From nobody Fri Apr 3 03:54:25 2026 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 796A8C32771 for ; Sun, 18 Sep 2022 08:03:12 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229783AbiIRIDK (ORCPT ); Sun, 18 Sep 2022 04:03:10 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:33804 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229616AbiIRIBf (ORCPT ); Sun, 18 Sep 2022 04:01:35 -0400 Received: from mail-yw1-x1149.google.com (mail-yw1-x1149.google.com [IPv6:2607:f8b0:4864:20::1149]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id A834C2656D for ; Sun, 18 Sep 2022 01:01:21 -0700 (PDT) Received: by mail-yw1-x1149.google.com with SMTP id 00721157ae682-348608c1cd3so230586327b3.10 for ; Sun, 18 Sep 2022 01:01:21 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20210112; h=content-transfer-encoding:cc:to:from:subject:references :mime-version:message-id:in-reply-to:date:from:to:cc:subject:date; bh=U43YKqHWj215EGLcMoze5D0BcuNypR2wHlQCyfq09BE=; b=Zi5tEMzGHXn0KcfY9hanumq4akKyayUThAqzjRPK2vLh7ajqle01c8rsAka8I4RF/P hGb5psS8mPFQCuBNoQRJkDYjsHKrTRmts0DvNvsw5QwaYnZ1tuY8DAzxvF7cYy7WNiQs VbuSOTHX1QyKRhVAQ4Szlnq9VF69GOArldvEUI20SEpfZ5v+5KrP4iCG3TtervpDhndN PxLdYDLl67VStV/VOviJT3XcGKffce3BELt0g32nRXcHJ2LX/niWbRyBwcEvIthGypKL BtnnqRPASiSFrmlM+qAXkRG1rrNnjCsS59ki4NZpYMvyz6NYNTkguXFJbA7d+RrpZE30 93Wg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=content-transfer-encoding:cc:to:from:subject:references :mime-version:message-id:in-reply-to:date:x-gm-message-state:from:to :cc:subject:date; bh=U43YKqHWj215EGLcMoze5D0BcuNypR2wHlQCyfq09BE=; b=vju9BVapYITM9+Xyx+I1H8qMvsWrL6tiu/fpUzumS0zc6z8k8xhYYS1jSZz4Jzpzv3 gViLyla9APthvTIk40XJrQNkIEPQubNoCRzqDbEpiNSQG00aY9TFuuvGT8fS6DZvZqW+ UnAeS8VuMpB51UGsk9JYaJShiqIxBea1DTeG2suYVuiNzTPuLHmaAyAcPHg6adhoid0t fvI+HyyPYNjbtxM6ixoa7MpGunmfCvRBdpEydsgDXFpPsuKDOizug0NJ35Fzg+xL1wgE fKONg9wJdKDCS2xyb8YIsSfo/GnJdl0YkvWaKFqQqawkrXiEvaVKAN8aUCjbfTbY7JiR PPNQ== X-Gm-Message-State: ACrzQf0N5QOzG/EBROo/8AxcF/j9MXAqMGuxv7Sf6VgQPDgrcNUsmkEO qsqoLfAEIpBttMcEf3ob6B4w8ZSR4/I= X-Google-Smtp-Source: AMsMyM66MNkQEaLvu5kyj8BMVzxMevTO3WrbIVmnzeqVnEKeIXSYb2bIrmckdOI5x8PNi3muKmXd2tQGQYA= X-Received: from yuzhao.bld.corp.google.com ([2620:15c:183:200:c05a:2e99:29cd:d157]) (user=yuzhao job=sendgmr) by 2002:a0d:eec2:0:b0:348:67bc:a1a3 with SMTP id x185-20020a0deec2000000b0034867bca1a3mr10293691ywe.148.1663488080721; Sun, 18 Sep 2022 01:01:20 -0700 (PDT) Date: Sun, 18 Sep 2022 02:00:10 -0600 In-Reply-To: <20220918080010.2920238-1-yuzhao@google.com> Message-Id: <20220918080010.2920238-14-yuzhao@google.com> Mime-Version: 1.0 References: <20220918080010.2920238-1-yuzhao@google.com> X-Mailer: git-send-email 2.37.3.968.ga6b4b080e4-goog Subject: [PATCH mm-unstable v15 13/14] mm: multi-gen LRU: admin guide From: Yu Zhao To: Andrew Morton Cc: Andi Kleen , Aneesh Kumar , Catalin Marinas , Dave Hansen , Hillf Danton , Jens Axboe , Johannes Weiner , Jonathan Corbet , Linus Torvalds , Matthew Wilcox , Mel Gorman , Michael Larabel , Michal Hocko , Mike Rapoport , Peter Zijlstra , Tejun Heo , Vlastimil Babka , Will Deacon , linux-arm-kernel@lists.infradead.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, x86@kernel.org, page-reclaim@google.com, Yu Zhao , Brian Geffon , Jan Alexander Steffens , Oleksandr Natalenko , Steven Barrett , Suleiman Souhlal , Daniel Byrne , Donald Carr , "=?UTF-8?q?Holger=20Hoffst=C3=A4tte?