Re: [PATCH 21/25] Use allocation flags as an index to the zone watermark

Mel Gorman
2009-04-22T10:23:20Z

On Wed, Apr 22, 2009 at 11:21:17AM +0100, Mel Gorman wrote:
> On Wed, Apr 22, 2009 at 09:26:10AM +0900, KOSAKI Motohiro wrote:
> > > ALLOC_WMARK_MIN, ALLOC_WMARK_LOW and ALLOC_WMARK_HIGH determin whether
> > > pages_min, pages_low or pages_high is used as the zone watermark when
> > > allocating the pages. Two branches in the allocator hotpath determine which
> > > watermark to use. This patch uses the flags as an array index and places
> > > the three watermarks in a union with an array so it can be offset. This
> > > means the flags can be used as an array index and reduces the branches
> > > taken.
> > >
> > > Signed-off-by: Mel Gorman <mel@csn.ul.ie>
> > > Reviewed-by: Christoph Lameter <cl@linux-foundation.org>
> > > ---
> > > include/linux/mmzone.h | 8 +++++++-
> > > mm/page_alloc.c | 18 ++++++++----------
> > > 2 files changed, 15 insertions(+), 11 deletions(-)
> > >
> > > diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> > > index f82bdba..c1fa208 100644
> > > --- a/include/linux/mmzone.h
> > > +++ b/include/linux/mmzone.h
> > > @@ -275,7 +275,13 @@ struct zone_reclaim_stat {
> > >
> > > struct zone {
> > > /* Fields commonly accessed by the page allocator */
> > > - unsigned long pages_min, pages_low, pages_high;
> > > + union {
> > > + struct {
> > > + unsigned long pages_min, pages_low, pages_high;
> > > + };
> > > + unsigned long pages_mark[3];
> > > + };
> > > +
> >
> > hmmm... I don't like union hack.
> > Why can't we change all caller to use page_mark?
> >
>
> Because pages_min, pages_low and pages_high are such well understood concepts
> and their current use is easy to userstand. It could all be changed to getters
> and setters with a patch to update all call sites or having symbolic names
> for the index and forcing the use of the array but when I started doing that,
> the code looked worse to my eye, not better. The union is a relatively
> small hack but well contained within the one place that cares.
>
> > > /*
> > > * We don't know if the memory that we're going to allocate will be freeable
> > > * or/and it will be released eventually, so to avoid totally wasting several
> > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > > index 376d848..e61867e 100644
> > > --- a/mm/page_alloc.c
> > > +++ b/mm/page_alloc.c
> > > @@ -1157,10 +1157,13 @@ failed:
> > > return NULL;
> > > }
> > >
> > > -#define ALLOC_NO_WATERMARKS 0x01 /* don't check watermarks at all */
> > > -#define ALLOC_WMARK_MIN 0x02 /* use pages_min watermark */
> > > -#define ALLOC_WMARK_LOW 0x04 /* use pages_low watermark */
> > > -#define ALLOC_WMARK_HIGH 0x08 /* use pages_high watermark */
> > > +/* The WMARK bits are used as an index zone->pages_mark */
> > > +#define ALLOC_WMARK_MIN 0x00 /* use pages_min watermark */
> > > +#define ALLOC_WMARK_LOW 0x01 /* use pages_low watermark */
> > > +#define ALLOC_WMARK_HIGH 0x02 /* use pages_high watermark */
> > > +#define ALLOC_NO_WATERMARKS 0x08 /* don't check watermarks at all */
> > > +#define ALLOC_WMARK_MASK 0x07 /* Mask to get the watermark bits */
> >
> > the mask only use two bit. but mask definition is three bit (0x07), why?
> >
>
> I was thinking ALLOC_NO_WATERMARKS-1 and that all the lower bits must be
> cleared and left the value of 0x08 to occupy same number of bits even though
> that wasn't necessary. As suggested I'll change this to
>
> ALLOC_NO_WATERMARKS 0x04
> WMARK_MASK (1-ALLOC_NO_WATERMARKS)
>

ALLOC_NO_WATERMARKS-1 obviously is what I meant

> >
> > > +
> > > #define ALLOC_HARDER 0x10 /* try to alloc harder */
> > > #define ALLOC_HIGH 0x20 /* __GFP_HIGH set */
> > > #ifdef CONFIG_CPUSETS
> > > @@ -1463,12 +1466,7 @@ zonelist_scan:
> > >
> > > if (!(alloc_flags & ALLOC_NO_WATERMARKS)) {
> > > unsigned long mark;
> > > - if (alloc_flags & ALLOC_WMARK_MIN)
> > > - mark = zone->pages_min;
> > > - else if (alloc_flags & ALLOC_WMARK_LOW)
> > > - mark = zone->pages_low;
> > > - else
> > > - mark = zone->pages_high;
> > > + mark = zone->pages_mark[alloc_flags & ALLOC_WMARK_MASK];
> > > if (!zone_watermark_ok(zone, order, mark,
> > > classzone_idx, alloc_flags)) {
> > > if (!zone_reclaim_mode ||
> > > --
> > > 1.5.6.5
> > >
> >
> >
> >
>
> --
> Mel Gorman
> Part-time Phd Student Linux Technology Center
> University of Limerick IBM Dublin Software Lab

--
Mel Gorman
Part-time Phd Student Linux Technology Center
University of Limerick IBM Dublin Software Lab