linux-kernel - Re: [PATCH bpf-next] bpf: Add a retry after refilling the free list when unit

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Message-ID: <a08493a4-632f-d66e-db26-727c9cf9e6c6@huaweicloud.com>
Date: Mon, 17 Feb 2025 10:19:53 +0800
From: Hou Tao <houtao@...weicloud.com>
To: Changwoo Min <changwoo@...lia.com>,
 Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: Alexei Starovoitov <ast@...nel.org>,
 Daniel Borkmann <daniel@...earbox.net>, Andrii Nakryiko <andrii@...nel.org>,
 Martin KaFai Lau <martin.lau@...ux.dev>, Eddy Z <eddyz87@...il.com>,
 Song Liu <song@...nel.org>, Yonghong Song <yonghong.song@...ux.dev>,
 John Fastabend <john.fastabend@...il.com>, KP Singh <kpsingh@...nel.org>,
 Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>,
 Jiri Olsa <jolsa@...nel.org>, Tejun Heo <tj@...nel.org>,
 Andrea Righi <arighi@...dia.com>, kernel-dev@...lia.com,
 bpf <bpf@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH bpf-next] bpf: Add a retry after refilling the free list
 when unit_alloc() fails

Hi,

On 2/17/2025 12:04 AM, Changwoo Min wrote:
> Hello,
>
> >  > What is sizeof(struct bpf_cpumask) in your system?
> >
> > In my system, sizeof(struct bpf_cpumask) is 1032.
> It was a wrong number. sizeof(struct bpf_cpumask) is actually 16.
>
> On 25. 2. 16. 00:16, Changwoo Min wrote:
>> Hello,
>>
>> On 25. 2. 15. 12:51, Alexei Starovoitov wrote:
>>  > On Fri, Feb 14, 2025 at 1:24 AM Changwoo Min <changwoo@...lia.com>
>> wrote:
>>  >>
>>  >> Hello Alexei,
>>  >>
>>  >> Thank you for the comments! I reordered your comments for ease of
>>  >> explanation.
>>  >>
>>  >> On 25. 2. 14. 02:45, Alexei Starovoitov wrote:
>>  >>> On Wed, Feb 12, 2025 at 12:49 AM Changwoo Min
>> <changwoo@...lia.com> wrote:
>>  >>
>>  >>> The commit log is too terse to understand what exactly is going on.
>>  >>> Pls share the call stack. What is the allocation size?
>>  >>> How many do you do in a sequence?
>>  >>
>>  >> The symptom is that an scx scheduler (scx_lavd) fails to load on
>>  >> an ARM64 platform on its first try. The second try succeeds. In
>>  >> the failure case, the kernel spits the following messages:
>>  >>
>>  >> [   27.431380] sched_ext: BPF scheduler "lavd" disabled (runtime
>> error)
>>  >> [   27.431396] sched_ext: lavd: ops.init() failed (-12)
>>  >> [   27.431401]    scx_ops_enable.isra.0+0x838/0xe48
>>  >> [   27.431413]    bpf_scx_reg+0x18/0x30
>>  >> [   27.431418]    bpf_struct_ops_link_create+0x144/0x1a0
>>  >> [   27.431427]    __sys_bpf+0x1560/0x1f98
>>  >> [   27.431433]    __arm64_sys_bpf+0x2c/0x80
>>  >> [   27.431439]    do_el0_svc+0x74/0x120
>>  >> [   27.431446]    el0_svc+0x80/0xb0
>>  >> [   27.431454]    el0t_64_sync_handler+0x120/0x138
>>  >> [   27.431460]    el0t_64_sync+0x174/0x178
>>  >>
>>  >> The ops.init() failed because the 5th bpf_cpumask_create() calls
>>  >> failed during the initialization of the BPF scheduler. The exact
>>  >> point where bpf_cpumask_create() failed is here [1]. That scx
>>  >> scheduler allocates 5 CPU masks to aid its scheduling decision.
>>  >
>>  > ...
>>  >
>>  >> In this particular scenario, the IRQ is not disabled. I just
>>  >
>>  > since irq-s are not disabled the unit_alloc() should have done:
>>  >          if (cnt < c->low_watermark)
>>  >                  irq_work_raise(c);
>>  >
>>  > and alloc_bulk() should have started executing after the first
>>  > calloc_cpumask(&active_cpumask);
>>  > to refill it from 3 to 64
>>
>> Is there any possibility that irq_work is not scheduled right away on
>> aarch64?

It is a IPI. I think its priority is higher than the current process.
>>
>>  >
>>  > What is sizeof(struct bpf_cpumask) in your system?
>>
>> In my system, sizeof(struct bpf_cpumask) is 1032.
>It was a wrong number. sizeof(struct bpf_cpumask) is actually 16.

It is indeed strange. The former guess is that bpf_cpumask may be
greater than 4K, so the refill in irq work may fail due to memory
fragment, but the allocation size is tiny.
>>
>>  >
>>  > Something doesn't add up. irq_work_queue() should be
>>  > instant when irq-s are not disabled.
>>  > This is not IRQ_WORK_LAZY.> Are you running PREEMPT_RT ?
>>
>> No, CONFIG_PREEMPT_RT is not set.

Could you please share the kernel .config file and the kernel version
for the problem ? And if you are running the test in a QEMU, please also
share the command line to run the qemu.
>>
>> Regards,
>> Changwoo Min
>>
>>
>
>
> .