netdev - Re: [PATCH net-next 3/3] tcp: add one skb cache for rx

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Date:   Thu, 21 Mar 2019 15:52:23 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Eric Dumazet <edumazet@...gle.com>,
        "David S . Miller" <davem@...emloft.net>
Cc:     netdev <netdev@...r.kernel.org>,
        Soheil Hassas Yeganeh <soheil@...gle.com>,
        Willem de Bruijn <willemb@...gle.com>,
        Florian Westphal <fw@...len.de>,
        Tom Herbert <tom@...bertland.com>
Subject: Re: [PATCH net-next 3/3] tcp: add one skb cache for rx



On 03/21/2019 03:17 PM, Eric Dumazet wrote:
> Often times, recvmsg() system calls and BH handling for a particular
> TCP socket are done on different cpus.

...

> Note that if rps/rfs is used, we do not enable this feature, because
> there is high chance that the same cpu is handling both the recvmsg()
> system call and the TCP rx path, but that another cpu did the skb
> allocations in the device driver right before the RPS/RFS logic.
> 
> To properly handle this case, it seems we would need to record
> on which cpu skb was allocated, and use a different channel
> to give skbs back to this cpu.

Oops a rebase went wrong and I missed the following bit,
this will be added in v2.

diff --git a/include/net/sock.h b/include/net/sock.h
index a7e936ce5a5ac935d90c47f6dd68bf9e8e47ba10..0840f4b27b91eddb205ff42c03f787e5914f755d 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -2439,7 +2439,7 @@ static inline void skb_setup_tx_timestamp(struct sk_buff *skb, __u16 tsflags)
 static inline void sk_eat_skb(struct sock *sk, struct sk_buff *skb)
 {
        __skb_unlink(skb, &sk->sk_receive_queue);
-       if (!sk->sk_rx_skb_cache) {
+       if (!static_branch_unlikely(&rps_needed) && !sk->sk_rx_skb_cache) {
                sk->sk_rx_skb_cache = skb;
                skb_orphan(skb);
                return;