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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id B88ECC433FE for ; Wed, 26 Oct 2022 11:03:09 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233453AbiJZLDI (ORCPT ); Wed, 26 Oct 2022 07:03:08 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:40110 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233326AbiJZLCt (ORCPT ); Wed, 26 Oct 2022 07:02:49 -0400 Received: from smtp-out2.suse.de (smtp-out2.suse.de [IPv6:2001:67c:2178:6::1d]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 3671FBCB88; Wed, 26 Oct 2022 04:02:41 -0700 (PDT) Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by smtp-out2.suse.de (Postfix) with ESMTPS id D7CEC1FDA4; Wed, 26 Oct 2022 11:02:39 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1666782159; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=adHMhRn3WIk1ySFAhVm2bNqHsYDSJM4TbkvH222iQq4=; b=RaDDXpbRe46YrwPfY4vYlvwAcgHjXPs2Y+UdGX6GUc0CDWdk02QHBrZVauVp8kX8vTnIWZ mCEN2MdReWMTroJHGH8F1G9k3mMyibN00+5cd/JK6WhUEL2Iz2512PKXtK01fu3fHW12rF ZFrPtuN06dsSVjDmkmMRAfqn2E11A+s= Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by imap2.suse-dmz.suse.de (Postfix) with ESMTPS id BA98E13A6E; Wed, 26 Oct 2022 11:02:39 +0000 (UTC) Received: from dovecot-director2.suse.de ([192.168.254.65]) by imap2.suse-dmz.suse.de with ESMTPSA id d/RWKs8TWWMhFAAAMHmgww (envelope-from ); Wed, 26 Oct 2022 11:02:39 +0000 Date: Wed, 26 Oct 2022 13:02:39 +0200 From: Michal Hocko To: Aneesh Kumar K V Cc: Feng Tang , Andrew Morton , Johannes Weiner , Tejun Heo , Zefan Li , Waiman Long , "Huang, Ying" , "linux-mm@kvack.org" , "cgroups@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "Hansen, Dave" , "Chen, Tim C" , "Yin, Fengwei" Subject: Re: [PATCH] mm/vmscan: respect cpuset policy during page demotion Message-ID: References: <20221026074343.6517-1-feng.tang@intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed 26-10-22 16:12:25, Aneesh Kumar K V wrote: > On 10/26/22 2:49 PM, Michal Hocko wrote: > > On Wed 26-10-22 16:00:13, Feng Tang wrote: > >> On Wed, Oct 26, 2022 at 03:49:48PM +0800, Aneesh Kumar K V wrote: > >>> On 10/26/22 1:13 PM, Feng Tang wrote: > >>>> In page reclaim path, memory could be demoted from faster memory tier > >>>> to slower memory tier. Currently, there is no check about cpuset's > >>>> memory policy, that even if the target demotion node is not allowd > >>>> by cpuset, the demotion will still happen, which breaks the cpuset > >>>> semantics. > >>>> > >>>> So add cpuset policy check in the demotion path and skip demotion > >>>> if the demotion targets are not allowed by cpuset. > >>>> > >>> > >>> What about the vma policy or the task memory policy? Shouldn't we respect > >>> those memory policy restrictions while demoting the page? > >> > >> Good question! We have some basic patches to consider memory policy > >> in demotion path too, which are still under test, and will be posted > >> soon. And the basic idea is similar to this patch. > > > > For that you need to consult each vma and it's owning task(s) and that > > to me sounds like something to be done in folio_check_references. > > Relying on memcg to get a cpuset cgroup is really ugly and not really > > 100% correct. Memory controller might be disabled and then you do not > > have your association anymore. > > > > I was looking at this recently and I am wondering whether we should worry about VM_SHARE > vmas. > > ie, page_to_policy() can just reverse lookup just one VMA and fetch the policy right? How would that help for private mappings shared between parent/child? Also reducing this to a single VMA is not really necessary as folio_check_references already does most of that work. What is really missing is to check for other memory policies (i.e. cpusets and per-task mempolicy). The later is what can get quite expensive. > if it VM_SHARE it will be a shared policy we can find using vma->vm_file? > > For non anonymous and anon vma not having any policy set it is owning task vma->vm_mm->owner task policy ? Please note that mm can be shared even outside of the traditional thread group so you would need to go into something like mm_update_next_owner > We don't worry about multiple tasks that can be possibly sharing that page right? Why not? > > This all can get quite expensive so the primary question is, does the > > existing behavior generates any real issues or is this more of an > > correctness exercise? I mean it certainly is not great to demote to an > > incompatible numa node but are there any reasonable configurations when > > the demotion target node is explicitly excluded from memory > > policy/cpuset? > > I guess vma policy is important. Applications want to make sure that they don't > have variable performance and they go to lengths to avoid that by using MEM_BIND. > So if they access the memory they always know access is satisfied from a specific > set of NUMA nodes. Swapin can cause performance impact but then all continued > access will be from a specific NUMA nodes. With slow memory demotion that is > not going to be the case. Large in-memory database applications are observed to > be sensitive to these access latencies. Yes, I do understand that from the correctness POV this is a problem. My question is whether this is a _practical_ problem worth really being fixed as it is not really a cheap fix. If there are strong node locality assumptions by the userspace then I would expect demotion to be disabled in the first place. -- Michal Hocko SUSE Labs