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=-5.2 required=3.0 tests=BAYES_00, 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 28EDEC47089 for ; Thu, 27 May 2021 13:34:49 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 0B2D061059 for ; Thu, 27 May 2021 13:34:49 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S236513AbhE0NgU (ORCPT ); Thu, 27 May 2021 09:36:20 -0400 Received: from mga03.intel.com ([134.134.136.65]:45779 "EHLO mga03.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235751AbhE0NgT (ORCPT ); Thu, 27 May 2021 09:36:19 -0400 IronPort-SDR: sTE+1sS++b1W7JcwWOErhXmVrxnN40Eb705s010dbmnIM788Xe4rKWx1fIBInw1uNWLWK9vLvS QgjCpzRBH1EA== X-IronPort-AV: E=McAfee;i="6200,9189,9996"; a="202767538" X-IronPort-AV: E=Sophos;i="5.82,334,1613462400"; d="scan'208";a="202767538" Received: from fmsmga002.fm.intel.com ([10.253.24.26]) by orsmga103.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 27 May 2021 06:34:42 -0700 IronPort-SDR: FWcnpGsV/3EM1TRBlZcvi/Q7p/9r0V0NT2prtrJY2JYJ2s0o+ZKZ74rmvOdF+f38llij4ZD8cu pRDfKlmJbEZQ== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.82,334,1613462400"; d="scan'208";a="480581513" Received: from shbuild999.sh.intel.com (HELO localhost) ([10.239.147.94]) by fmsmga002.fm.intel.com with ESMTP; 27 May 2021 06:34:37 -0700 Date: Thu, 27 May 2021 21:34:36 +0800 From: Feng Tang To: Michal Hocko Cc: linux-mm@kvack.org, Andrew Morton , David Rientjes , Dave Hansen , Ben Widawsky , linux-kernel@vger.kernel.org, Andrea Arcangeli , Mel Gorman , Mike Kravetz , Randy Dunlap , Vlastimil Babka , Andi Kleen , Dan Williams , ying.huang@intel.com Subject: Re: [PATCH v1 4/4] mm/mempolicy: kill MPOL_F_LOCAL bit Message-ID: <20210527133436.GD7743@shbuild999.sh.intel.com> References: <1622005302-23027-1-git-send-email-feng.tang@intel.com> <1622005302-23027-5-git-send-email-feng.tang@intel.com> <20210527121041.GA7743@shbuild999.sh.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.24 (2015-08-30) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, May 27, 2021 at 02:26:24PM +0200, Michal Hocko wrote: > On Thu 27-05-21 20:10:41, Feng Tang wrote: > > On Thu, May 27, 2021 at 10:20:08AM +0200, Michal Hocko wrote: > > > On Wed 26-05-21 13:01:42, Feng Tang wrote: > > > > Now the only remaining case of a real 'local' policy faked by > > > > 'prefer' policy plus MPOL_F_LOCAL bit is: > > > > > > > > A valid 'prefer' policy with a valid 'preferred' node is 'rebind' > > > > to a nodemask which doesn't contains the 'preferred' node, then it > > > > will handle allocation with 'local' policy. > > > > > > > > Add a new 'MPOL_F_LOCAL_TEMP' bit for this case, and kill the > > > > MPOL_F_LOCAL bit, which could simplify the code much. > > > > > > As I've pointed out in the reply to the previous patch. It would have > > > been much better if most of the MPOL_F_LOCAL usage was gone by this > > > patch. > > > > > > I also dislike a new MPOL_F_LOCAL_TEMP. This smells like sneaking the > > > hack back in after you have painstakingly removed it. So this looks like > > > a step backwards to me. I also do not understand why do we need the > > > rebind callback for local policy at all. There is no node mask for local > > > so what is going on here? > > > > This is the special case 4 for 'perfer' policy with MPOL_F_STATIC_NODES > > flag set, say it prefer node 1, when it is later 'refind' to a new > > nodemask node 2-3, according to current code it will be add the > > MPOL_F_LOCAL bit and performs 'local' policy acctually. And in future > > it is 'rebind' again with a nodemask 1-2, it will be restored back > > to 'prefer' policy with preferred node 1. > > Honestly I still do not follow the actual problem. I was confused too, and don't know the original thought behind it. This case 4 was just imagined by reading the code. > A preferred node is a > _hint_. If you rebind the task to a different cpuset then why should we > actually care? The allocator will fallback to the closest node according > to the distance metric. Maybe the original code was trying to handle > that in some way but I really do fail to understand that code and I > strongly suspect it is more likely to overengineered rather than backed > by a real usecase. I might be wrong here but then this is an excellent > opportunity to clarify all those subtleties. >From the code, the original special handling may be needed in 3 cases: get_policy_nodemask() policy_node() mempolicy_slab_node() to not return the preset prefer_nid. Thanks, Feng > -- > Michal Hocko > SUSE Labs