Messages in this thread | | | Subject | Re: [PATCH] sched/fair: Don't balance task to its current running CPU | From | Yicong Yang <> | Date | Mon, 29 May 2023 19:02:13 +0800 |
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Hi Vincent,
On 2023/5/26 18:34, Vincent Guittot wrote: > On Fri, 26 May 2023 at 10:18, Yicong Yang <yangyicong@huawei.com> wrote: >> >> On 2023/5/25 23:13, Vincent Guittot wrote: >>> On Wed, 24 May 2023 at 09:21, Yicong Yang <yangyicong@huawei.com> wrote: >>>> >>>> From: Yicong Yang <yangyicong@hisilicon.com> >>>> >>>> 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@hisilicon.com> >>>> --- >>>> Thanks Valentin for the knowledge of migration disable. Previous discussion can >>>> be found at >>>> https://lore.kernel.org/all/20230313065759.39698-1-yangyicong@huawei.com/ >>>> >>>> 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 ? >>> >> >> It's a 2P 4 NUMA machine, the groups in the top NUMA domains are overlapped, for example for CPU64: >> >> [ 3.147038] CPU64 attaching sched-domain(s): >> [ 3.147040] domain-0: span=64-67 level=CLS >> [ 3.147043] groups: 64:{ span=64 cap=1023 }, 65:{ span=65 cap=1023 }, 66:{ span=66 cap=1023 }, 67:{ span=67 } >> [ 3.147056] domain-1: span=64-95 level=MC >> [ 3.147059] groups: 64:{ span=64-67 cap=4093 }, 68:{ span=68-71 cap=4096 }, 72:{ span=72-75 cap=4096 }, 76:{ span=76-79 cap=4096 }, 80:{ span=80-83 cap=4096 }, 84:{ span=84-87 cap=4096 }, 88:{ span=88-91 cap=4096 }, 92:{ span=92-95 cap=4096 } >> [ 3.147085] domain-2: span=64-127 level=NUMA >> [ 3.147087] groups: 64:{ span=64-95 cap=32765 }, 96:{ span=96-127 cap=32767 } >> [ 3.147095] domain-3: span=0-31,64-127 level=NUMA >> [ 3.147098] groups: 64:{ span=64-127 cap=65532 }, 0:{ span=0-31 cap=32767 } >> [ 3.147106] domain-4: span=0-127 level=NUMA >> [ 3.147109] groups: 64:{ span=0-31,64-127 mask=64-95 cap=98300 }, 32:{ span=0-63 mask=32-63 cap=65531 } >> > > Thanks for confirming this. > > So I wonder if a better solution would be to make env->dst_grpmask = > group_balance_cpu(sd->groups) instead of > sched_group_span(sd->groups),. The behavior remains the same for non > overlapping groups because group_balance_cpu(sd->groups) == > sched_group_span(sd->groups) in this case and for overlapping group, > we will try to find a dst_cpu that is not contained in src/busiest > group and the load balance will effectively pull load from the > busiest_group >
I think this make sense to me. We've already limited the dst_cpu within the group_balance_mask(sd->groups) in should_we_balance() for periodical balance (but not for idle balance). The sg->sgc->cpumask is commented as "balance mask", so only the cpus in sg->sgc->cpumask can pull the task in the load balance. The newidle CPU maybe an exception, but also need to limit the new_dst_cpu int the sg->sgc->cpumask.
> >>>> env->flags |= LBF_DST_PINNED; >>>> env->new_dst_cpu = cpu; >>>> break; >>>> -- >>>> 2.24.0 >>>> >>> . >>> > . >
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