From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753999AbcDYJdT (ORCPT ); Mon, 25 Apr 2016 05:33:19 -0400 Received: from mx2.suse.de ([195.135.220.15]:55334 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753673AbcDYJdS (ORCPT ); Mon, 25 Apr 2016 05:33:18 -0400 Subject: Re: [PATCH 01/28] mm, page_alloc: Only check PageCompound for high-order pages To: Mel Gorman , Andrew Morton References: <1460710760-32601-1-git-send-email-mgorman@techsingularity.net> <1460710760-32601-2-git-send-email-mgorman@techsingularity.net> Cc: Jesper Dangaard Brouer , Linux-MM , LKML From: Vlastimil Babka Message-ID: <571DE45B.2050504@suse.cz> Date: Mon, 25 Apr 2016 11:33:15 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.7.2 MIME-Version: 1.0 In-Reply-To: <1460710760-32601-2-git-send-email-mgorman@techsingularity.net> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 Acked-by: Vlastimil Babka 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. > + bool compound = PageCompound(page); > + int i; > + > + VM_BUG_ON_PAGE(compound && compound_order(page) != order, page); > + > + for (i = 1; i < (1 << order); i++) { > + if (compound) > + bad += free_tail_pages_check(page, page + i); > + bad += free_pages_check(page + i); > + } > + } > if (PageAnon(page)) > page->mapping = NULL; > bad += free_pages_check(page); > - for (i = 1; i < (1 << order); i++) { > - if (compound) > - bad += free_tail_pages_check(page, page + i); > - bad += free_pages_check(page + i); > - } > if (bad) > return false; > >