linux-kernel - Re: [net-next PATCH 1/5] net: Do not record sender_cpu as napi

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Date:   Thu, 16 Mar 2017 15:50:18 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Alexander Duyck <alexander.duyck@...il.com>
Cc:     Netdev <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Samudrala, Sridhar" <sridhar.samudrala@...el.com>,
        Eric Dumazet <edumazet@...gle.com>,
        David Miller <davem@...emloft.net>
Subject: Re: [net-next PATCH 1/5] net: Do not record sender_cpu as napi_id
 in socket receive paths

On Thu, 2017-03-16 at 15:33 -0700, Alexander Duyck wrote:
> On Thu, Mar 16, 2017 at 3:05 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:

> > It is not clear why this patch is needed .
> >
> > What you describe here is the case we might receive packets for a socket
> > coming from different interfaces ?
> >
> > If skb->napi_id is a sender_cpu, why should we prevent overwriting the
> > sk_napi_id with it, knowing that busy polling will simply ignore the
> > invalid value ?
> >
> > Do not get me wrong :
> >
> > I simply try to understand why the test about napi_id validity is now
> > done twice :
> >
> > 1) At the time we are writing into sk->sk_napi_id
> 
> I would argue that this is the one piece we were missing.
> 
> > 2) At busy polling time when we read sk->sk_napi_id
> 
> Unless there was something recently added I don't think this was ever
> checked.  Instead we start digging into the hash looking for the ID
> that won't ever be there.  Maybe we should add something to napi_by_id
> that just returns NULL in these cases.

But this is exactly what should happen.

For invalid ID, we return NULL from napi_by_id()

No need to add code for that, since the function is meant to deal with
valid cases.



> On top of that I think there end up being several spots where once we
> lock in a non-NAPI ID it is stuck such as the sk_mark_napi_id_once
> call.  I figure we are better off locking in an actual NAPI ID rather
> than getting a sender_cpu stuck in there.

Are you referring to sk_mark_napi_id_once() ?

Since this is only used by UDP, I would be OK to avoid the 'locking' for
'sender_cpu'  ids.