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

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Message-ID: <6632e26d-996c-432e-956f-5be178722e5b@igalia.com>
Date: Sun, 16 Feb 2025 00:16:43 +0900
From: Changwoo Min <changwoo@...lia.com>
To: 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

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?

 >
 > What is sizeof(struct bpf_cpumask) in your system?

In my system, sizeof(struct bpf_cpumask) is 1032.

 >
 > 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.

Regards,
Changwoo Min