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

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Message-ID: <CAADnVQL5dt7_S-zFSh-ps7uPfL2ofYs0vo1fFuFBwiz0=DV2Vw@mail.gmail.com>
Date: Fri, 14 Feb 2025 19:51:46 -0800
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Changwoo Min <changwoo@...lia.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

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.

What is sizeof(struct bpf_cpumask) in your system?

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 ?
That would be the only explanation.