netdev - Re: regression: unregister

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Date:	Mon, 25 May 2009 10:04:20 +0200
From:	Damien Wyart <damien.wyart@...e.fr>
To:	Eric Dumazet <dada1@...mosbay.com>
Cc:	Benjamin LaHaise <bcrl@...et.ca>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Denys Fedoryschenko <denys@...p.net.lb>,
	netdev@...r.kernel.org, linux kernel <linux-kernel@...r.kernel.org>
Subject: Re: regression: unregister_netdev() unusably slow

Hello,

* Eric Dumazet <dada1@...mosbay.com> [2009-05-25 07:22]:
> Time to include Paul and lkml in the discussion, and find a better
> solution than one provided in February.

> > bf51935f3e988e0ed6f34b55593e5912f990750a is first bad commit
> > commit bf51935f3e988e0ed6f34b55593e5912f990750a
> > Author: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > 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();

> 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 ?

Ran some tests on 2.6.30-rc7 with bf51935f reverted, and I am still
seeing the problems I originally reported back in February, so I guess
64ca5ab9 is not enough to fully solve all the issues... Note that I am
using CONFIG_TREE_RCU=y (was already true in my February reports).

Feel free to ask if more testing is needed.

-- 
Damien Wyart
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