From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1422914AbeCBI06 (ORCPT ); Fri, 2 Mar 2018 03:26:58 -0500 Received: from mga09.intel.com ([134.134.136.24]:19901 "EHLO mga09.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1422857AbeCBI05 (ORCPT ); Fri, 2 Mar 2018 03:26:57 -0500 X-Amp-Result: UNSCANNABLE X-Amp-File-Uploaded: False X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.47,411,1515484800"; d="scan'208";a="30902997" Date: Fri, 2 Mar 2018 16:27:56 +0800 From: Aaron Lu To: Andrew Morton Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Huang Ying , Dave Hansen , Kemi Wang , Tim Chen , Andi Kleen , Michal Hocko , Vlastimil Babka , Mel Gorman , Matthew Wilcox , David Rientjes Subject: Re: [PATCH v4 3/3] mm/free_pcppages_bulk: prefetch buddy while not holding lock Message-ID: <20180302082756.GC6356@intel.com> References: <20180301062845.26038-1-aaron.lu@intel.com> <20180301062845.26038-4-aaron.lu@intel.com> <20180301160950.b561d6b8b561217bad511229@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180301160950.b561d6b8b561217bad511229@linux-foundation.org> User-Agent: Mutt/1.9.2 (2017-12-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Mar 01, 2018 at 04:09:50PM -0800, Andrew Morton wrote: > On Thu, 1 Mar 2018 14:28:45 +0800 Aaron Lu wrote: > > > When a page is freed back to the global pool, its buddy will be checked > > to see if it's possible to do a merge. This requires accessing buddy's > > page structure and that access could take a long time if it's cache cold. > > > > This patch adds a prefetch to the to-be-freed page's buddy outside of > > zone->lock in hope of accessing buddy's page structure later under > > zone->lock will be faster. Since we *always* do buddy merging and check > > an order-0 page's buddy to try to merge it when it goes into the main > > allocator, the cacheline will always come in, i.e. the prefetched data > > will never be unused. > > > > In the meantime, there are two concerns: > > 1 the prefetch could potentially evict existing cachelines, especially > > for L1D cache since it is not huge; > > 2 there is some additional instruction overhead, namely calculating > > buddy pfn twice. > > > > For 1, it's hard to say, this microbenchmark though shows good result but > > the actual benefit of this patch will be workload/CPU dependant; > > For 2, since the calculation is a XOR on two local variables, it's expected > > in many cases that cycles spent will be offset by reduced memory latency > > later. This is especially true for NUMA machines where multiple CPUs are > > contending on zone->lock and the most time consuming part under zone->lock > > is the wait of 'struct page' cacheline of the to-be-freed pages and their > > buddies. > > > > Test with will-it-scale/page_fault1 full load: > > > > kernel Broadwell(2S) Skylake(2S) Broadwell(4S) Skylake(4S) > > v4.16-rc2+ 9034215 7971818 13667135 15677465 > > patch2/3 9536374 +5.6% 8314710 +4.3% 14070408 +3.0% 16675866 +6.4% > > this patch 10338868 +8.4% 8544477 +2.8% 14839808 +5.5% 17155464 +2.9% > > Note: this patch's performance improvement percent is against patch2/3. > > > > ... > > > > @@ -1150,6 +1153,18 @@ static void free_pcppages_bulk(struct zone *zone, int count, > > continue; > > > > list_add_tail(&page->lru, &head); > > + > > + /* > > + * We are going to put the page back to the global > > + * pool, prefetch its buddy to speed up later access > > + * under zone->lock. It is believed the overhead of > > + * calculating buddy_pfn here can be offset by reduced > > + * memory latency later. > > + */ > > + pfn = page_to_pfn(page); > > + buddy_pfn = __find_buddy_pfn(pfn, 0); > > + buddy = page + (buddy_pfn - pfn); > > + prefetch(buddy); > > What is the typical list length here? Maybe it's approximately the pcp > batch size which is typically 128 pages? Most of time it is pcp->batch, unless when pcp's pages need to be all drained like in drain_local_pages(zone). The pcp->batch has a default value of 31 and its upper limit is 96 for x86_64. For this test, it is 31 here, I didn't manipulate /proc/sys/vm/percpu_pagelist_fraction to change it. With this said, the count here could be pcp->count when pcp's pages need to be all drained and though pcp->count's default value is (6*pcp->batch)=186, user can increase that value through the above mentioned procfs interface and the resulting pcp->count could be too big for prefetch. Ying also mentioned this today and suggested adding an upper limit here to avoid prefetching too much. Perhaps just prefetch the last pcp->batch pages if count here > pcp->batch? Since pcp->batch has an upper limit, we won't need to worry prefetching too much. > > If so, I'm a bit surprised that it is effective to prefetch 128 page > frames before using any them for real. I guess they'll fit in the L2 > cache. Thoughts?