxxxxxxxxx>
>> ---
>>  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.

> Reviewed-by: Chen Yu <yu.c.chen@xxxxxxxxx>

Thanks,
Yicong

> 
> thanks,
> Chenyu 
> 
> .
>