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

[Eric W. Biederman]

Rafael Tinoco <rafael.tinoco@xxxxxxxxxxxxx> writes:

> Paul E. McKenney, Eric Biederman, David Miller (and/or anyone else interested):
>
> It was brought to my attention that netns creation/execution might
> have suffered scalability/performance regression after v3.8.
>
> I would like you, or anyone interested, to review these charts/data
> and check if there is something that could be discussed/said before I
> move further.
>
> The following script was used for all the tests and charts generation:

> ====
> #!/bin/bash
> IP=/sbin/ip
>
> function add_fake_router_uuid() {
>     j=`uuidgen`
>     $IP netns add bar-${j}
>     $IP netns exec bar-${j} $IP link set lo up
>     $IP netns exec bar-${j} sysctl -w net.ipv4.ip_forward=1 > /dev/null
>     k=`echo $j | cut -b -11`
>     $IP link add qro-${k} type veth peer name qri-${k} netns bar-${j}
>     $IP link add qgo-${k} type veth peer name qgi-${k} netns bar-${j}
> }
>
> for i in `seq 1 $1`; do
>     if [ `expr $i % 250` -eq 0 ]; then
>         echo "$i by `date +%s`"
>     fi
>     add_fake_router_uuid
> done

[snip long explanation]

> I was able to see that, from the script above, the following lines
> causes major impact on netns scalability/performance:
>
> 1) ip netns add -> huge performance regression:
>
>  1 cpu: no regression
>  4 cpu: regression for NOCB_CPU_ALL
>
>  obs: regression from 250 netns/sec to 50 netns/sec on 500 netns
> already created mark

copy_netns except possibly in the per_net callbacks does not use
rcu so I am mystified.  So a little more digging to figure out which
rcu usage is causing the problem would be very interesting.

> 2) ip netns exec -> some performance regression
>
>  1 cpu: no regression
>  4 cpu: regression for NOCB_CPU_ALL
>
>  obs: regression from 40 netns (+1 exec per netns creation) to 20
> netns/sec on 500 netns created mark

The performance regression is probably in setns().
switch_task_namespaces is occassionaly a choke point.

At one point I was playing with ideas on how to use the task lock
instead of rcu to protect nsproxy.  As the original reason we could
not use task_lock appeared to have disappeared.

That could be worth playing with.


> ========
>
> FULL NOTE: http://people.canonical.com/~inaddy/lp1328088/
>
> ** Assumption: RCU callbacks being offloaded to multiple cpus
> (cpumask_setall) caused regression in
> copy_net_ns<-created_new_namespaces or unshare(clone_newnet).
>
> ** Next Steps: I'll probably begin to function_graph netns creation execution

Eric


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