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=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,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 E5B28C35242 for ; Wed, 12 Feb 2020 02:25:45 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id C85202082F for ; Wed, 12 Feb 2020 02:25:45 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727710AbgBLCZj (ORCPT ); Tue, 11 Feb 2020 21:25:39 -0500 Received: from mga07.intel.com ([134.134.136.100]:31415 "EHLO mga07.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727535AbgBLCZi (ORCPT ); Tue, 11 Feb 2020 21:25:38 -0500 X-Amp-Result: UNKNOWN X-Amp-Original-Verdict: FILE UNKNOWN X-Amp-File-Uploaded: False Received: from orsmga007.jf.intel.com ([10.7.209.58]) by orsmga105.jf.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 11 Feb 2020 18:25:38 -0800 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.70,428,1574150400"; d="scan'208";a="222139494" Received: from richard.sh.intel.com (HELO localhost) ([10.239.159.54]) by orsmga007.jf.intel.com with ESMTP; 11 Feb 2020 18:25:36 -0800 Date: Wed, 12 Feb 2020 10:25:55 +0800 From: Wei Yang To: Mel Gorman Cc: Wei Yang , akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, shakeelb@google.com, yang.shi@linux.alibaba.com Subject: Re: [RFC Patch] mm/vmscan.c: not inherit classzone_idx from previous reclaim Message-ID: <20200212022554.GA7855@richard> Reply-To: Wei Yang References: <20200209074145.31389-1-richardw.yang@linux.intel.com> <20200211104223.GL3466@techsingularity.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200211104223.GL3466@techsingularity.net> User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Feb 11, 2020 at 10:42:23AM +0000, Mel Gorman wrote: >On Sun, Feb 09, 2020 at 03:41:45PM +0800, Wei Yang wrote: >> Before commit e716f2eb24de ("mm, vmscan: prevent kswapd sleeping >> prematurely due to mismatched classzone_idx"), classzone_idx could have >> two possibilities on a new loop based on whether there is a wakeup >> during reclaiming: >> >> * 0 if no wakeup >> * the classzone_idx request by wakeup >> >> As described in the changelog, this commit is willing to change the >> first case to (MAX_NR_ZONES - 1) to avoid some premature sleep. But it >> does not achieve the goal. >> >> There are two versions of kswapd_classzone_idx() since this change: >> >> * commit e716f2eb24de ("mm, vmscan: prevent kswapd sleeping >> prematurely due to mismatched classzone_idx") >> * commit dffcac2cb88e ("mm/vmscan.c: prevent useless kswapd loops") >> >> Both of them would return the classzone_idx we passed as the 2nd >> parameter when (pgdat->kswapd_classzone_idx == MAX_NR_ZONES). This >> means if there is no wakeup during reclaiming, we would use >> classzone_idx in previous round to sleep. >> > >This is somewhat intended. > >> This patch fixes the logic by using (MAX_NR_ZONES - 1) for the first >> case. >> > >Ok, what is the user-visible impact that is fixed by this patch or is >this based on code review only? Please describe the test case exactly >and the before and after results. I ask because this area is a magnet for >regressions and intuitive ideas often lead to counter-intuitive results. > This is based on code review only. I know your concern. This is an area more like art then engineering :-) Would you mind sharing some idea why we intend to inherit the classzone_idx? And for kswapd_order, we would restart at 0 if no wakeup during reclaim. I am curious about the idea behind this design :-) >-- >Mel Gorman >SUSE Labs -- Wei Yang Help you, Help me