From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qk1-f197.google.com (mail-qk1-f197.google.com [209.85.222.197]) by kanga.kvack.org (Postfix) with ESMTP id 3B5FB6B303C for ; Fri, 23 Nov 2018 03:46:56 -0500 (EST) Received: by mail-qk1-f197.google.com with SMTP id s70so11359761qks.4 for ; Fri, 23 Nov 2018 00:46:56 -0800 (PST) Received: from mx1.redhat.com (mx1.redhat.com. [209.132.183.28]) by mx.google.com with ESMTPS id q11si5343380qvb.83.2018.11.23.00.46.55 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 23 Nov 2018 00:46:55 -0800 (PST) Subject: Re: [PATCH v2] mm, hotplug: move init_currently_empty_zone() under zone_span_lock protection References: <20181120014822.27968-1-richard.weiyang@gmail.com> <20181122101241.7965-1-richard.weiyang@gmail.com> <18088694-22c8-b09b-f500-4932b6199004@redhat.com> <20181123084201.GA8625@dhcp22.suse.cz> From: David Hildenbrand Message-ID: Date: Fri, 23 Nov 2018 09:46:52 +0100 MIME-Version: 1.0 In-Reply-To: <20181123084201.GA8625@dhcp22.suse.cz> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Michal Hocko Cc: Wei Yang , osalvador@suse.de, akpm@linux-foundation.org, linux-mm@kvack.org On 23.11.18 09:42, Michal Hocko wrote: > On Thu 22-11-18 16:26:40, David Hildenbrand wrote: >> On 22.11.18 11:12, Wei Yang wrote: >>> During online_pages phase, pgdat->nr_zones will be updated in case this >>> zone is empty. >>> >>> Currently the online_pages phase is protected by the global lock >>> mem_hotplug_begin(), which ensures there is no contention during the >>> update of nr_zones. But this global lock introduces scalability issues. >>> >>> This patch is a preparation for removing the global lock during >>> online_pages phase. Also this patch changes the documentation of >>> node_size_lock to include the protectioin of nr_zones. >> >> I looked into locking recently, and there is more to it. >> >> Please read: >> >> commit dee6da22efac451d361f5224a60be2796d847b51 >> Author: David Hildenbrand >> Date: Tue Oct 30 15:10:44 2018 -0700 >> >> memory-hotplug.rst: add some details about locking internals >> >> Let's document the magic a bit, especially why device_hotplug_lock is >> required when adding/removing memory and how it all play together with >> requests to online/offline memory from user space. >> >> Short summary: Onlining/offlining of memory requires the device_hotplug_lock >> as of now. > > Well, I would tend to disagree here. You might be describing the current > state of art but the device_hotplug_lock doesn't make much sense for the > memory hotplug in principle. There is absolutely nothing in the core MM There are collisions with CPU hotplug that require this lock (when nodes come and go as far as I remember). And there is the problematic lock inversion that can happen when adding/remving memory. This all has to be sorted out, we'll have to see if we really need it for onlining/offlining, though, however ... > that would require this lock. The current state just uses a BKL in some > sense and we really want to get rid of that longterm. This patch is a tiny > step in that direction and I suspect many more will need to come on the > way. We really want to end up with a clear scope of each lock being > taken. A project for a brave soul... ... for now I don't consider "optimize for parallel onlining/offlining/adding/removing of memory and cpus" really necessary. What is necessary indeed is to not slowdown the whole system just because some memory is coming/going. Therefore I agree, this patch is a step into the right direction. -- Thanks, David / dhildenb