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Date: Thu, 09 Sep 2010 07:23:14 +0200
From: Eric Dumazet <eric.dumazet@...il.com>
To: Jesse Gross <jesse@...ira.com>
Cc: David Miller <davem@...emloft.net>, netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next-2.6] net: inet_add_protocol() can use cmpxchg()
Le mercredi 08 septembre 2010 à 16:49 -0700, Jesse Gross a écrit :
> On Wed, Sep 8, 2010 at 3:56 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> > Use cmpxchg() to get rid of spinlocks in inet_add_protocol() and
> > friends.
> >
> > inet_protos[] & inet6_protos[] are moved to read_mostly section
> >
> > Signed-off-by: Eric Dumazet <eric.dumazet@...il.com>
>
> What's the benefit to this? It's hard to imagine that add/deleting
> protocols is highly contended. On the other hand, a simple spinlock
> is very easy to look at and verify correct, while this takes a little
> more thought.
Good question.
Did you know that _bh() was not necessary for example at this point ?
Its a leftover of an RCU conversion, done many years ago. It _was_
working of course, but it really hurts my mind. We could use
stop_machine or other brute force thing too ;)
Before patch :
# size net/ipv4/protocol.o net/ipv6/protocol.o
text data bss dec hex filename
340 0 2052 2392 958 net/ipv4/protocol.o
342 0 2052 2394 95a net/ipv6/protocol.o
After patch :
# size net/ipv4/protocol.o net/ipv6/protocol.o
text data bss dec hex filename
220 2048 0 2268 8dc net/ipv4/protocol.o
222 2048 0 2270 8de net/ipv6/protocol.o
45 lines removed, 240 bytes of text removed, I found this interesting.
Removing the ____cacheline_aligned_in_smp is also good, because
the .data alignement, is reduced and linker is not force to propagate
this alignement to upper .o files, with many holes.
If you take a look at most patches flying around, this one at least took
me to think a bit ;)
Thanks
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