linux-kernel - Re: [PATCH 13/15] sched: Set preferred NUMA node based on number of private faults

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Date:	Mon, 8 Jul 2013 10:23:03 +0100
From:	Mel Gorman <mgorman@...e.de>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...nel.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 13/15] sched: Set preferred NUMA node based on number of
 private faults

On Sat, Jul 06, 2013 at 12:41:07PM +0200, Peter Zijlstra wrote:
> On Sat, Jul 06, 2013 at 12:09:00AM +0100, Mel Gorman wrote:
> > +++ b/include/linux/mm.h
> > @@ -582,11 +582,11 @@ static inline pte_t maybe_mkwrite(pte_t pte, struct vm_area_struct *vma)
> >   * sets it, so none of the operations on it need to be atomic.
> >   */
> >  
> > -/* Page flags: | [SECTION] | [NODE] | ZONE | [LAST_NID] | ... | FLAGS | */
> > +/* Page flags: | [SECTION] | [NODE] | ZONE | [LAST_NIDPID] | ... | FLAGS | */
> >  #define SECTIONS_PGOFF		((sizeof(unsigned long)*8) - SECTIONS_WIDTH)
> >  #define NODES_PGOFF		(SECTIONS_PGOFF - NODES_WIDTH)
> >  #define ZONES_PGOFF		(NODES_PGOFF - ZONES_WIDTH)
> > -#define LAST_NID_PGOFF		(ZONES_PGOFF - LAST_NID_WIDTH)
> > +#define LAST_NIDPID_PGOFF	(ZONES_PGOFF - LAST_NIDPID_WIDTH)
> 
> I saw the same with Ingo's patch doing the similar thing. But why do we fuse
> these two into a single field? Would it not make more sense to have them be
> separate fields?
> 
> Yes I get we update and read them together, and we could still do that with
> appropriate helper function, but they are two independent values stored in the
> page flags.
> 

There were two reasons. First, it is because we update and read them
together. Second, it's all or nothing if this field is included in the
page->flags or not. I know this could also be done with helpers and
other tricks but I did not think it would be any easier to understand.

-- 
Mel Gorman
SUSE Labs
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