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

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Date:   Thu, 18 Jun 2020 16:44:44 -0700
From:   "Nambiar, Amritha" <amritha.nambiar@...el.com>
To:     Eric Dumazet <edumazet@...gle.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 6/18/2020 3:29 PM, Eric Dumazet wrote:
> 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>
> 

Thanks for the review. Should I send a v2 with this change?

> 
> 
> 
> 
> 
>>
>>   #endif
>>   }
>>
>>