Re: regression: unregister_netdev() unusably slow

From: Eric Dumazet
Date: Mon May 25 2009 - 01:22:33 EST

Benjamin LaHaise a écrit :
> On Mon, May 25, 2009 at 12:47:39AM +0200, Eric Dumazet wrote:
>> There is a strong dependancy against HZ
>> BTW, I am using TREE_RCU
> I'm using CLASSIC_RCU. The bisect just completed, and it points to RCU.
> It makes some degree of sense since I'm testing on an otherwise idle
> machine. That said, where is fixing it going to make sense? I'm not
> opposed to having device unregister take a few timer ticks, but there
> has to be some way of exposing parallelism to the system, and since the
> synchronize_net() calls are done under rntl_lock(), none is possible at
> present. Hrm.

Thanks Ben, this bisection indeed confirms how nasty synchronize_rcu() is :)

Time to include Paul and lkml in the discussion, and find a better solution than
one provided in February.

> -ben
> bf51935f3e988e0ed6f34b55593e5912f990750a is first bad commit
> commit bf51935f3e988e0ed6f34b55593e5912f990750a
> Author: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> Date: Tue Feb 17 06:01:30 2009 -0800
> x86, rcu: fix strange load average and ksoftirqd behavior
> Damien Wyart reported high ksoftirqd CPU usage (20%) on an
> otherwise idle system.
> The function-graph trace Damien provided:
> ...
> diff --git a/arch/x86/kernel/process_32.c b/arch/x86/kernel/process_32.c
> index a546f55..bd4da2a 100644
> --- a/arch/x86/kernel/process_32.c
> +++ b/arch/x86/kernel/process_32.c
> @@ -104,9 +104,6 @@ void cpu_idle(void)
> check_pgt_cache();
> rmb();
> - if (rcu_pending(cpu))
> - rcu_check_callbacks(cpu, 0);
> -
> if (cpu_is_offline(cpu))
> play_dead();
> --

Paul, this commit makes net device unregister very slow (more than 100 ms
if CONFIG_NO_HZ is set), while it used to be pretty fast in previous kernels.

Quoting Ben :
" I just ran a few L2TP tests against 2.6.30-rc7, and it looks like network
device deletion has become unusably slow. At least in, deleting
1000 network interfaces takes less than 2 seconds of real time. The same
test run under 2.6.30-rc7 is taking hundreds of seconds to delete 1000
interfaces at a rate of about 5 per second. The interfaces all share the
same local ip address, but each have a single route to a unique client
ip address."

Device unregister is a synchronize_rcu() abuser (three calls to dismantle
a vlan...) so delaying rcu callbacks can be pretty expensive for it.

I wonder if the real root of the problem was not discovered in the meantime,
by commit 64ca5ab913f1594ef316556e65f5eae63ff50cee
rcu: increment quiescent state counter in ksoftirqd()

Maybe this commit solved Damien Wyart problem as well, and we can revert
commit bf51935f3e988e0ed6f34b55593e5912f990750a ?

Thank you

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