From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx135.postini.com [74.125.245.135]) by kanga.kvack.org (Postfix) with SMTP id 7F5946B0036 for ; Wed, 14 Aug 2013 23:46:53 -0400 (EDT) Message-ID: <520C4EFF.8040305@huawei.com> Date: Thu, 15 Aug 2013 11:46:07 +0800 From: Xishi Qiu MIME-Version: 1.0 Subject: Re: [PATCH] mm: skip the page buddy block instead of one page 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> <20130815024427.GA2718@gmail.com> In-Reply-To: <20130815024427.GA2718@gmail.com> Content-Type: text/plain; charset="ISO-8859-1" Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Minchan Kim Cc: Mel Gorman , Andrew Morton , riel@redhat.com, aquini@redhat.com, linux-mm@kvack.org, LKML , Xishi Qiu On 2013/8/15 10:44, Minchan Kim wrote: > 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 Hi Minchan, I mean if the private is set to a large number, it will skip 2^private pages, not (pageblock_nr_pages -1). I find somewhere will use page->private, such as fs. Here is the comment about parivate. /* Mapping-private opaque data: * usually used for buffer_heads * if PagePrivate set; used for * swp_entry_t if PageSwapCache; * indicates order in the buddy * system if PG_buddy is set. */ Thanks, Xishi Qiu > 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. >>> >> >> >> > -- 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