On Mon, Apr 25, 2016 at 11:33:15AM +0200, Vlastimil Babka wrote: > On 04/15/2016 10:58 AM, Mel Gorman wrote: > >order-0 pages by definition cannot be compound so avoid the check in the > >fast path for those pages. > > > >Signed-off-by: Mel Gorman <[email protected]> > > Acked-by: Vlastimil Babka <[email protected]> > > Suggestion to improve below: > > >--- > > mm/page_alloc.c | 25 +++++++++++++++++-------- > > 1 file changed, 17 insertions(+), 8 deletions(-) > > > >diff --git a/mm/page_alloc.c b/mm/page_alloc.c > >index 59de90d5d3a3..5d205bcfe10d 100644 > >--- a/mm/page_alloc.c > >+++ b/mm/page_alloc.c > >@@ -1024,24 +1024,33 @@ void __meminit reserve_bootmem_region(unsigned long > >start, unsigned long end) > > > > static bool free_pages_prepare(struct page *page, unsigned int order) > > { > >- bool compound = PageCompound(page); > >- int i, bad = 0; > >+ int bad = 0; > > > > VM_BUG_ON_PAGE(PageTail(page), page); > >- VM_BUG_ON_PAGE(compound && compound_order(page) != order, page); > > > > trace_mm_page_free(page, order); > > kmemcheck_free_shadow(page, order); > > kasan_free_pages(page, order); > > > >+ /* > >+ * Check tail pages before head page information is cleared to > >+ * avoid checking PageCompound for order-0 pages. > >+ */ > >+ if (order) { > > Sticking unlikely() here results in: > > add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-30 (-30) > function old new delta > free_pages_prepare 771 741 -30 > > And from brief comparison of disassembly it really seems it's moved the > compound handling towards the end of the function, which should be nicer for > the instruction cache, branch prediction etc. And since this series is about > microoptimization, I think the extra step is worth it. >
I dithered on this a bit and could not convince myself that the order case really is unlikely. It depends on the situation as we could be tearing down a large THP-backed mapping. SLUB is also using compound pages so it's both workload and configuration dependent whether this path is really likely or not. -- Mel Gorman SUSE Labs
