linux-kernel - Re: [PATCH] net: skip RPS if packet is already on target CPU

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Message-ID: <CANn89iLx-4dTB9fFgfrsXQ8oA0Z+TpBWNk4b91PPS1o=oypuBQ@mail.gmail.com>
Date: Tue, 29 Oct 2024 20:02:03 +0100
From: Eric Dumazet <edumazet@...gle.com>
To: Caleb Sander Mateos <csander@...estorage.com>
Cc: "David S. Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>, Simon Horman <horms@...nel.org>, netdev@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] net: skip RPS if packet is already on target CPU

On Tue, Oct 29, 2024 at 7:27 PM Caleb Sander Mateos
<csander@...estorage.com> wrote:
>
> If RPS is enabled, all packets with a CPU flow hint are enqueued to the
> target CPU's input_pkt_queue and process_backlog() is scheduled on that
> CPU to dequeue and process the packets. If ARFS has already steered the
> packets to the correct CPU, this additional queuing is unnecessary and
> the spinlocks involved incur significant CPU overhead.
>
> In netif_receive_skb_internal() and netif_receive_skb_list_internal(),
> check if the CPU flow hint get_rps_cpu() returns is the current CPU. If
> so, bypass input_pkt_queue and immediately process the packet(s) on the
> current CPU.
>
> Signed-off-by: Caleb Sander Mateos <csander@...estorage.com>

Current implementation was a conscious choice. This has been discussed
several times.

By processing packets inline, you are actually increasing latencies of
packets queued to other cpus.