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List:       linux-netdev
Subject:    Re: [BUG?] the first bind() of an AF_PACKET socket to an interface is slow
From:       Tom Gundersen <teg () jklm ! no>
Date:       2014-03-31 21:24:30
Message-ID: CAG-2HqWLGNg5RXLCFq5YN3NKmQLWU9N2mLgMZ3d3kCSs7cPx-Q () mail ! gmail ! com
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On Mon, Mar 31, 2014 at 10:16 PM, Daniel Borkmann <dborkman@redhat.com> wrote:
> On 03/31/2014 08:43 PM, Tom Gundersen wrote:
> > 
> > Hi,
> > 
> > I'm observing some strange behavior using bind().
> > 
> > For any given process, and any given interface, the first time I bind
> > a socket to the interface it takes a very long time. Subsequent binds
> > to the same interface in the same process are instantaneous.
> > 
> > The time it takes seems independent of the interface, so also affects
> > the loopback interface, but binding to all interfaces (ifindex=0) is
> > instantaneous.
> > 
> > It is also peculiar to note that the time it takes to bind is
> > seemingly randomly chosen on my machine from 9ms, 19ms, 29ms, 39ms,
> > 49ms and 59 ms, but I never observed any other values.
> > 
> > The attached test program illustrates the problem, binding to
> > ifindex=0 (all) and ifindex=1 (loopback).
> > 
> > Sample run:
> > # strace -f -T -ebind ./bind
> > bind(3, {sa_family=AF_PACKET, proto=0x800, if0, pkttype=0x40 /* ? */,
> > addr(6)={1328, ffffffffffff}, 20) = 0 <0.000036>
> > bind(3, {sa_family=AF_PACKET, proto=0x800, if0, pkttype=0x40 /* ? */,
> > addr(6)={1328, ffffffffffff}, 20) = 0 <0.000033>
> > bind(3, {sa_family=AF_PACKET, proto=0x800, if1, pkttype=0x40 /* ? */,
> > addr(6)={1328, ffffffffffff}, 20) = 0 <0.059706>
> > bind(3, {sa_family=AF_PACKET, proto=0x800, if1, pkttype=0x40 /* ? */,
> > addr(6)={1328, ffffffffffff}, 20) = 0 <0.000111>
> > 
> > 
> > Can anyone enlighten me to what is going on? Is this expected behavior
> > or a bug? Any suggestions for a work-around?
> 
> 
> Expected behaviour as you can save in some cases a synchronize_net() call:
> 
> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=902fefb82ef72a50c78cb4a20cc954b037a98d1c
> 

Great, that explains what I need to do. Thanks!

Cheers,

Tom
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