netdev - Re: [PATCH] net: allow netdev_wait

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Message-ID: <4AE30E1B.5080008@gmail.com>
Date:	Sat, 24 Oct 2009 16:24:27 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	paulmck@...ux.vnet.ibm.com
CC:	Octavian Purdila <opurdila@...acom.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

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.

Do you think the "on_each_cpu(flush_backlog, dev, 1);"
we perform right before calling netdev_wait_allrefs() could be changed
somehow to speedup rcu callbacks ? Maybe we ould avoid sending IPI twice to
cpus ?

Thanks

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