From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx170.postini.com [74.125.245.170]) by kanga.kvack.org (Postfix) with SMTP id 93BC26B0032 for ; Wed, 14 Aug 2013 22:44:38 -0400 (EDT) Received: by mail-ie0-f182.google.com with SMTP id tp5so455927ieb.13 for ; Wed, 14 Aug 2013 19:44:38 -0700 (PDT) Date: Thu, 15 Aug 2013 11:44:27 +0900 From: Minchan Kim Subject: Re: [PATCH] mm: skip the page buddy block instead of one page Message-ID: <20130815024427.GA2718@gmail.com> References: <520B0B75.4030708@huawei.com> <20130814085711.GK2296@suse.de> <20130814155205.GA2706@gmail.com> <20130814161642.GM2296@suse.de> <20130814163921.GC2706@gmail.com> <20130814180012.GO2296@suse.de> <520C3DD2.8010905@huawei.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <520C3DD2.8010905@huawei.com> Sender: owner-linux-mm@kvack.org List-ID: To: Xishi Qiu Cc: Mel Gorman , Andrew Morton , riel@redhat.com, aquini@redhat.com, linux-mm@kvack.org, LKML Hi Xishi, On Thu, Aug 15, 2013 at 10:32:50AM +0800, Xishi Qiu wrote: > On 2013/8/15 2:00, Mel Gorman wrote: > > >>> Even if the page is still page buddy, there is no guarantee that it's > >>> the same page order as the first read. It could have be currently > >>> merging with adjacent buddies for example. There is also a really > >>> small race that a page was freed, allocated with some number stuffed > >>> into page->private and freed again before the second PageBuddy check. > >>> It's a bit of a hand grenade. How much of a performance benefit is there > >> > >> 1. Just worst case is skipping pageblock_nr_pages > > > > No, the worst case is that page_order returns a number that is > > completely garbage and low_pfn goes off the end of the zone > > > >> 2. Race is really small > >> 3. Higher order page allocation customer always have graceful fallback. > >> > > Hi Minchan, > I think in this case, we may get the wrong value from page_order(page). > > 1. page is in page buddy > > > if (PageBuddy(page)) { > > 2. someone allocated the page, and set page->private to another value > > > int nr_pages = (1 << page_order(page)) - 1; > > 3. someone freed the page > > > if (PageBuddy(page)) { > > 4. we will skip wrong pages So, what's the result by that? As I said, it's just skipping (pageblock_nr_pages -1) at worst case and the case you mentioned is right academically and I and Mel already pointed out that. But how often could that happen in real practice? I believe such is REALLY REALLY rare. So, as Mel said, if you have some workloads to see the benefit from this patch, I think we could accept the patch. Could you try and respin with the number? I guess big contigous memory range or memory-hotplug which are full of free pages in embedded CPU which is rather slower than server or desktop side could have benefit. Thanks. > > > nr_pages = min(nr_pages, MAX_ORDER_NR_PAGES - 1); > > low_pfn += nr_pages; > > continue; > > } > > } > > > > It's still race-prone meaning that it really should be backed by some > > performance data justifying it. > > > > > -- Kind regards, Minchan Kim -- 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: email@kvack.org