netdev - Re: Scalability of interface creation and deletion

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Date:	Sun, 08 May 2011 15:27:07 +0100
From:	Alex Bligh <alex@...x.org.uk>
To:	paulmck@...ux.vnet.ibm.com
cc:	Eric Dumazet <eric.dumazet@...il.com>, netdev@...r.kernel.org,
	Alex Bligh <alex@...x.org.uk>
Subject: Re: Scalability of interface creation and deletion

Paul,

>> Yes, really 20-49us and 50-99us, not ms. Raw data attached :-)
>>
>> I'm guessing there are circumstances where there is an early exit.
>
> Well, if you were onlining and offlining CPUs, then if there was only
> one CPU online, this could happen.

No, I wasn't doing that.

>  And there really is only one CPU
> online during boot, so if your measurements included early boot time,
> this could easily explain these very short timings.

No, I waited a few minutes after boot for the system to stabilize, and
all CPUs were definitely online.

The patch to the kernel I am running is below.

>> There is nothing much going on these systems (idle, no other users,
>> just normal system daemons).
>
> And normal system daemons might cause this, right?

Yes. Everything is normal, except I did
 service udev stop
 unshare -n bash
which together stop the system running interface scripts when
interfaces are created (as upstart and upstart-udev-bridge are
now integrated, you can't kill upstart, so you have to rely on
unshare -n to stop the events being propagated). That's just
to avoid measuring the time it takes to execute the scripts.

-- 
Alex Bligh

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index dd4aea8..e401018 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -1518,6 +1518,7 @@ EXPORT_SYMBOL_GPL(call_rcu_bh);
 void synchronize_sched(void)
 {
        struct rcu_synchronize rcu;
+       ktime_t time_start = ktime_get();

        if (rcu_blocking_is_gp())
                return;
@@ -1529,6 +1530,7 @@ void synchronize_sched(void)
        /* Wait for it. */
        wait_for_completion(&rcu.completion);
        destroy_rcu_head_on_stack(&rcu.head);
+       pr_err("synchronize_sched() in %lld us\n", 
ktime_us_delta(ktime_get(), time_start));
 }
 EXPORT_SYMBOL_GPL(synchronize_sched);

diff --git a/net/core/dev.c b/net/core/dev.c
index 856b6ee..013f627 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -5164,7 +5164,9 @@ static void rollback_registered_many(struct list_head 
*head)
        dev = list_first_entry(head, struct net_device, unreg_list);
        call_netdevice_notifiers(NETDEV_UNREGISTER_BATCH, dev);

+       pr_err("begin rcu_barrier()\n");
        rcu_barrier();
+       pr_err("end rcu_barrier()\n");

        list_for_each_entry(dev, head, unreg_list)
                dev_put(dev);
@@ -5915,8 +5917,10 @@ EXPORT_SYMBOL(free_netdev);
  */
 void synchronize_net(void)
 {
+       pr_err("begin synchronize_net()\n");
        might_sleep();
        synchronize_rcu();
+       pr_err("end synchronize_net()\n");
 }
 EXPORT_SYMBOL(synchronize_net);


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