Messages in this thread | | | From | Vincent Guittot <> | Date | Fri, 26 May 2023 12:34:38 +0200 | Subject | Re: [PATCH] sched/fair: Don't balance task to its current running CPU |
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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
> >> env->flags |= LBF_DST_PINNED; > >> env->new_dst_cpu = cpu; > >> break; > >> -- > >> 2.24.0 > >> > > . > >
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