linux-kernel - Re: [GIT PULL v2] Networking for 6.7

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Message-ID: <11e2e744-4bc7-45b1-aaca-298b5e4ee281@linux.dev>
Date:   Thu, 9 Nov 2023 10:09:46 -0800
From:   Yonghong Song <yonghong.song@...ux.dev>
To:     "Kirill A. Shutemov" <kirill@...temov.name>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Hou Tao <houtao1@...wei.com>, Jakub Kicinski <kuba@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "David S. Miller" <davem@...emloft.net>,
        Network Development <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Paolo Abeni <pabeni@...hat.com>, yonghong.song@...ux.dev
Subject: Re: [GIT PULL v2] Networking for 6.7


On 11/9/23 8:14 AM, Kirill A. Shutemov wrote:
> On Thu, Nov 09, 2023 at 08:01:39AM -0800, Alexei Starovoitov wrote:
>> On Thu, Nov 9, 2023 at 7:49 AM Kirill A. Shutemov <kirill@...temov.name> wrote:
>>> On Tue, Oct 31, 2023 at 02:09:48PM -0700, Jakub Kicinski wrote:
>>>>        bpf: Add support for non-fix-size percpu mem allocation
>>> Recent changes in BPF increased per-CPU memory consumption a lot.
>>>
>>> On virtual machine with 288 CPUs, per-CPU consumtion increased from 111 MB
>>> to 969 MB, or 8.7x.
>>>
>>> I've bisected it to the commit 41a5db8d8161 ("bpf: Add support for
>>> non-fix-size percpu mem allocation"), which part of the pull request.
>> Hmm. This is unexpected. Thank you for reporting.
>>
>> How did you measure this 111 MB vs 969 MB ?
>> Pls share the steps to reproduce.
> Boot VMM with 288 (qemu-system-x86_64 -smp 288) and check Percpu: field of
> /proc/meminfo.

I did some experiments with my VM. My VM currently supports up to 255 cpus,
so I tried 4/32/252 number of cpus. For a particular number of cpus, two
experiments are done:
   (1). bpf-percpu-mem-prefill
   (2). no-bpf-percpu-mem-prefill

For 4 cpu:
    bpf-percpu-mem-prefill:
      Percpu:             2000 kB
    no-bpf-percpu-mem-prefill:
      Percpu:             1808 kB

    bpf-percpu-mem-prefill percpu cost: (2000 - 1808)/4 KB = 48KB

For 32 cpus:
    bpf-percpu-mem-prefill:
      Percpu:            25344 kB
    no-bpf-percpu-mem-prefill:
      Percpu:            14464 kB

    bpf-percpu-mem-prefill percpu cost: (25344 - 14464)/4 KB = 340KB

For 252 cpus:
    bpf-percpu-mem-prefill:
      Percpu:           230912 kB
    no-bpf-percpu-mem-prefill:
      Percpu:            57856 kB
  
    bpf-percpu-mem-prefill percpu cost: (230912 - 57856)/4 KB = 686KB

I am not able to reproduce the dramatic number from 111 MB to 969 MB.
My number with 252 cpus is from ~58MB to ~231MB.

I appears that percpu allocation cost goes up when the number of cpus
is increased.

I will continue to debug this. Thanks!

>