netdev - Re: [RFC PATCH net-next] net: add an entry for CONFIG_NET_RX_BUSY

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Message-ID: <CANn89i+dYsvrVwWCRX=B1ZyL3nZUjnNtaQ5rfizDOV5XhHV2dQ@mail.gmail.com>
Date: Tue, 23 Jul 2024 16:57:19 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: Jason Xing <kerneljasonxing@...il.com>
Cc: davem@...emloft.net, kuba@...nel.org, pabeni@...hat.com, horms@...nel.org, 
	netdev@...r.kernel.org, Jason Xing <kernelxing@...cent.com>
Subject: Re: [RFC PATCH net-next] net: add an entry for CONFIG_NET_RX_BUSY_POLL

On Tue, Jul 23, 2024 at 3:57 PM Jason Xing <kerneljasonxing@...il.com> wrote:
>
> From: Jason Xing <kernelxing@...cent.com>
>
> When I was doing performance test on unix_poll(), I found out that
> accessing sk->sk_ll_usec when calling sock_poll()->sk_can_busy_loop()
> occupies too much time, which causes around 16% degradation. So I
> decided to turn off this config, which cannot be done apparently
> before this patch.

Too many CONFIG_ options, distros will enable it anyway.

In my builds, offset of sk_ll_usec is 0xe8.

Are you using some debug options or an old tree ?

I can not understand how a 16% degradation can occur, reading a field
in a cache line which contains read mostly fields for af_unix socket.

I think you need to provide more details / analysis, and perhaps come
to a different conclusion.

>
> Signed-off-by: Jason Xing <kernelxing@...cent.com>
> ---
> More data not much related if you're interested:
>   5.82 │      mov   0x18(%r13),%rdx
>   0.03 │      mov   %rsi,%r12
>   1.76 │      mov   %rdi,%rbx
>        │    sk_can_busy_loop():
>   0.50 │      mov   0x104(%rdx),%r14d
>  41.30 │      test  %r14d,%r14d
> Note: I run 'perf record -e  L1-dcache-load-misses' to diagnose
> ---
>  net/Kconfig | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/net/Kconfig b/net/Kconfig
> index d27d0deac0bf..1f1b793984fe 100644
> --- a/net/Kconfig
> +++ b/net/Kconfig
> @@ -335,8 +335,10 @@ config CGROUP_NET_CLASSID
>           being used in cls_cgroup and for netfilter matching.
>
>  config NET_RX_BUSY_POLL
> -       bool
> +       bool "Low latency busy poll timeout"
>         default y if !PREEMPT_RT || (PREEMPT_RT && !NETCONSOLE)
> +       help
> +         Approximate time in us to spin waiting for packets on the device queue.

Wrong comment. It is a y/n choice, no 'usec' at this stage.

>
>  config BQL
>         bool
> --
> 2.37.3
>