linux-kernel - Re: [RFC] dynticks: dynticks_idle is only modified locally use this

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Date:	Tue, 2 Sep 2014 19:11:52 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC] dynticks: dynticks_idle is only modified locally use
 this_cpu ops

On Tue, Sep 02, 2014 at 06:22:52PM -0500, Christoph Lameter wrote:
> On Tue, 2 Sep 2014, Paul E. McKenney wrote:
> 
> > Yep, these two have been on my "when I am feeling insanely gutsy" list
> > for quite some time.
> >
> > But I have to ask...  On x86, is a pair of mfence instructions really
> > cheaper than an atomic increment?
> 
> Not sure why you would need an mfence instruction?

Because otherwise RCU can break.  As soon as the grace-period machinery
sees that the value of this variable is even, it assumes a quiescent
state.  If there are no memory barriers, the non-quiescent code might
not have completed executing, and your kernel's actuarial statistics
become sub-optimal.

							Thanx, Paul

> > > If the first patch I send gets merged then a lot of other this_cpu related
> > > optimizations become possible regardless of the ones in the RFC.
> >
> > Yep, I am queuing that one.
> 
> Great.
> 
> > But could you please do future patches against the rcu/dev branch of
> > git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git?
> > I had to hand-apply due to conflicts.  Please see below for my version,
> > and please check to make sure that I didn't mess something up in the
> > translation.
> 
> Looks ok. Will use the correct tree next time.
> 
> 

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