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Date:	Thu, 19 Jun 2014 16:11:47 -0500 (CDT)
From:	Christoph Lameter <cl@...two.org>
To:	Tejun Heo <tj@...nel.org>
cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	David Howells <dhowells@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC] percpu: add data dependency barrier in percpu
 accessors and operations

On Thu, 19 Jun 2014, Tejun Heo wrote:

> On Thu, Jun 19, 2014 at 03:42:07PM -0500, Christoph Lameter wrote:
> > In that case special care needs to be taken to get this right. True.
> >
> > I typically avoid these scenarios by sending an IPI with a pointer to the
> > data structure. The modification is done by the cpu for which the per cpu
> > data is local.
> >
> > Maybe rewrite the code to avoid writing to other processors percpu data
> > would be the right approach?
>
> It depends on the specific use case but in general no.  IPIs would be
> far more expensive than making use of proper barriers in vast majority
> of cases especially when the "hot" side is data dependency barrier,
> IOW, nothing.  Also, we are talking about extremely low frequency
> events like init and recycling after reinit.  Regular per-cpu
> operation isn't really the subject here.

The aim of having percpu data is to have the ability for a processor to
access memory dedicated to that processor in the fastest way possible by
avoiding synchronization. You are beginning to add synchronization
elements into the accesses of a processor to memory dedicated to its sole
use.

Remote write events are contrary to that design and are exceedingly rare.
An IPI is justifiable for such a rare event. At least in my use cases I
have always found that to be sufficient. Well, I designed the data
structures in a way that made this possible because of the design criteria
that did not allow me remote write access to other processors per cpu
data.

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