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

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Date:	Fri, 10 Feb 2012 02:39:14 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Avi Kivity <avi@...hat.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 03:41:45PM -0800, Paul E. McKenney wrote:
> On Thu, Feb 09, 2012 at 07:22:19PM +0100, Frederic Weisbecker wrote:
> > 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?
> 
> As long as the code doesn't enter RCU read-side critical sections in
> the time between rcu_idle_enter() and rcu_idle_exit(), this should
> work fine.

This should work fine yeah but further the correctness, I wonder if this
is going to be a win.

We use rcu_idle_enter() in idle to avoid to keep the tick for RCU. But
what about falling into guest mode? I guess the tick is kept there
so is it going to be a win in throughput or something to use rcu_idle_enter()?
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