From mboxrd@z Thu Jan 1 00:00:00 1970 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 865DEC433FE for ; Tue, 12 Apr 2022 02:16:20 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 0BE9D6B0073; Mon, 11 Apr 2022 22:16:20 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 045B56B0074; Mon, 11 Apr 2022 22:16:19 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id DDC286B0075; Mon, 11 Apr 2022 22:16:19 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (relay.hostedemail.com [64.99.140.28]) by kanga.kvack.org (Postfix) with ESMTP id CBFF46B0073 for ; Mon, 11 Apr 2022 22:16:19 -0400 (EDT) Received: from smtpin02.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay09.hostedemail.com (Postfix) with ESMTP id A30A22443D for ; Tue, 12 Apr 2022 02:16:19 +0000 (UTC) X-FDA: 79346612478.02.54292C0 Received: from dfw.source.kernel.org (dfw.source.kernel.org [139.178.84.217]) by imf18.hostedemail.com (Postfix) with ESMTP id 0E7071C0004 for ; Tue, 12 Apr 2022 02:16:18 +0000 (UTC) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by dfw.source.kernel.org (Postfix) with ESMTPS id 1F8EA616B0; Tue, 12 Apr 2022 02:16:18 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 09DBAC385A4; Tue, 12 Apr 2022 02:16:16 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linux-foundation.org; s=korg; t=1649729777; bh=+vTX2+LmH++7Pu5Shh4Uo4lXVzWdHkKYI41k6YSLJJE=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=v7M7INNLgEZQE6SBiglzE66UBjgkcXhuHBB5+i1GqjY3bnVXvwddo7ZV+t64wV8jW rQswAF4M1KJZDWoB+yWTRxmIa8yOkVwrjoqEzbqgpuH20d7tceiFye8aJhHv2U6dXc cVYYtMP4kr5qoIrc7sGwiQxWx7FrtUji3t7YTkZY= Date: Mon, 11 Apr 2022 19:16:15 -0700 From: Andrew Morton To: Yu Zhao Cc: Stephen Rothwell , linux-mm@kvack.org, Andi Kleen , Aneesh Kumar , Barry Song <21cnbao@gmail.com>, Catalin Marinas , Dave Hansen , Hillf Danton , Jens Axboe , Jesse Barnes , Johannes Weiner , Jonathan Corbet , Linus Torvalds , Matthew Wilcox , Mel Gorman , Michael Larabel , Michal Hocko , Mike Rapoport , Rik van Riel , Vlastimil Babka , Will Deacon , Ying Huang , linux-arm-kernel@lists.infradead.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, page-reclaim@google.com, x86@kernel.org, Brian Geffon , Jan Alexander Steffens , Oleksandr Natalenko , Steven Barrett , Suleiman Souhlal , Daniel Byrne , Donald Carr , =?ISO-8859-1?Q?"Holger_Hoffst=E4tte"?= , Konstantin Kharlamov , Shuang Zhai , Sofia Trinh , Vaibhav Jain Subject: Re: [PATCH v10 05/14] mm: multi-gen LRU: groundwork Message-Id: <20220411191615.a34959bdcc25ef3f9c16a7ce@linux-foundation.org> In-Reply-To: <20220407031525.2368067-6-yuzhao@google.com> References: <20220407031525.2368067-1-yuzhao@google.com> <20220407031525.2368067-6-yuzhao@google.com> X-Mailer: Sylpheed 3.7.0 (GTK+ 2.24.33; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: 0E7071C0004 X-Stat-Signature: 6rq4mgrk4to563yxs7uhzhasct91uats Authentication-Results: imf18.hostedemail.com; dkim=pass header.d=linux-foundation.org header.s=korg header.b=v7M7INNL; dmarc=none; spf=pass (imf18.hostedemail.com: domain of akpm@linux-foundation.org designates 139.178.84.217 as permitted sender) smtp.mailfrom=akpm@linux-foundation.org X-Rspam-User: X-HE-Tag: 1649729778-774184 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: On Wed, 6 Apr 2022 21:15:17 -0600 Yu Zhao wrote: > Evictable pages are divided into multiple generations for each lruvec. > The youngest generation number is stored in lrugen->max_seq for both > anon and file types as they are aged on an equal footing. The oldest > generation numbers are stored in lrugen->min_seq[] separately for anon > and file types as clean file pages can be evicted regardless of swap > constraints. These three variables are monotonically increasing. > > ... > > +static inline bool lru_gen_del_folio(struct lruvec *lruvec, struct folio *folio, bool reclaiming) There's a lot of function inlining here. Fortunately the compiler will ignore it all, because some of it looks wrong. Please review (and remeasure!). If inlining is reqlly justified, use __always_inline, and document the reasons for doing so. > +{ > + int gen; > + unsigned long old_flags, new_flags; > + > + do { > + new_flags = old_flags = READ_ONCE(folio->flags); > + if (!