netdev - Re: [RFC] net: remove erroneous sk null assignment in timestamping

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Date:	Sat, 08 Oct 2011 10:57:18 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Johannes Berg <johannes@...solutions.net>
Cc:	Richard Cochran <richardcochran@...il.com>,
	David Miller <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [RFC] net: remove erroneous sk null assignment in timestamping

Le samedi 08 octobre 2011 à 10:16 +0200, Johannes Berg a écrit :

> I'm not terribly familiar with struct sock. Looking at it, I'm a bit
> confused by skb_orphan() -- it doesn't put the sock reference. So are
> sockets not refcounted for skbs in this way? They seem to use
> sock_wfree() which does a bit more than this it seems, and I don't see
> it using sk_refcnt anywhere so I'm a bit confused now.

Check following commit changelog to get some information on this.

commit 2b85a34e911bf483c27cfdd124aeb1605145dc80
Author: Eric Dumazet <eric.dumazet@...il.com>
Date:   Thu Jun 11 02:55:43 2009 -0700

    net: No more expensive sock_hold()/sock_put() on each tx
    
    One of the problem with sock memory accounting is it uses
    a pair of sock_hold()/sock_put() for each transmitted packet.
    
    This slows down bidirectional flows because the receive path
    also needs to take a refcount on socket and might use a different
    cpu than transmit path or transmit completion path. So these
    two atomic operations also trigger cache line bounces.
    
    We can see this in tx or tx/rx workloads (media gateways for example),
    where sock_wfree() can be in top five functions in profiles.
    
    We use this sock_hold()/sock_put() so that sock freeing
    is delayed until all tx packets are completed.
    
    As we also update sk_wmem_alloc, we could offset sk_wmem_alloc
    by one unit at init time, until sk_free() is called.
    Once sk_free() is called, we atomic_dec_and_test(sk_wmem_alloc)
    to decrement initial offset and atomicaly check if any packets
    are in flight.
    
    skb_set_owner_w() doesnt call sock_hold() anymore
    
    sock_wfree() doesnt call sock_put() anymore, but check if sk_wmem_alloc
    reached 0 to perform the final freeing.
    
    Drawback is that a skb->truesize error could lead to unfreeable sockets, or
    even worse, prematurely calling __sk_free() on a live socket.
    
    Nice speedups on SMP. tbench for example, going from 2691 MB/s to 2711 MB/s
    on my 8 cpu dev machine, even if tbench was not really hitting sk_refcnt
    contention point. 5 % speedup on a UDP transmit workload (depends
    on number of flows), lowering TX completion cpu usage.


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