netdev - Re: [PATCH] net: allow netdev_wait

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Message-ID: <20091024135214.GB6638@linux.vnet.ibm.com>
Date:	Sat, 24 Oct 2009 06:52:14 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Eric Dumazet <eric.dumazet@...il.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

On Sat, Oct 24, 2009 at 10:49:55AM +0200, Eric Dumazet wrote:
> Paul E. McKenney a écrit :
> > On Sat, Oct 24, 2009 at 06:35:53AM +0200, Eric Dumazet wrote:
> > 
> >> Maybe we could call it once only, if we had to call 1 times
> >> the jiffie delay ?
> > 
> > This could be a very useful approach!
> > 
> > However, please keep in mind that although synchronize_rcu_expedited()
> > forces a grace period, it does nothing to speed the invocation of other
> > RCU callbacks.  In short, synchronize_rcu_expedited() is a faster version
> > of synchronize_rcu(), but doesn't necessarily help other synchronize_rcu()
> > or call_rcu() invocations.
> > 
> > The reason I point this out is that it looks to me that the code below is
> > waiting for some other task which is in turn waiting on a grace period.
> > But I don't know this code, so could easily be confused.
> > 
> 
> Normally, we need a synchronize_rcu() calls, but I feel its bit more than really
> needed here.
> 
> 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.

							Thanx, Paul

> messages:Oct 21 19:13:14 svivoipvnx001-00 kernel: [ 2515.580259] synchronize_net() 4045596 ns
> messages:Oct 21 19:13:14 svivoipvnx001-00 kernel: [ 2515.588262] synchronize_net() 7769327 ns
> messages:Oct 21 19:13:14 svivoipvnx001-00 kernel: [ 2515.625014] synchronize_net() 4772052 ns
> messages:Oct 21 19:13:14 svivoipvnx001-00 kernel: [ 2515.633008] synchronize_net() 7773896 ns
> messages:Oct 21 19:13:14 svivoipvnx001-00 kernel: [ 2515.669260] synchronize_net() 3958141 ns
> messages:Oct 21 19:13:14 svivoipvnx001-00 kernel: [ 2515.677259] synchronize_net() 7755817 ns
> messages:Oct 21 19:13:14 svivoipvnx001-00 kernel: [ 2515.712011] synchronize_net() 2502544 ns
> messages:Oct 21 19:13:14 svivoipvnx001-00 kernel: [ 2515.720011] synchronize_net() 7767748 ns
> messages:Oct 21 19:13:14 svivoipvnx001-00 kernel: [ 2515.754259] synchronize_net() 2087946 ns
> messages:Oct 21 19:13:14 svivoipvnx001-00 kernel: [ 2515.762258] synchronize_net() 7738054 ns
> messages:Oct 21 19:13:14 svivoipvnx001-00 kernel: [ 2515.796011] synchronize_net() 3392760 ns
> messages:Oct 21 19:13:15 svivoipvnx001-00 kernel: [ 2515.808025] synchronize_net() 11814619 ns
> messages:Oct 21 19:13:15 svivoipvnx001-00 kernel: [ 2515.848010] synchronize_net() 8970220 ns
> messages:Oct 21 19:13:15 svivoipvnx001-00 kernel: [ 2515.856015] synchronize_net() 7800782 ns
> messages:Oct 21 19:13:15 svivoipvnx001-00 kernel: [ 2515.893008] synchronize_net() 6650174 ns
> messages:Oct 21 19:13:15 svivoipvnx001-00 kernel: [ 2515.897012] synchronize_net() 3744808 ns
> messages:Oct 21 19:13:15 svivoipvnx001-00 kernel: [ 2515.940202] synchronize_net() 8354366 ns
> messages:Oct 21 19:13:15 svivoipvnx001-00 kernel: [ 2515.952137] synchronize_net() 11693215 ns
> messages:Oct 21 19:13:15 svivoipvnx001-00 kernel: [ 2515.985010] synchronize_net() 2355970 ns
> messages:Oct 21 19:13:15 svivoipvnx001-00 kernel: [ 2515.989009] synchronize_net() 3771419 ns
> messages:Oct 21 19:13:15 svivoipvnx001-00 kernel: [ 2516.028137] synchronize_net() 7661195 ns
> messages:Oct 21 19:13:15 svivoipvnx001-00 kernel: [ 2516.036152] synchronize_net() 7800056 ns
> messages:Oct 21 19:13:15 svivoipvnx001-00 kernel: [ 2516.083135] synchronize_net() 6774026 ns
> messages:Oct 21 19:13:15 svivoipvnx001-00 kernel: [ 2516.089145] synchronize_net() 5727189 ns
> messages:Oct 21 19:13:15 svivoipvnx001-00 kernel: [ 2516.130385] synchronize_net() 10133932 ns
> messages:Oct 21 19:13:15 svivoipvnx001-00 kernel: [ 2516.134399] synchronize_net() 3773058 ns
> messages:Oct 21 19:13:15 svivoipvnx001-00 kernel: [ 2516.170136] synchronize_net() 4479194 ns
> messages:Oct 21 19:13:15 svivoipvnx001-00 kernel: [ 2516.178138] synchronize_net() 7710466 ns
> messages:Oct 21 19:13:15 svivoipvnx001-00 kernel: [ 2516.217198] synchronize_net() 4323437 ns
> messages:Oct 21 19:13:15 svivoipvnx001-00 kernel: [ 2516.226206] synchronize_net() 8723108 ns
> messages:Oct 21 19:13:15 svivoipvnx001-00 kernel: [ 2516.268013] synchronize_net() 6221155 ns
> messages:Oct 21 19:13:15 svivoipvnx001-00 kernel: [ 2516.280007] synchronize_net() 11719297 ns
> messages:Oct 21 19:13:15 svivoipvnx001-00 kernel: [ 2516.324008] synchronize_net() 11654511 ns
> messages:Oct 21 19:13:15 svivoipvnx001-00 kernel: [ 2516.332009] synchronize_net() 7744182 ns
> 
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