On Mon, 4 Aug 2014 10:55:22 +0200 Vlastimil Babka <[email protected]> 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. > > Testing with stress-highalloc from mmtests shows a 15% reduction in number of > pages scanned by migration scanner. The reduction is >60% with __GFP_NO_KSWAPD > allocations, along with success rates better by few percent. > This change is also a prerequisite for a later patch which is detecting when > a cc->order block of pages contains non-buddy pages that cannot be isolated, > and the scanner should thus skip to the next block immediately. What is this "later patch"? Or is the changelog stale? -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
