Re: [PATCH] sched/fair: Don't balance task to its current running CPU

[Vincent Guittot]

On Wed, 24 May 2023 at 09:21, Yicong Yang <yangyicong@xxxxxxxxxx> wrote:
>
> From: Yicong Yang <yangyicong@xxxxxxxxxxxxx>
>
> We've run into the case that the balancer tries to balance a migration
> disabled task and trigger the warning in set_task_cpu() like below:
>
>  ------------[ 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 ]---
>
> Further investigation shows that the warning is superfluous, the migration
> disabled task is just going to be migrated to its current running CPU.
> This is because that on load balance if the dst_cpu is not allowed by the
> task, we'll re-select a new_dst_cpu as a candidate. If no task can be
> balanced to dst_cpu we'll try to balance the task to the new_dst_cpu
> instead. In this case when the migration disabled task is not on CPU it
> only allows to run on its current CPU, load balance will select its
> current CPU as new_dst_cpu and later triggers the the warning above.
>
> This patch tries to solve this by not select the task's current running
> CPU as new_dst_cpu in the load balance.
>
> Signed-off-by: Yicong Yang <yangyicong@xxxxxxxxxxxxx>
> ---
> Thanks Valentin for the knowledge of migration disable. Previous discussion can
> be found at
> https://lore.kernel.org/all/20230313065759.39698-1-yangyicong@xxxxxxxxxx/
>
>  kernel/sched/fair.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 7a1b1f855b96..3c4f3a244c1d 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -8456,7 +8456,8 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env)
>
>                 /* Prevent to re-select dst_cpu via env's CPUs: */
>                 for_each_cpu_and(cpu, env->dst_grpmask, env->cpus) {
> -                       if (cpumask_test_cpu(cpu, p->cpus_ptr)) {
> +                       if (cpumask_test_cpu(cpu, p->cpus_ptr) &&
> +                           cpu != env->src_cpu) {

So I'm a bit surprised that src_cpu can be part of the dst_grpmask and
selected as new_dst_cpu. The only reason would be some numa
overlapping domains. Is it the case for you ?

>                                 env->flags |= LBF_DST_PINNED;
>                                 env->new_dst_cpu = cpu;
>                                 break;
> --
> 2.24.0
>