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SubjectRe: [PATCH 4/5] page allocator: Pre-emptively wake kswapd when high-order watermarks are hit
On Tue, Oct 27, 2009 at 11:42:58AM +0900, KOSAKI Motohiro wrote:
> > When a high-order allocation fails, kswapd is kicked so that it reclaims
> > at a higher-order to avoid direct reclaimers stall and to help GFP_ATOMIC
> > allocations. Something has changed in recent kernels that affect the timing
> > where high-order GFP_ATOMIC allocations are now failing with more frequency,
> > particularly under pressure.
> >
> > This patch pre-emptively checks if watermarks have been hit after a
> > high-order allocation completes successfully. If the watermarks have been
> > reached, kswapd is woken in the hope it fixes the watermarks before the
> > next GFP_ATOMIC allocation fails.
> >
> > Warning, this patch is somewhat of a band-aid. If this makes a difference,
> > it still implies that something has changed that is either causing more
> > GFP_ATOMIC allocations to occur (such as the case with iwlagn wireless
> > driver) or make them more likely to fail.
>
> hmm, I'm confused. this description addressed generic high order allocation.
> but,
>
> >
> > Signed-off-by: Mel Gorman <mel@csn.ul.ie>
> > ---
> > mm/page_alloc.c | 33 ++++++++++++++++++++++-----------
> > 1 files changed, 22 insertions(+), 11 deletions(-)
> >
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 7f2aa3e..851df40 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -1596,6 +1596,17 @@ try_next_zone:
> > return page;
> > }
> >
> > +static inline
> > +void wake_all_kswapd(unsigned int order, struct zonelist *zonelist,
> > + enum zone_type high_zoneidx)
> > +{
> > + struct zoneref *z;
> > + struct zone *zone;
> > +
> > + for_each_zone_zonelist(zone, z, zonelist, high_zoneidx)
> > + wakeup_kswapd(zone, order);
> > +}
> > +
> > static inline int
> > should_alloc_retry(gfp_t gfp_mask, unsigned int order,
> > unsigned long pages_reclaimed)
> > @@ -1730,18 +1741,18 @@ __alloc_pages_high_priority(gfp_t gfp_mask, unsigned int order,
>
> __alloc_pages_high_priority() is only called if ALLOC_NO_WATERMARKS.
> ALLOC_NO_WATERMARKS mean PF_MEMALLOC or TIF_MEMDIE and GFP_ATOMIC don't make
> nested alloc_pages() (= don't make PF_MEMALLOC case).
> Then, I haven't understand why this patch improve iwlagn GFP_ATOMIC case.
>
> hmm, maybe I missed something. I see the code again tommorow.
>

The description is misleading but in the patches current form, it makes
a different to Tobias's testing. I still haven't figured out why.

>
> > congestion_wait(BLK_RW_ASYNC, HZ/50);
> > } while (!page && (gfp_mask & __GFP_NOFAIL));
> >
> > - return page;
> > -}
> > -
> > -static inline
> > -void wake_all_kswapd(unsigned int order, struct zonelist *zonelist,
> > - enum zone_type high_zoneidx)
> > -{
> > - struct zoneref *z;
> > - struct zone *zone;
> > + /*
> > + * If after a high-order allocation we are now below watermarks,
> > + * pre-emptively kick kswapd rather than having the next allocation
> > + * fail and have to wake up kswapd, potentially failing GFP_ATOMIC
> > + * allocations or entering direct reclaim
> > + */
> > + if (unlikely(order) && page && !zone_watermark_ok(preferred_zone, order,
> > + preferred_zone->watermark[ALLOC_WMARK_LOW],
> > + zone_idx(preferred_zone), ALLOC_WMARK_LOW))
> > + wake_all_kswapd(order, zonelist, high_zoneidx);
> >
> > - for_each_zone_zonelist(zone, z, zonelist, high_zoneidx)
> > - wakeup_kswapd(zone, order);
> > + return page;
> > }
> >
> > static inline int
> > --
> > 1.6.3.3
> >
> > --
> > To unsubscribe, send a message with 'unsubscribe linux-mm' in
> > the body to majordomo@kvack.org. For more info on Linux MM,
> > see: http://www.linux-mm.org/ .
> > Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
>
>
>

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


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