Re: [net-next PATCH 1/5] net: Do not record sender_cpu as napi_id in socket receive paths

On Thu, Mar 16, 2017 at 3:50 PM, Eric Dumazet [off-list ref] wrote:

On Thu, 2017-03-16 at 15:33 -0700, Alexander Duyck wrote:

quoted

On Thu, Mar 16, 2017 at 3:05 PM, Eric Dumazet [off-list ref] wrote:

quoted

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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.

quoted

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.

I don't know.  My concern here is about the cost of going through all
that code just for something that we know shouldn't be valid.  If
nothing else I might update sk_can_busy_loop so that it doesn't try
busy looping on a sk_napi_id that is NR_CPU or less.

quoted

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.

What I probably can do is go through and replace all the spots where
we where checking for sk_napi_id being 0, and instead replace it with
a check against NR_CPUS.