netdev - Re: [net-next PATCH] net: Avoid overwriting valid skb->napi

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Date:   Thu, 18 Jun 2020 15:29:30 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     Amritha Nambiar <amritha.nambiar@...el.com>
Cc:     netdev <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        Alexander Duyck <alexander.h.duyck@...el.com>,
        Eliezer Tamir <eliezer.tamir@...ux.intel.com>,
        "Samudrala, Sridhar" <sridhar.samudrala@...el.com>
Subject: Re: [net-next PATCH] net: Avoid overwriting valid skb->napi_id

On Thu, Jun 18, 2020 at 2:21 PM Amritha Nambiar
<amritha.nambiar@...el.com> wrote:
>
> This will be useful to allow busy poll for tunneled traffic. In case of
> busy poll for sessions over tunnels, the underlying physical device's
> queues need to be polled.
>
> Tunnels schedule NAPI either via netif_rx() for backlog queue or
> schedule the gro_cell_poll(). netif_rx() propagates the valid skb->napi_id
> to the socket. OTOH, gro_cell_poll() stamps the skb->napi_id again by
> calling skb_mark_napi_id() with the tunnel NAPI which is not a busy poll
> candidate.


Yes the tunnel NAPI id should be 0 really (look for NAPI_STATE_NO_BUSY_POLL)

> This was preventing tunneled traffic to use busy poll. A valid
> NAPI ID in the skb indicates it was already marked for busy poll by a
> NAPI driver and hence needs to be copied into the socket.
>
> Signed-off-by: Amritha Nambiar <amritha.nambiar@...el.com>
> ---
>  include/net/busy_poll.h |    6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/include/net/busy_poll.h b/include/net/busy_poll.h
> index 86e028388bad..b001fa91c14e 100644
> --- a/include/net/busy_poll.h
> +++ b/include/net/busy_poll.h
> @@ -114,7 +114,11 @@ static inline void skb_mark_napi_id(struct sk_buff *skb,
>                                     struct napi_struct *napi)
>  {
>  #ifdef CONFIG_NET_RX_BUSY_POLL
> -       skb->napi_id = napi->napi_id;
> +       /* If the skb was already marked with a valid NAPI ID, avoid overwriting
> +        * it.
> +        */
> +       if (skb->napi_id < MIN_NAPI_ID)
> +               skb->napi_id = napi->napi_id;


It should be faster to not depend on MIN_NAPI_ID (aka NR_CPUS + 1)

    if (napi->napi_id)
       skb->napi_id = napi->napi_id;

Probably not a big deal.

Reviewed-by: Eric Dumazet <edumazet@...gle.com>






>
>  #endif
>  }
>
>