=" , Konstantin Kharlamov , Shuang Zhai , Sofia Trinh , Vaibhav Jain Content-Transfer-Encoding: quoted-printable Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Type: text/plain; charset="utf-8" Add an admin guide. Signed-off-by: Yu Zhao Acked-by: Brian Geffon Acked-by: Jan Alexander Steffens (heftig) Acked-by: Oleksandr Natalenko Acked-by: Steven Barrett Acked-by: Suleiman Souhlal Tested-by: Daniel Byrne Tested-by: Donald Carr Tested-by: Holger Hoffst=C3=A4tte Tested-by: Konstantin Kharlamov Tested-by: Shuang Zhai Tested-by: Sofia Trinh Tested-by: Vaibhav Jain --- Documentation/admin-guide/mm/index.rst | 1 + Documentation/admin-guide/mm/multigen_lru.rst | 162 ++++++++++++++++++ mm/Kconfig | 3 +- mm/vmscan.c | 4 + 4 files changed, 169 insertions(+), 1 deletion(-) create mode 100644 Documentation/admin-guide/mm/multigen_lru.rst diff --git a/Documentation/admin-guide/mm/index.rst b/Documentation/admin-g= uide/mm/index.rst index 1bd11118dfb1..d1064e0ba34a 100644 --- a/Documentation/admin-guide/mm/index.rst +++ b/Documentation/admin-guide/mm/index.rst @@ -32,6 +32,7 @@ the Linux memory management. idle_page_tracking ksm memory-hotplug + multigen_lru nommu-mmap numa_memory_policy numaperf diff --git a/Documentation/admin-guide/mm/multigen_lru.rst b/Documentation/= admin-guide/mm/multigen_lru.rst new file mode 100644 index 000000000000..33e068830497 --- /dev/null +++ b/Documentation/admin-guide/mm/multigen_lru.rst @@ -0,0 +1,162 @@ +.. SPDX-License-Identifier: GPL-2.0 + +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D +Multi-Gen LRU +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D +The multi-gen LRU is an alternative LRU implementation that optimizes +page reclaim and improves performance under memory pressure. Page +reclaim decides the kernel's caching policy and ability to overcommit +memory. It directly impacts the kswapd CPU usage and RAM efficiency. + +Quick start +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D +Build the kernel with the following configurations. + +* ``CONFIG_LRU_GEN=3Dy`` +* ``CONFIG_LRU_GEN_ENABLED=3Dy`` + +All set! + +Runtime options +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D +``/sys/kernel/mm/lru_gen/`` contains stable ABIs described in the +following subsections. + +Kill switch +----------- +``enabled`` accepts different values to enable or disable the +following components. Its default value depends on +``CONFIG_LRU_GEN_ENABLED``. All the components should be enabled +unless some of them have unforeseen side effects. Writing to +``enabled`` has no effect when a component is not supported by the +hardware, and valid values will be accepted even when the main switch +is off. + +=3D=3D=3D=3D=3D=3D =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D +Values Components +=3D=3D=3D=3D=3D=3D =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D +0x0001 The main switch for the multi-gen LRU. +0x0002 Clearing the accessed bit in leaf page table entries in large + batches, when MMU sets it (e.g., on x86). This behavior can + theoretically worsen lock contention (mmap_lock). If it is + disabled, the multi-gen LRU will suffer a minor performance + degradation for workloads that contiguously map hot pages, + whose accessed bits can be otherwise cleared by fewer larger + batches. +0x0004 Clearing the accessed bit in non-leaf page table entries as + well, when MMU sets it (e.g., on x86). This behavior was not + verified on x86 varieties other than Intel