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=-5.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 autolearn=no 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 3ACBCC2B9F7 for ; Mon, 24 May 2021 15:58:56 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 1D80B6109F for ; Mon, 24 May 2021 15:58:56 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234856AbhEXQAV (ORCPT ); Mon, 24 May 2021 12:00:21 -0400 Received: from mga17.intel.com ([192.55.52.151]:52730 "EHLO mga17.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235318AbhEXPzF (ORCPT ); Mon, 24 May 2021 11:55:05 -0400 IronPort-SDR: B10Q/kr/k+f0Z2AOnhHEbA0qVZpeiipepfGRLjQJDsVkDgo6nLwSEGhO/8w+G4N6B/zblYI3lb Enk660/wRZfA== X-IronPort-AV: E=McAfee;i="6200,9189,9993"; a="182282892" X-IronPort-AV: E=Sophos;i="5.82,325,1613462400"; d="scan'208";a="182282892" Received: from fmsmga002.fm.intel.com ([10.253.24.26]) by fmsmga107.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 24 May 2021 08:52:05 -0700 IronPort-SDR: DWfQDheTDtxp1tyv+SuoREIy9eL5yGlXo6GQHvi+7bXoGN1HloKO9er3LLh+1DW52MA2eEKr+V bRqL6DynZdsA== X-IronPort-AV: E=Sophos;i="5.82,325,1613462400"; d="scan'208";a="478605771" Received: from caknoll-mobl.amr.corp.intel.com (HELO [10.209.100.8]) ([10.209.100.8]) by fmsmga002-auth.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 24 May 2021 08:52:05 -0700 Subject: Re: [PATCH 3/6] mm/page_alloc: Adjust pcp->high after CPU hotplug events To: Mel Gorman Cc: Linux-MM , Dave Hansen , Matthew Wilcox , Vlastimil Babka , Michal Hocko , Nicholas Piggin , LKML References: <20210521102826.28552-1-mgorman@techsingularity.net> <20210521102826.28552-4-mgorman@techsingularity.net> <20210524090726.GB30378@techsingularity.net> From: Dave Hansen Autocrypt: addr=dave.hansen@intel.com; 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Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: <20210524090726.GB30378@techsingularity.net> Content-Type: text/plain; charset=US-ASCII Content-Language: en-US Content-Transfer-Encoding: 7BIT Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 5/24/21 2:07 AM, Mel Gorman wrote: > On Fri, May 21, 2021 at 03:13:35PM -0700, Dave Hansen wrote: >> On 5/21/21 3:28 AM, Mel Gorman wrote: >>> The PCP high watermark is based on the number of online CPUs so the >>> watermarks must be adjusted during CPU hotplug. At the time of >>> hot-remove, the number of online CPUs is already adjusted but during >>> hot-add, a delta needs to be applied to update PCP to the correct >>> value. After this patch is applied, the high watermarks are adjusted >>> correctly. >>> >>> # grep high: /proc/zoneinfo | tail -1 >>> high: 649 >>> # echo 0 > /sys/devices/system/cpu/cpu4/online >>> # grep high: /proc/zoneinfo | tail -1 >>> high: 664 >>> # echo 1 > /sys/devices/system/cpu/cpu4/online >>> # grep high: /proc/zoneinfo | tail -1 >>> high: 649 >> This is actually a comment more about the previous patch, but it doesn't >> really become apparent until the example above. >> >> In your example, you mentioned increased exit() performance by using >> "vm.percpu_pagelist_fraction to increase the pcp->high value". That's >> presumably because of the increased batching effects and fewer lock >> acquisitions. >> > Yes > >> But, logically, doesn't that mean that, the more CPUs you have in a >> node, the *higher* you want pcp->high to be? If we took this to the >> extreme and had an absurd number of CPUs in a node, we could end up with >> a too-small pcp->high value. >> > I see your point but I don't think increasing pcp->high for larger > numbers of CPUs is the right answer because then reclaim can be > triggered simply because too many PCPs have pages. > > To address your point requires much deeper surgery. ... > There is value to doing something like this but it's beyond what this > series is trying to do and doing the work without introducing regressions > would be very difficult. Agreed, such a solution is outside of the scope of what this set is trying to do. It would be nice to touch on this counter-intuitive property in the changelog, and *maybe* add a WARN_ON_ONCE() if we hit an edge case. Maybe WARN_ON_ONCE() if pcp->high gets below pcp->batch*SOMETHING.