netdev - Re: [PATCH] net: allow netdev_wait

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Date:	Sun, 25 Oct 2009 02:49:00 +0300
From:	Octavian Purdila <opurdila@...acom.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	paulmck@...ux.vnet.ibm.com, Benjamin LaHaise <bcrl@...et.ca>,
	netdev@...r.kernel.org, Cosmin Ratiu <cratiu@...acom.com>
Subject: Re: [PATCH] net: allow netdev_wait_allrefs() to run faster

On Saturday 24 October 2009 17:24:27 you wrote:
> Paul E. McKenney a écrit :
> > On Sat, Oct 24, 2009 at 10:49:55AM +0200, Eric Dumazet wrote:
> >> On my dev machine, a synchronize_rcu() lasts between 2 an 12 ms
> >
> > That sounds like the right range, depending on what else is happening
> > on the machine at the time.
> >
> > The synchronize_rcu_expedited() primitive would run in the 10s-100s
> > of microseconds.  It involves a pair of wakeups and a pair of context
> > switches on each CPU.
> 
> Hmm... I'll make some experiments Monday and post results, but it seems
>  very promising.
> 

Got some time today and did some experiments myself. The test is deleting 1000 
dummy interfaces (interface status down, no IP/IPv6 addresses assigned) on a 
UP non-preempt ppc750 @800Mhz system.

1. Ben's patch:

real    0m 3.42s
user    0m 0.00s
sys     0m 0.00s

2. Eric's schedule_timeout_uninterruptible(1);

real    0m 3.00s
user    0m 0.00s
sys     0m 0.00s

3. Simple synchronize_rcu_expedited()

This doesn't seem to work well with the UP non-preempt case since 
synchronize_rcu_expedited() is a noop in this case - turning 
netdev_wait_allrefs() into a while(1) loop.

tavi




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