netdev - Re: [PATCH] net/udp: do not touch skb->peeked unless really needed

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Date:   Tue, 06 Dec 2016 11:35:22 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Paolo Abeni <pabeni@...hat.com>
Cc:     David Miller <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH] net/udp: do not touch skb->peeked unless really needed

On Tue, 2016-12-06 at 20:16 +0100, Paolo Abeni wrote:
> On Tue, 2016-12-06 at 10:58 -0800, Eric Dumazet wrote:
> > On Tue, 2016-12-06 at 19:31 +0100, Paolo Abeni wrote:
> > 
> > > cacheline 2 boundary (128 bytes) is 8 bytes before sk_lock: cacheline 2
> > > includes also skc_refcnt and skc_rxhash from __sk_common (I use 'pahole
> > > -E ...' to get the full blown output). skc_rxhash is read for each
> > > packet in inet_recvmsg()/sock_rps_record_flow() if CONFIG_RPS is set. I
> > > get a cache miss per packet there and inet_recvmsg() in my test takes
> > > about 8% of the whole u/s processing time.
> > 
> > Wait a minute, this sk->sk_rxhash should only be read on connected
> > socket. Relying on it being 0 was okay only if we did not care
> > of false sharing. And UDP sockets used to grab socket refcount, so we
> > had false sharing a _lot_ in the past.
> 
> Thank you for the pointer.
> 
> > We must fix this if not already done properly.
> > 
> > Can you take care of this problem ?
> 
> I'll try, but it can be very soon: I'll have limited time and bad
> internet connection up to next week.

Do not worry, I had a better idea anyway. I am testing it ;)