linux-kernel - Re: Possible netns creation and execution performance/scalability regression since v3.8 due to rcu callbacks being offloaded to multiple cpus

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Message-ID: <87r42v2mjy.fsf@x220.int.ebiederm.org>
Date:	Wed, 11 Jun 2014 13:55:45 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	paulmck@...ux.vnet.ibm.com
Cc:	Dave Chiluk <chiluk@...onical.com>,
	Rafael Tinoco <rafael.tinoco@...onical.com>,
	linux-kernel@...r.kernel.org, davem@...emloft.net,
	Christopher Arges <chris.j.arges@...onical.com>,
	Jay Vosburgh <jay.vosburgh@...onical.com>
Subject: Re: Possible netns creation and execution performance/scalability regression since v3.8 due to rcu callbacks being offloaded to multiple cpus

"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> writes:

> On Wed, Jun 11, 2014 at 01:27:07PM -0500, Dave Chiluk wrote:
>> On 06/11/2014 11:18 AM, Paul E. McKenney wrote:
>> > On Wed, Jun 11, 2014 at 10:46:00AM -0500, David Chiluk wrote:
>> >> Now think about what happens when a gateway goes down, the namespaces
>> >> need to be migrated, or a new machine needs to be brought up to replace
>> >> it.  When we're talking about 3000 namespaces, the amount of time it
>> >> takes simply to recreate the namespaces becomes very significant.
>> >>
>> >> The script is a stripped down example of what exactly is being done on
>> >> the neutron gateway in order to create namespaces.
>> > 
>> > Are the namespaces torn down and recreated one at a time, or is there some
>> > syscall, ioctl(), or whatever that allows bulk tear down and recreating?
>> > 
>> > 							Thanx, Paul
>> 
>> In the normal running case, the namespaces are created one at a time, as
>> new customers create a new set of VMs on the cloud.
>> 
>> However, in the case of failover to a new neutron gateway the namespaces
>> are created all at once using the ip command (more or less serially).
>> 
>> As far as I know there is no syscall or ioctl that allows bulk tear down
>> and recreation.  if such a beast exists that might be helpful.
>
> The solution might be to create such a beast.  I might be able to shave
> a bit of time off of this benchmark, but at the cost of significant
> increases in RCU's CPU consumption.  A bulk teardown/recreation API could
> reduce the RCU grace-period overhead by several orders of magnitude by
> having a single RCU grace period cover a few thousand changes.
>
> This is why other bulk-change syscalls exist.
>
> Just out of curiosity, what syscalls does the ip command use?

You can look in iproute2 ip/ipnetns.c

But rought ip netns add does:

unshare(CLONE_NEWNET);
mkdir /var/run/netns/<name>
mount --bind /proc/self/ns/net /var/run/netns/<name>

I don't know if there is any sensible way to batch that work.

(The unshare gets you into copy_net_ns in net/core/net_namespace.c
 and to find all of the code it can call you have to trace all
 of the register_pernet_subsys and register_pernet_device calls).

At least for creation I would like to see if we can make all of the
rcu_callback synchronize_rcu calls go away.  That seems preferable
to batching at creation time.

Eric
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