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

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Message-ID: <1481050715.18162.604.camel@edumazet-glaptop3.roam.corp.google.com>
Date:   Tue, 06 Dec 2016 10:58:35 -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 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.

We must fix this if not already done properly.

Can you take care of this problem ?

Thanks !