linux-kernel - Re: [v7 0/8] Reduce cross CPU IPI interference

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Date:	Thu, 9 Feb 2012 19:22:19 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Avi Kivity <avi@...hat.com>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Christoph Lameter <cl@...ux.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Gilad Ben-Yossef <gilad@...yossef.com>,
	linux-kernel@...r.kernel.org, Chris Metcalf <cmetcalf@...era.com>,
	linux-mm@...ck.org, Pekka Enberg <penberg@...nel.org>,
	Matt Mackall <mpm@...enic.com>,
	Sasha Levin <levinsasha928@...il.com>,
	Rik van Riel <riel@...hat.com>,
	Andi Kleen <andi@...stfloor.org>, Mel Gorman <mel@....ul.ie>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Michal Nazarewicz <mina86@...a86.com>,
	Kosaki Motohiro <kosaki.motohiro@...il.com>,
	Milton Miller <miltonm@....com>
Subject: Re: [v7 0/8] Reduce cross CPU IPI interference

On Thu, Feb 09, 2012 at 06:05:07PM +0200, Avi Kivity wrote:
> On 02/09/2012 05:22 PM, Frederic Weisbecker wrote:
> > > > 
> > > > Looks like there are new rcu_user_enter() and rcu_user_exit() APIs which
> > > > we can use.  Hopefully they subsume rcu_virt_note_context_switch() so we
> > > > only need one set of APIs.
> > > 
> > > Now that you mention it, that is a good goal.  However, it requires
> > > coordination with Frederic's code as well, so some investigation
> > > is required.  Bad things happen if you tell RCU you are idle when you
> > > really are not and vice versa!
> > > 
> > > 							Thanx, Paul
> > > 
> >
> > Right. Avi I need to know more about what you need. rcu_virt_note_context_switch()
> > notes a quiescent state while rcu_user_enter() shuts down RCU (it's in fact the same
> > thing than rcu_idle_enter() minus the is_idle_cpu() checks).
> 
> I don't know enough about RCU to say if it's okay or not (I typically
> peek at the quick quiz answers).  However, switching to guest mode is
> very similar to exiting to user mode: we're guaranteed not to be in an
> rcu critical section, and to remain so until the guest exits back to
> us.

Awesome!

> What guarantees does rcu_user_enter() provide?  With luck guest
> entry satisifies them all.

So rcu_user_enter() puts the CPU into RCU idle mode, which means the CPU
won't need to be part of the global RCU grace period completion. This
prevents it to depend on the timer tick (although for now you keep it)
and to complete some RCU specific work during the tick.

Paul, do you think that would be a win?
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