netdev - Re: [PATCH net] net: fix raising a softirq on the current cpu with rps enabled

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Message-ID: <CANn89iJaVrObJNDC9TrnSUC3XQeo-zfmUXLVrNVcsbRDPuSNtA@mail.gmail.com>
Date:   Sat, 25 Mar 2023 08:56:54 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     Jason Xing <kerneljasonxing@...il.com>
Cc:     davem@...emloft.net, kuba@...nel.org, pabeni@...hat.com,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        Jason Xing <kernelxing@...cent.com>
Subject: Re: [PATCH net] net: fix raising a softirq on the current cpu with
 rps enabled

On Sat, Mar 25, 2023 at 8:26 AM Jason Xing <kerneljasonxing@...il.com> wrote:
>
> From: Jason Xing <kernelxing@...cent.com>
>
> Since we decide to put the skb into a backlog queue of another
> cpu, we should not raise the softirq for the current cpu. When
> to raise a softirq is based on whether we have more data left to
> process later. As to the current cpu, there is no indication of
> more data enqueued, so we do not need this action. After enqueuing
> to another cpu, net_rx_action() function will call ipi and then
> another cpu will raise the softirq as expected.
>
> Also, raising more softirqs which set the corresponding bit field
> can make the IRQ mechanism think we probably need to start ksoftirqd
> on the current cpu. Actually it shouldn't happen.
>
> Fixes: 0a9627f2649a ("rps: Receive Packet Steering")
> Signed-off-by: Jason Xing <kernelxing@...cent.com>
> ---
>  net/core/dev.c | 2 --
>  1 file changed, 2 deletions(-)
>
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 1518a366783b..bfaaa652f50c 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -4594,8 +4594,6 @@ static int napi_schedule_rps(struct softnet_data *sd)
>         if (sd != mysd) {
>                 sd->rps_ipi_next = mysd->rps_ipi_list;
>                 mysd->rps_ipi_list = sd;
> -
> -               __raise_softirq_irqoff(NET_RX_SOFTIRQ);
>                 return 1;
>         }
>  #endif /* CONFIG_RPS */
> --
> 2.37.3
>

This is not going to work in some cases. Please take a deeper look.

I have to run, if you (or others) do not find the reason, I will give
more details when I am done traveling.