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 1B8ABC352A3 for ; Tue, 11 Feb 2020 10:42:28 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id EA232206D7 for ; Tue, 11 Feb 2020 10:42:27 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728402AbgBKKm1 (ORCPT ); Tue, 11 Feb 2020 05:42:27 -0500 Received: from outbound-smtp17.blacknight.com ([46.22.139.234]:41926 "EHLO outbound-smtp17.blacknight.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727805AbgBKKm0 (ORCPT ); Tue, 11 Feb 2020 05:42:26 -0500 Received: from mail.blacknight.com (pemlinmail06.blacknight.ie [81.17.255.152]) by outbound-smtp17.blacknight.com (Postfix) with ESMTPS id 27C0A1C2C4D for ; Tue, 11 Feb 2020 10:42:25 +0000 (GMT) Received: (qmail 23464 invoked from network); 11 Feb 2020 10:42:25 -0000 Received: from unknown (HELO techsingularity.net) (mgorman@techsingularity.net@[84.203.18.57]) by 81.17.254.9 with ESMTPSA (AES256-SHA encrypted, authenticated); 11 Feb 2020 10:42:24 -0000 Date: Tue, 11 Feb 2020 10:42:23 +0000 From: Mel Gorman To: Wei Yang Cc: 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: <20200211104223.GL3466@techsingularity.net> References: <20200209074145.31389-1-richardw.yang@linux.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline In-Reply-To: <20200209074145.31389-1-richardw.yang@linux.intel.com> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. -- Mel Gorman SUSE Labs