On Mon, 9 Jun 2014, Vlastimil Babka wrote: > The migration scanner skips PageBuddy pages, but does not consider their order > as checking page_order() is generally unsafe without holding the zone->lock, > and acquiring the lock just for the check wouldn't be a good tradeoff. > > Still, this could avoid some iterations over the rest of the buddy page, and > if we are careful, the race window between PageBuddy() check and page_order() > is small, and the worst thing that can happen is that we skip too much and miss > some isolation candidates. This is not that bad, as compaction can already fail > for many other reasons like parallel allocations, and those have much larger > race window. > > This patch therefore makes the migration scanner obtain the buddy page order > and use it to skip the whole buddy page, if the order appears to be in the > valid range. > > It's important that the page_order() is read only once, so that the value used > in the checks and in the pfn calculation is the same. But in theory the > compiler can replace the local variable by multiple inlines of page_order(). > Therefore, the patch introduces page_order_unsafe() that uses ACCESS_ONCE to > prevent this. > > Preliminary results with stress-highalloc from mmtests show a 10% reduction in > number of pages scanned by migration scanner. This change is also important to > later allow detecting when a cc->order block of pages cannot be compacted, and > the scanner should skip to the next block instead of wasting time. > > Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx> > Cc: Minchan Kim <minchan@xxxxxxxxxx> > Cc: Mel Gorman <mgorman@xxxxxxx> > Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx> > Cc: Michal Nazarewicz <mina86@xxxxxxxxxx> > Cc: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx> > Cc: Christoph Lameter <cl@xxxxxxxxx> > Cc: Rik van Riel <riel@xxxxxxxxxx> > Cc: David Rientjes <rientjes@xxxxxxxxxx> > --- > V2: fix low_pfn > end_pfn check; comments > kept page_order_unsafe() approach for now > Please see http://marc.info/?l=linux-mm&m=140235272808846, I'd love to be proved wrong. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>