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 X-Spam-Level: X-Spam-Status: No, score=-15.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 9A0CEC433E6 for ; Fri, 26 Feb 2021 01:17:47 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 59F0564F26 for ; Fri, 26 Feb 2021 01:17:47 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229946AbhBZBRg (ORCPT ); Thu, 25 Feb 2021 20:17:36 -0500 Received: from mail.kernel.org ([198.145.29.99]:49368 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229949AbhBZBRd (ORCPT ); Thu, 25 Feb 2021 20:17:33 -0500 Received: by mail.kernel.org (Postfix) with ESMTPSA id C8A7864F03; Fri, 26 Feb 2021 01:16:51 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linux-foundation.org; s=korg; t=1614302212; bh=eXy8vO4itDYN5MUfc4uV2DiZdadEtVemdC7JXXQ2rcc=; h=Date:From:To:Subject:In-Reply-To:From; b=bAyk94YIIWhEW9RhY56mTqAjtuHR6+Ssbib//ildQeT0TMd02iuee3ftnEupKJ/i4 MoVRHko7ex61QR8yhVwQfk27b2zh26C79GajIaCs+B3wO4FbA1nOjNc9Y6zEP8AZJr vCG8X35jpIdjHYSQvvLNP/4RIyl9aKdXSrVCDH3U= Date: Thu, 25 Feb 2021 17:16:51 -0800 From: Andrew Morton To: akpm@linux-foundation.org, hannes@cmpxchg.org, linux-mm@kvack.org, mm-commits@vger.kernel.org, torvalds@linux-foundation.org Subject: [patch 024/118] mm: vmstat: add some comments on internal storage of byte items Message-ID: <20210226011651.UY-aB_mq-%akpm@linux-foundation.org> In-Reply-To: <20210225171452.713967e96554bb6a53e44a19@linux-foundation.org> User-Agent: s-nail v14.8.16 Precedence: bulk Reply-To: linux-kernel@vger.kernel.org List-ID: X-Mailing-List: mm-commits@vger.kernel.org From: Johannes Weiner Subject: mm: vmstat: add some comments on internal storage of byte items Byte-accounted items are used for slab object accounting at the cgroup level, because the objects in a slab page can belong to different cgroups. At the global level these items always change in multiples of whole slab pages. The vmstat code exploits this and stores these items as pages internally, which allows for more compact per-cpu data. This optimization isn't self-evident from the asserts and the division in the stat update functions. Provide the reader with some context. Link: https://lkml.kernel.org/r/20210202184411.118614-1-hannes@cmpxchg.org Signed-off-by: Johannes Weiner Signed-off-by: Andrew Morton --- include/linux/vmstat.h | 6 ++++++ mm/vmstat.c | 12 ++++++++++++ 2 files changed, 18 insertions(+) --- a/include/linux/vmstat.h~mm-vmstat-add-some-comments-on-internal-storage-of-byte-items +++ a/include/linux/vmstat.h @@ -313,6 +313,12 @@ static inline void __mod_node_page_state enum node_stat_item item, int delta) { if (vmstat_item_in_bytes(item)) { + /* + * Only cgroups use subpage accounting right now; at + * the global level, these items still change in + * multiples of whole pages. Store them as pages + * internally to keep the per-cpu counters compact. + */ VM_WARN_ON_ONCE(delta & (PAGE_SIZE - 1)); delta >>= PAGE_SHIFT; } --- a/mm/vmstat.c~mm-vmstat-add-some-comments-on-internal-storage-of-byte-items +++ a/mm/vmstat.c @@ -342,6 +342,12 @@ void __mod_node_page_state(struct pglist long t; if (vmstat_item_in_bytes(item)) { + /* + * Only cgroups use subpage accounting right now; at + * the global level, these items still change in + * multiples of whole pages. Store them as pages + * internally to keep the per-cpu counters compact. + */ VM_WARN_ON_ONCE(delta & (PAGE_SIZE - 1)); delta >>= PAGE_SHIFT; } @@ -551,6 +557,12 @@ static inline void mod_node_state(struct long o, n, t, z; if (vmstat_item_in_bytes(item)) { + /* + * Only cgroups use subpage accounting right now; at + * the global level, these items still change in + * multiples of whole pages. Store them as pages + * internally to keep the per-cpu counters compact. + */ VM_WARN_ON_ONCE(delta & (PAGE_SIZE - 1)); delta >>= PAGE_SHIFT; } _