and AMD. If it is + disabled, the multi-gen LRU will suffer a negligible + performance degradation. +[yYnN] Apply to all the components above. +=3D=3D=3D=3D=3D=3D =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D + +E.g., +:: + + echo y >/sys/kernel/mm/lru_gen/enabled + cat /sys/kernel/mm/lru_gen/enabled + 0x0007 + echo 5 >/sys/kernel/mm/lru_gen/enabled + cat /sys/kernel/mm/lru_gen/enabled + 0x0005 + +Thrashing prevention +-------------------- +Personal computers are more sensitive to thrashing because it can +cause janks (lags when rendering UI) and negatively impact user +experience. The multi-gen LRU offers thrashing prevention to the +majority of laptop and desktop users who do not have ``oomd``. + +Users can write ``N`` to ``min_ttl_ms`` to prevent the working set of +``N`` milliseconds from getting evicted. The OOM killer is triggered +if this working set cannot be kept in memory. In other words, this +option works as an adjustable pressure relief valve, and when open, it +terminates applications that are hopefully not being used. + +Based on the average human detectable lag (~100ms), ``N=3D1000`` usually +eliminates intolerable janks due to thrashing. Larger values like +``N=3D3000`` make janks less noticeable at the risk of premature OOM +kills. + +The default value ``0`` means disabled. + +Experimental features +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D +``/sys/kernel/debug/lru_gen`` accepts commands described in the +following subsections. Multiple command lines are supported, so does +concatenation with delimiters ``,`` and ``;``. + +``/sys/kernel/debug/lru_gen_full`` provides additional stats for +debugging. ``CONFIG_LRU_GEN_STATS=3Dy`` keeps historical stats from +evicted generations in this file. + +Working set estimation +---------------------- +Working set estimation measures how much memory an application needs +in a given time interval, and it is usually done with little impact on +the performance of the application. E.g., data centers want to +optimize job scheduling (bin packing) to improve memory utilizations. +When a new job comes in, the job scheduler needs to find out whether +each server it manages can allocate a certain amount of memory for +this new job before it can pick a candidate. To do so, the job +scheduler needs to estimate the working sets of the existing jobs. + +When it is read, ``lru_gen`` returns a histogram of numbers of pages +accessed over different time intervals for each memcg and node. +``MAX_NR_GENS`` decides the number of bins for each histogram. The +histograms are noncumulative. +:: + + memcg memcg_id memcg_path + node node_id + min_gen_nr age_in_ms nr_anon_pages nr_file_pages + ... + max_gen_nr age_in_ms nr_anon_pages nr_file_pages + +Each bin contains an estimated number of pages that have been accessed +within ``age_in_ms``. E.g., ``min_gen_nr`` contains the coldest pages +and ``max_gen_nr`` contains the hottest pages, since ``age_in_ms`` of +the former is the largest and that of the latter is the smallest. + +Users can write the following command to ``lru_gen`` to create a new +generation ``max_gen_nr+1``: + + ``+ memcg_id node_id max_gen_nr [can_swap [force_scan]]`` + +``can_swap`` defaults to the swap setting and, if it is set to ``1``, +it forces the scan of anon pages when swap is off, and vice versa. +``force_scan`` defaults to ``1`` and, if it is set to ``0``, it +employs heuristics