Re: [RFC] dynticks: dynticks_idle is only modified locally use this_cpu ops

From: Paul E. McKenney
Date: Tue Sep 02 2014 - 22:12:06 EST

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://
> > 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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