netdev - Re: [PATCH net-next 4/4] net: optimize ____napi_schedule() to avoid extra NET_RX

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Date:   Thu, 30 Mar 2023 17:50:28 +0800
From:   Jason Xing <kerneljasonxing@...il.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Jason Xing <kernelxing@...cent.com>, netdev@...r.kernel.org,
        eric.dumazet@...il.com
Subject: Re: [PATCH net-next 4/4] net: optimize ____napi_schedule() to avoid
 extra NET_RX_SOFTIRQ

On Wed, Mar 29, 2023 at 7:53 AM Eric Dumazet <edumazet@...gle.com> wrote:
>
> ____napi_schedule() adds a napi into current cpu softnet_data poll_list,
> then raises NET_RX_SOFTIRQ to make sure net_rx_action() will process it.
>
> Idea of this patch is to not raise NET_RX_SOFTIRQ when being called indirectly
> from net_rx_action(), because we can process poll_list from this point,
> without going to full softirq loop.
>
> This needs a change in net_rx_action() to make sure we restart
> its main loop if sd->poll_list was updated without NET_RX_SOFTIRQ
> being raised.
>
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Cc: Jason Xing <kernelxing@...cent.com>
> ---
>  net/core/dev.c | 22 ++++++++++++++++++----
>  1 file changed, 18 insertions(+), 4 deletions(-)
>
> diff --git a/net/core/dev.c b/net/core/dev.c
> index f34ce93f2f02e7ec71f5e84d449fa99b7a882f0c..0c4b21291348d4558f036fb05842dab023f65dc3 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -4360,7 +4360,11 @@ static inline void ____napi_schedule(struct softnet_data *sd,
>         }
>
>         list_add_tail(&napi->poll_list, &sd->poll_list);
> -       __raise_softirq_irqoff(NET_RX_SOFTIRQ);
> +       /* If not called from net_rx_action()
> +        * we have to raise NET_RX_SOFTIRQ.
> +        */
> +       if (!sd->in_net_rx_action)
> +               __raise_softirq_irqoff(NET_RX_SOFTIRQ);
>  }
>
>  #ifdef CONFIG_RPS
> @@ -6648,6 +6652,7 @@ static __latent_entropy void net_rx_action(struct softirq_action *h)
>         LIST_HEAD(list);
>         LIST_HEAD(repoll);
>
> +start:
>         sd->in_net_rx_action = true;
>         local_irq_disable();
>         list_splice_init(&sd->poll_list, &list);
> @@ -6659,9 +6664,18 @@ static __latent_entropy void net_rx_action(struct softirq_action *h)
>                 skb_defer_free_flush(sd);
>
>                 if (list_empty(&list)) {
> -                       sd->in_net_rx_action = false;
> -                       if (!sd_has_rps_ipi_waiting(sd) && list_empty(&repoll))
> -                               goto end;
> +                       if (list_empty(&repoll)) {
> +                               sd->in_net_rx_action = false;
> +                               barrier();
> +                               /* We need to check if ____napi_schedule()
> +                                * had refilled poll_list while
> +                                * sd->in_net_rx_action was true.
> +                                */
> +                               if (!list_empty(&sd->poll_list))
> +                                       goto start;

I noticed that since we decide to go back and restart this loop, it
would be better to check the time_limit. More than that,
skb_defer_free_flush() can consume some time which is supposed to take
into account.

Just for your consideration.

> +                               if (!sd_has_rps_ipi_waiting(sd))
> +                                       goto end;
> +                       }
>                         break;
>                 }
>
> --
> 2.40.0.348.gf938b09366-goog
>