to reduce the overhead, which is likely to reduce +the coverage as well. + +A typical use case is that a job scheduler runs this command at a +certain time interval to create new generations, and it ranks the +servers it manages based on the sizes of their cold pages defined by +this time interval. + +Proactive reclaim +----------------- +Proactive reclaim induces page reclaim when there is no memory +pressure. It usually targets cold pages only. E.g., when a new job +comes in, the job scheduler wants to proactively reclaim cold pages on +the server it selected, to improve the chance of successfully landing +this new job. + +Users can write the following command to ``lru_gen`` to evict +generations less than or equal to ``min_gen_nr``. + + ``- memcg_id node_id min_gen_nr [swappiness [nr_to_reclaim]]`` + +``min_gen_nr`` should be less than ``max_gen_nr-1``, since +``max_gen_nr`` and ``max_gen_nr-1`` are not fully aged (equivalent to +the active list) and therefore cannot be evicted. ``swappiness`` +overrides the default value in ``/proc/sys/vm/swappiness``. +``nr_to_reclaim`` limits the number of pages to evict. + +A typical use case is that a job scheduler runs this command before it +tries to land a new job on a server. If it fails to materialize enough +cold pages because of the overestimation, it retries on the next +server according to the ranking result obtained from the working set +estimation step. This less forceful approach limits the impacts on the +existing jobs. diff --git a/mm/Kconfig b/mm/Kconfig index ab6ef5115eb8..ceec438c0741 100644 --- a/mm/Kconfig +++ b/mm/Kconfig @@ -1125,7 +1125,8 @@ config LRU_GEN # make sure folio->flags has enough spare bits depends on 64BIT || !SPARSEMEM || SPARSEMEM_VMEMMAP help - A high performance LRU implementation to overcommit memory. + A high performance LRU implementation to overcommit memory. See + Documentation/admin-guide/mm/multigen_lru.rst for details. =20 config LRU_GEN_ENABLED bool "Enable by default" diff --git a/mm/vmscan.c b/mm/vmscan.c index 7657d54c9c42..1456f133f256 100644 --- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -5310,6 +5310,7 @@ static ssize_t show_min_ttl(struct kobject *kobj, str= uct kobj_attribute *attr, c return sprintf(buf, "%u\n", jiffies_to_msecs(READ_ONCE(lru_gen_min_ttl))); } =20 +/* see Documentation/admin-guide/mm/multigen_lru.rst for details */ static ssize_t store_min_ttl(struct kobject *kobj, struct kobj_attribute *= attr, const char *buf, size_t len) { @@ -5343,6 +5344,7 @@ static ssize_t show_enabled(struct kobject *kobj, str= uct kobj_attribute *attr, c return snprintf(buf, PAGE_SIZE, "0x%04x\n", caps); } =20 +/* see Documentation/admin-guide/mm/multigen_lru.rst for details */ static ssize_t store_enabled(struct kobject *kobj, struct kobj_attribute *= attr, const char *buf, size_t len) { @@ -5490,6 +5492,7 @@ static void lru_gen_seq_show_full(struct seq_file *m,= struct lruvec *lruvec, seq_putc(m, '\n'); } =20 +/* see Documentation/admin-guide/mm/multigen_lru.rst for details */ static int lru_gen_seq_show(struct seq_file *m, void *v) { unsigned long seq; @@ -5648,6 +5651,7 @@ static int run_cmd(char cmd, int memcg_id, int nid, u= nsigned long seq, return err; } =20 +/* see Documentation/admin-guide/mm/multigen_lru.rst for details */ static ssize_t lru_gen_seq_write(struct file *file, const char __user *src, size_t len, loff_t *pos) { --=20 2.37.3.968.ga6b4b080e4-goog