From patchwork Mon Feb 21 09:41:19 2022 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Mel Gorman X-Patchwork-Id: 12753298 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id 48895C433F5 for ; Mon, 21 Feb 2022 09:41:45 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id C76C08D0003; Mon, 21 Feb 2022 04:41:44 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id BFF398D0001; Mon, 21 Feb 2022 04:41:44 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id AA0928D0003; Mon, 21 Feb 2022 04:41:44 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (relay.hostedemail.com [64.99.140.27]) by kanga.kvack.org (Postfix) with ESMTP id 9862A8D0001 for ; Mon, 21 Feb 2022 04:41:44 -0500 (EST) Received: from smtpin02.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay01.hostedemail.com (Postfix) with ESMTP id 3A63D60817 for ; Mon, 21 Feb 2022 09:41:44 +0000 (UTC) X-FDA: 79166294928.02.9EDF62A Received: from outbound-smtp52.blacknight.com (outbound-smtp52.blacknight.com [46.22.136.236]) by imf01.hostedemail.com (Postfix) with ESMTP id DAC1640005 for ; Mon, 21 Feb 2022 09:41:42 +0000 (UTC) Received: from mail.blacknight.com (pemlinmail03.blacknight.ie [81.17.254.16]) by outbound-smtp52.blacknight.com (Postfix) with ESMTPS id B39B7FB611 for ; Mon, 21 Feb 2022 09:41:41 +0000 (GMT) Received: (qmail 32136 invoked from network); 21 Feb 2022 09:41:41 -0000 Received: from unknown (HELO stampy.112glenside.lan) (mgorman@techsingularity.net@[84.203.17.223]) by 81.17.254.9 with ESMTPA; 21 Feb 2022 09:41:41 -0000 From: Mel Gorman To: Andrew Morton Cc: Aaron Lu , Dave Hansen , Vlastimil Babka , Michal Hocko , Jesper Dangaard Brouer , LKML , Linux-MM , Mel Gorman Subject: [PATCH 1/1] mm/page_alloc: Do not prefetch buddies during bulk free Date: Mon, 21 Feb 2022 09:41:19 +0000 Message-Id: <20220221094119.15282-2-mgorman@techsingularity.net> X-Mailer: git-send-email 2.26.2 In-Reply-To: <20220221094119.15282-1-mgorman@techsingularity.net> References: <20220221094119.15282-1-mgorman@techsingularity.net> MIME-Version: 1.0 Authentication-Results: imf01.hostedemail.com; dkim=none; dmarc=none; spf=pass (imf01.hostedemail.com: domain of mgorman@techsingularity.net designates 46.22.136.236 as permitted sender) smtp.mailfrom=mgorman@techsingularity.net X-Rspam-User: X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: DAC1640005 X-Stat-Signature: cynyqt4hh8xywuc3mr7iuwb1tzhymkp1 X-HE-Tag: 1645436502-275656 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: free_pcppages_bulk() has taken two passes through the pcp lists since commit 0a5f4e5b4562 ("mm/free_pcppages_bulk: do not hold lock when picking pages to free") due to deferring the cost of selecting PCP lists until the zone lock is held. As the list processing now takes place under the zone lock, it's less clear that this will always benefit for two reasons. 1. There is a guaranteed cost to calculating the buddy which definitely has to be calculated again. However, as the zone lock is held and there is no deferring of buddy merging, there is no guarantee that the prefetch will have completed when the second buddy calculation takes place and buddies are being merged. With or without the prefetch, there may be further stalls depending on how many pages get merged. In other words, a stall due to merging is inevitable and at best only one stall might be avoided at the cost of calculating the buddy location twice. 2. As the zone lock is held, prefetch_nr makes less sense as once prefetch_nr expires, the cache lines of interest have already been merged. The main concern is that there is a definite cost to calculating the buddy location early for the prefetch and it is a "maybe win" depending on whether the CPU prefetch logic and memory is fast enough. Remove the prefetch logic on the basis that reduced instructions in a path is always a saving where as the prefetch might save one memory stall depending on the CPU and memory. In most cases, this has marginal benefit as the calculations are a small part of the overall freeing of pages. However, it was detectable on at least one machine. 5.17.0-rc3 5.17.0-rc3 mm-highpcplimit-v2r1 mm-noprefetch-v1r1 Min elapsed 630.00 ( 0.00%) 610.00 ( 3.17%) Amean elapsed 639.00 ( 0.00%) 623.00 * 2.50%* Max elapsed 660.00 ( 0.00%) 660.00 ( 0.00%) Suggested-by: Aaron Lu Signed-off-by: Mel Gorman Reviewed-by: Vlastimil Babka Reviewed-by: Aaron Lu --- mm/page_alloc.c | 24 ------------------------ 1 file changed, 24 deletions(-) diff --git a/mm/page_alloc.c b/mm/page_alloc.c index de9f072d23bd..2d5cc098136d 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -1432,15 +1432,6 @@ static bool bulkfree_pcp_prepare(struct page *page) } #endif /* CONFIG_DEBUG_VM */ -static inline void prefetch_buddy(struct page *page, unsigned int order) -{ - unsigned long pfn = page_to_pfn(page); - unsigned long buddy_pfn = __find_buddy_pfn(pfn, order); - struct page *buddy = page + (buddy_pfn - pfn); - - prefetch(buddy); -} - /* * Frees a number of pages from the PCP lists * Assumes all pages on list are in same zone. @@ -1453,7 +1444,6 @@ static void free_pcppages_bulk(struct zone *zone, int count, int min_pindex = 0; int max_pindex = NR_PCP_LISTS - 1; unsigned int order; - int prefetch_nr = READ_ONCE(pcp->batch); bool isolated_pageblocks; struct page *page; @@ -1508,20 +1498,6 @@ static void free_pcppages_bulk(struct zone *zone, int count, if (bulkfree_pcp_prepare(page)) continue; - /* - * 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 - * an additional test and calculating buddy_pfn here - * can be offset by reduced memory latency later. To - * avoid excessive prefetching due to large count, only - * prefetch buddy for the first pcp->batch nr of pages. - */ - if (prefetch_nr) { - prefetch_buddy(page, order); - prefetch_nr--; - } - /* MIGRATE_ISOLATE page should not go to pcplists */ VM_BUG_ON_PAGE(is_migrate_isolate(mt), page); /* Pageblock could have been isolated meanwhile */