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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 1D6BAC433EF for ; Tue, 26 Apr 2022 22:39:49 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1355891AbiDZWmz (ORCPT ); Tue, 26 Apr 2022 18:42:55 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:47364 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S240418AbiDZWmy (ORCPT ); Tue, 26 Apr 2022 18:42:54 -0400 Received: from mail-vs1-xe34.google.com (mail-vs1-xe34.google.com [IPv6:2607:f8b0:4864:20::e34]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 735FF1785A7 for ; Tue, 26 Apr 2022 15:39:45 -0700 (PDT) Received: by mail-vs1-xe34.google.com with SMTP id z144so133776vsz.13 for ; Tue, 26 Apr 2022 15:39:45 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20210112; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=07Zd/16O2HE5Be6dl0YhxA/WYrWu/Ra4RQxC/o3rHN4=; b=mXqqHglO8XEfpNHe4xKmQqH8vvmuEmWI2bFNvmyTbbCSV0ya7UVXg/CycRw8F18FwN UcSuyVAxAkkxGzNmJMQQLPhr7tKKwVRzQg5TEQVjgMzzOOilYUIOThNtoL67+JZ+ZyhN 5Pajba/FLU+hVEX2+YLmzV+ru2ChYCNly/8fbTWXtQoLaEtfEOoL7/pUYffGG7YWBfwo IwcYTlGqHA67e2TC163DJobE7uBZLHCJTXOOq/kEfJ0EBn/W8KeKWzAk1RDA7rhSVdju YJznNyD0LtFe11RwAAZLedbIeNvEpcI7u+tykPQ0OZYSXc4Ih1OudNrgdkLaGY0wVuJs QhnQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=07Zd/16O2HE5Be6dl0YhxA/WYrWu/Ra4RQxC/o3rHN4=; b=D1NY5Az+iz5d9M2ozJnZcQrTKw2x/LKqpHNox67FyJIob9AfA3ljxYu++lCZRGU4W3 DFbNw9DEuGZyWu4TejKVRi6iEloL1v9SFcP6j/mB47BdQlmTieV41Ufo5hHzW8SZJwuW VMhSORffOv7Swb+kPz18doav7zPD+57TajyD7FKR59/s1U6nyPfe/g6OSU6Bgb/73a++ rsv7GcNtxa3J5dICgFPQJ9HkFkYMhMg37UiFl6ENMv+VV2BcHd/cMNnFjDP05DIXzOmW Gv2q+zQEYsmuiUwzR9iNAzdO7CdWAKjO3jDkKKIVG9FcqSnaeZ4fGqLvycgNEYJRLQZZ CovA== X-Gm-Message-State: AOAM530jiNg1JCstXR76kmV9KcQkRKdZFY0rTeJvyxCBSEB09+6fXwEz hnP/+caLZUOJEvh71R3se263uCZ/H7Vc+gfqb5nCMg== X-Google-Smtp-Source: ABdhPJz+toQujxv6dyO8lpqxNaRIpc4rYh12ObWjdCPFl2wj3PCxuXZcbz1W3wRfVwURFc/mI9NOxhiwyLvLEQzX21c= X-Received: by 2002:a05:6102:158a:b0:32a:56ea:3fba with SMTP id g10-20020a056102158a00b0032a56ea3fbamr7477528vsv.84.1651012784332; Tue, 26 Apr 2022 15:39:44 -0700 (PDT) MIME-Version: 1.0 References: <20220407031525.2368067-1-yuzhao@google.com> <20220407031525.2368067-6-yuzhao@google.com> <20220411191615.a34959bdcc25ef3f9c16a7ce@linux-foundation.org> In-Reply-To: <20220411191615.a34959bdcc25ef3f9c16a7ce@linux-foundation.org> From: Yu Zhao Date: Tue, 26 Apr 2022 16:39:07 -0600 Message-ID: Subject: Re: [PATCH v10 05/14] mm: multi-gen LRU: groundwork To: Andrew Morton Cc: Stephen Rothwell , Linux-MM , 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 , "open list:DOCUMENTATION" , linux-kernel , Kernel Page Reclaim v2 , "the arch/x86 maintainers" , Brian Geffon , Jan Alexander Steffens , Oleksandr Natalenko , Steven Barrett , Suleiman Souhlal , Daniel Byrne , Donald Carr , =?UTF-8?Q?Holger_Hoffst=C3=A4tte?= , Konstantin Kharlamov , Shuang Zhai , Sofia Trinh , Vaibhav Jain Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Apr 11, 2022 at 8:16 PM Andrew Morton wrote: > > 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. I totally expect modern compilers to make better decisions than I do. And personally, I'd never use __always_inline; instead, I'd strongly recommend FDO/LTO. > > +{ > > + 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! Right. set_mask_bits() should suffice here. > > +#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? Will update: "i.e., gen". > > + * 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? Will update: yes, active/inactive LRU sizes in /proc/vmstat. > > + * 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"? Will update. > > + * 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. The !MAXSMP is to work around [1], which I haven't had the time to fix. That BUILD_BUG_ON() shouldn't assert sizeof(struct page) == 64 since the true size depends on WANT_PAGE_VIRTUAL as well as LAST_CPUPID_NOT_IN_PAGE_FLAGS. My plan is here [2]. [1] https://lore.kernel.org/r/20190905154603.10349-4-aneesh.kumar@linux.ibm.com/ [2] https://lore.kernel.org/r/Ygl1Gf+ATBuI%2Fm2q@google.com/ 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 bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 6533AC433F5 for ; Tue, 26 Apr 2022 22:41:04 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender: Content-Transfer-Encoding:Content-Type:List-Subscribe:List-Help:List-Post: List-Archive:List-Unsubscribe:List-Id:Cc:To:Subject:Message-ID:Date:From: In-Reply-To:References:MIME-Version:Reply-To:Content-ID:Content-Description: Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID: List-Owner; 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Tue, 26 Apr 2022 15:39:44 -0700 (PDT) MIME-Version: 1.0 References: <20220407031525.2368067-1-yuzhao@google.com> <20220407031525.2368067-6-yuzhao@google.com> <20220411191615.a34959bdcc25ef3f9c16a7ce@linux-foundation.org> In-Reply-To: <20220411191615.a34959bdcc25ef3f9c16a7ce@linux-foundation.org> From: Yu Zhao Date: Tue, 26 Apr 2022 16:39:07 -0600 Message-ID: Subject: Re: [PATCH v10 05/14] mm: multi-gen LRU: groundwork To: Andrew Morton Cc: Stephen Rothwell , Linux-MM , 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 , "open list:DOCUMENTATION" , linux-kernel , Kernel Page Reclaim v2 , "the arch/x86 maintainers" , Brian Geffon , Jan Alexander Steffens , Oleksandr Natalenko , Steven Barrett , Suleiman Souhlal , Daniel Byrne , Donald Carr , =?UTF-8?Q?Holger_Hoffst=C3=A4tte?= , Konstantin Kharlamov , Shuang Zhai , Sofia Trinh , Vaibhav Jain X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20220426_153947_890671_ED6CEC2E X-CRM114-Status: GOOD ( 41.78 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Mon, Apr 11, 2022 at 8:16 PM Andrew Morton wrote: > > 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. I totally expect modern compilers to make better decisions than I do. And personally, I'd never use __always_inline; instead, I'd strongly recommend FDO/LTO. > > +{ > > + 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! Right. set_mask_bits() should suffice here. > > +#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? Will update: "i.e., gen". > > + * 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? Will update: yes, active/inactive LRU sizes in /proc/vmstat. > > + * 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"? Will update. > > + * 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. The !MAXSMP is to work around [1], which I haven't had the time to fix. That BUILD_BUG_ON() shouldn't assert sizeof(struct page) == 64 since the true size depends on WANT_PAGE_VIRTUAL as well as LAST_CPUPID_NOT_IN_PAGE_FLAGS. My plan is here [2]. [1] https://lore.kernel.org/r/20190905154603.10349-4-aneesh.kumar@linux.ibm.com/ [2] https://lore.kernel.org/r/Ygl1Gf+ATBuI%2Fm2q@google.com/ _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel