linux-kernel - Re: getaddrinfo slowdown in 3.17.1, due to getifaddrs

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Date:	Fri, 17 Oct 2014 09:28:03 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Thomas Graf <tgraf@...g.ch>
Cc:	"Steinar H. Gunderson" <sgunderson@...foot.com>,
	linux-kernel@...r.kernel.org,
	"David S. Miller" <davem@...emloft.net>,
	Nikolay Aleksandrov <nikolay@...hat.com>
Subject: Re: getaddrinfo slowdown in 3.17.1, due to getifaddrs

On Fri, 2014-10-17 at 07:25 +0100, Thomas Graf wrote:
> On 10/17/14 at 02:34am, Steinar H. Gunderson wrote:
> > On Fri, Oct 17, 2014 at 02:21:32AM +0200, Steinar H. Gunderson wrote:
> > > Hi,
> > > 
> > > We recently upgraded a machine from 3.14.5 to 3.17.1, and a Perl script we're
> > > running to poll SNMP suddenly needed ten times as much time to complete.
> > 
> > e341694e3eb57fcda9f1adc7bfea42fe080d8d7a looks like it might cause something
> > like this (it certainly added the synchronize_net() call). Cc-ing people on 
> > that commit; quoting the entire rest of the message for reference.
> 
> I think the only option at this point is to re-add the nltable lock to
> netlink_lookup() so we can drop the synchronize_net() until we find a
> way to RCUify socket destruction. I will cook up a patch today unless
> somebody can come up with a smarter way to work around needing the
> synchronize_net().

I had a patch to add conditional RCUify socket destruction for some kind
of sockets (opt-in at protocol level).

I needed this to remove too expensive UDP socket refcount inc/dec (so
get rid of SLAB_DESTROY_BY_RCU, and instead use call_rcu() or kfree_rcu
thing)

But I couldn't finish and submit this for 3.18, and I believe I got it
wrong anyway. Cant remember exact details right now, maybe later in the
day, once my headache is better.


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