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 3:05 PM, Eric Dumazet [off-list ref] wrote:

On Thu, 2017-03-16 at 11:32 -0700, Alexander Duyck wrote:

quoted

From: Sridhar Samudrala <sridhar.samudrala@intel.com>

Fix sk_mark_napi_id() and sk_mark_napi_id_once() to set sk_napi_id only if
skb->napi_id is a valid value.

This happens in loopback paths where skb->napi_id is not updated in
rx path and holds sender_cpu that is set in xmit path.

Signed-off-by: Sridhar Samudrala <sridhar.samudrala@intel.com>
Signed-off-by: Alexander Duyck <redacted>
---
 include/net/busy_poll.h |    5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/include/net/busy_poll.h b/include/net/busy_poll.h
index c0452de83086..67991635953e 100644
--- a/include/net/busy_poll.h
+++ b/include/net/busy_poll.h

@@ -116,7 +116,8 @@ static inline bool sk_busy_loop(struct sock *sk, int nonblock)
 static inline void sk_mark_napi_id(struct sock *sk, const struct sk_buff *skb)
 {
 #ifdef CONFIG_NET_RX_BUSY_POLL
-     sk->sk_napi_id = skb->napi_id;
+     if (skb->napi_id > (u32)NR_CPUS)
+             sk->sk_napi_id = skb->napi_id;
 #endif
 }

@@ -125,7 +126,7 @@ static inline void sk_mark_napi_id_once(struct sock *sk,
                                      const struct sk_buff *skb)
 {
 #ifdef CONFIG_NET_RX_BUSY_POLL
-     if (!sk->sk_napi_id)
+     if (!sk->sk_napi_id && (skb->napi_id > (u32)NR_CPUS))
              sk->sk_napi_id = skb->napi_id;
 #endif
 }

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.

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.