netdev - Re: [PATCH v2 net-next 2/2] tcp: reduce cpu usage when SO

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Message-ID: <1466621479.6850.75.camel@edumazet-glaptop3.roam.corp.google.com>
Date:	Wed, 22 Jun 2016 11:51:19 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Jason Baron <jbaron@...mai.com>
Cc:	davem@...emloft.net, netdev@...r.kernel.org
Subject: Re: [PATCH v2 net-next 2/2] tcp: reduce cpu usage when SO_SNDBUF is
 set

On Wed, 2016-06-22 at 11:43 -0700, Eric Dumazet wrote:
> On Wed, 2016-06-22 at 14:18 -0400, Jason Baron wrote:
> > 
> 
> > 
> > For 1/2, the getting the correct memory barrier, should I re-submit
> > that as a separate patch?
> 
> Are you sure a full memory barrier (smp_mb() is needed ?
> 
> Maybe smp_wmb() would be enough ?
> 
> (And smp_rmb() in tcp_poll() ?)

Well, in tcp_poll() smp_mb__after_atomic() is fine as it follows 
set_bit(SOCK_NOSPACE, &sk->sk_socket->flags);

(although we might add a comment why we should keep
sk_set_bit(SOCKWQ_ASYNC_NOSPACE, sk) before the set_bit() !)

But presumably smp_wmb() would be enough in tcp_check_space()