(new_flags & LRU_GEN_MASK)) > + return false; > + > + VM_BUG_ON_FOLIO(folio_test_active(folio), folio); > + VM_BUG_ON_FOLIO(folio_test_unevictable(folio), folio); > + > + gen = ((new_flags & LRU_GEN_MASK) >> LRU_GEN_PGOFF) - 1; > + > + new_flags &= ~LRU_GEN_MASK; > + /* for shrink_page_list() */ > + if (reclaiming) > + new_flags &= ~(BIT(PG_referenced) | BIT(PG_reclaim)); > + else if (lru_gen_is_active(lruvec, gen)) > + new_flags |= BIT(PG_active); > + } while (cmpxchg(&folio->flags, old_flags, new_flags) != old_flags); Clearly the cmpxchg loop is handling races against a concurrent updater. But it's unclear who that updater is, what are the dynamics here and why the designer didn't use, say, spin_lock(). The way to clarify such thigs is with code comments! > > +#endif /* !__GENERATING_BOUNDS_H */ > + > +/* > + * Evictable pages are divided into multiple generations. The youngest and the > + * oldest generation numbers, max_seq and min_seq, are monotonically increasing. > + * They form a sliding window of a variable size [MIN_NR_GENS, MAX_NR_GENS]. An > + * offset within MAX_NR_GENS, gen, indexes the LRU list of the corresponding The "within MAX_NR_GENS, gen," text here is unclear? > + * generation. The gen counter in folio->flags stores gen+1 while a page is on > + * one of lrugen->lists[]. Otherwise it stores 0. > + * > + * A page is added to the youngest generation on faulting. The aging needs to > + * check the accessed bit at least twice before handing this page over to the > + * eviction. The first check takes care of the accessed bit set on the initial > + * fault; the second check makes sure this page hasn't been used since then. > + * This process, AKA second chance, requires a minimum of two generations, > + * hence MIN_NR_GENS. And to maintain ABI compatibility with the active/inactive Where is the ABI compatibility issue? Is it in some way in which the legacy LRU is presented to userspace? > + * LRU, these two generations are considered active; the rest of generations, if > + * they exist, are considered inactive. See lru_gen_is_active(). PG_active is > + * always cleared while a page is on one of lrugen->lists[] so that the aging > + * needs not to worry about it. And it's set again when a page considered active > + * is isolated for non-reclaiming purposes, e.g., migration. See > + * lru_gen_add_folio() and lru_gen_del_folio(). > + * > + * MAX_NR_GENS is set to 4 so that the multi-gen LRU can support twice of the "twice the number of"? > + * categories of the active/inactive LRU when keeping track of accesses through > + * page tables. It requires order_base_2(MAX_NR_GENS+1) bits in folio->flags. > + */ Helpful comment, overall. > > ... > > --- a/mm/Kconfig > +++ b/mm/Kconfig > @@ -909,6 +909,14 @@ config ANON_VMA_NAME > area from being merged with adjacent virtual memory areas due to the > difference in their name. > > +config LRU_GEN > + bool "Multi-Gen LRU" > + depends on MMU > + # the following options can use up the spare bits in page flags > + depends on !MAXSMP && (64BIT || !SPARSEMEM || SPARSEMEM_VMEMMAP) > + help > + A high performance LRU implementation to overcommit memory. > + > source "mm/damon/Kconfig" This is a problem. I had to jump through hoops just to be able to compile-test this. Turns out I had to figure out how to disable MAXSMP. Can we please figure out a way to ensure that more testers are at least compile testing this? Allnoconfig, defconfig, allyesconfig, allmodconfig. Also, I suggest that we actually make MGLRU the default while in linux-next. > > ... >