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=-7.4 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, MENTIONS_GIT_HOSTING,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=unavailable 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 A0925C2D0DB for ; Wed, 22 Jan 2020 18:16:04 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 4DAD821835 for ; Wed, 22 Jan 2020 18:16:04 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="b1VSse4F" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 4DAD821835 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id D81C56B0005; Wed, 22 Jan 2020 13:16:03 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id D32A76B0006; Wed, 22 Jan 2020 13:16:03 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id BFABC6B0007; Wed, 22 Jan 2020 13:16:03 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0240.hostedemail.com [216.40.44.240]) by kanga.kvack.org (Postfix) with ESMTP id AABF56B0005 for ; Wed, 22 Jan 2020 13:16:03 -0500 (EST) Received: from smtpin04.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay04.hostedemail.com (Postfix) with SMTP id 4E34040F0 for ; Wed, 22 Jan 2020 18:16:03 +0000 (UTC) X-FDA: 76406074206.04.way24_8c18bab9ef733 X-HE-Tag: way24_8c18bab9ef733 X-Filterd-Recvd-Size: 12835 Received: from us-smtp-delivery-1.mimecast.com (us-smtp-2.mimecast.com [207.211.31.81]) by imf08.hostedemail.com (Postfix) with ESMTP for ; Wed, 22 Jan 2020 18:16:02 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1579716962; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references:autocrypt:autocrypt; bh=aYDoYCqh0OtZe0lF7uX7a/4uwn5JcyIry0p1fbUQD68=; b=b1VSse4FGC9qD/9UbaF/AcGRPeJTkSpPpDw5t7vX4A6qI/GKfiDfzxzABzf8mN6Yj6rbhp sNO703okvnYxoLuxGP9f2M6eVZ3xSieCxvWeIMSk6IMLVorAlZL4V8xxdz8odwm6nEfyGR ZbSZDJAPEzb9NUL/kZ5oXB/eS+nr5NY= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-263-BInrnXY6OL2cKxIQT_2XaQ-1; Wed, 22 Jan 2020 13:15:58 -0500 X-MC-Unique: BInrnXY6OL2cKxIQT_2XaQ-1 Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.phx2.redhat.com [10.5.11.16]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 57384DB60; Wed, 22 Jan 2020 18:15:53 +0000 (UTC) Received: from [10.36.117.21] (ovpn-117-21.ams2.redhat.com [10.36.117.21]) by smtp.corp.redhat.com (Postfix) with ESMTP id B6FA75C1BB; Wed, 22 Jan 2020 18:15:48 +0000 (UTC) Subject: Re: [PATCH RFC v1] mm: is_mem_section_removable() overhaul To: Michal Hocko Cc: Dan Williams , Linux Kernel Mailing List , Linux MM , Benjamin Herrenschmidt , Paul Mackerras , Michael Ellerman , Greg Kroah-Hartman , "Rafael J. Wysocki" , Andrew Morton , Leonardo Bras , Nathan Lynch , Allison Randal , Nathan Fontenot , Thomas Gleixner , Stephen Rothwell , Anshuman Khandual , lantianyu1986@gmail.com, linuxppc-dev References: <25a94f61-46a1-59a6-6b54-8cc6b35790d2@redhat.com> <20200120074816.GG18451@dhcp22.suse.cz> <20200121120714.GJ29276@dhcp22.suse.cz> <20200122104230.GU29276@dhcp22.suse.cz> <98b6c208-b4dd-9052-43f6-543068c649cc@redhat.com> <816ddd66-c90b-76f1-f4a0-72fe41263edd@redhat.com> <20200122164618.GY29276@dhcp22.suse.cz> From: David Hildenbrand Autocrypt: addr=david@redhat.com; prefer-encrypt=mutual; keydata= mQINBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABtCREYXZpZCBIaWxk ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT6JAlgEEwEIAEICGwMFCQlmAYAGCwkIBwMCBhUI AgkKCwQWAgMBAh4BAheAFiEEG9nKrXNcTDpGDfzKTd4Q9wD/g1oFAl3pImkCGQEACgkQTd4Q 9wD/g1o+VA//SFvIHUAvul05u6wKv/pIR6aICPdpF9EIgEU448g+7FfDgQwcEny1pbEzAmiw zAXIQ9H0NZh96lcq+yDLtONnXk/bEYWHHUA014A1wqcYNRY8RvY1+eVHb0uu0KYQoXkzvu+s Dncuguk470XPnscL27hs8PgOP6QjG4jt75K2LfZ0eAqTOUCZTJxA8A7E9+XTYuU0hs7QVrWJ jQdFxQbRMrYz7uP8KmTK9/Cnvqehgl4EzyRaZppshruKMeyheBgvgJd5On1wWq4ZUV5PFM4x II3QbD3EJfWbaJMR55jI9dMFa+vK7MFz3rhWOkEx/QR959lfdRSTXdxs8V3zDvChcmRVGN8U Vo93d1YNtWnA9w6oCW1dnDZ4kgQZZSBIjp6iHcA08apzh7DPi08jL7M9UQByeYGr8KuR4i6e RZI6xhlZerUScVzn35ONwOC91VdYiQgjemiVLq1WDDZ3B7DIzUZ4RQTOaIWdtXBWb8zWakt/ ztGhsx0e39Gvt3391O1PgcA7ilhvqrBPemJrlb9xSPPRbaNAW39P8ws/UJnzSJqnHMVxbRZC Am4add/SM+OCP0w3xYss1jy9T+XdZa0lhUvJfLy7tNcjVG/sxkBXOaSC24MFPuwnoC9WvCVQ ZBxouph3kqc4Dt5X1EeXVLeba+466P1fe1rC8MbcwDkoUo65Ag0EVcufkQEQAOfX3n0g0fZz Bgm/S2zF/kxQKCEKP8ID+Vz8sy2GpDvveBq4H2Y34XWsT1zLJdvqPI4af4ZSMxuerWjXbVWb T6d4odQIG0fKx4F8NccDqbgHeZRNajXeeJ3R7gAzvWvQNLz4piHrO/B4tf8svmRBL0ZB5P5A 2uhdwLU3NZuK22zpNn4is87BPWF8HhY0L5fafgDMOqnf4guJVJPYNPhUFzXUbPqOKOkL8ojk CXxkOFHAbjstSK5Ca3fKquY3rdX3DNo+EL7FvAiw1mUtS+5GeYE+RMnDCsVFm/C7kY8c2d0G NWkB9pJM5+mnIoFNxy7YBcldYATVeOHoY4LyaUWNnAvFYWp08dHWfZo9WCiJMuTfgtH9tc75 7QanMVdPt6fDK8UUXIBLQ2TWr/sQKE9xtFuEmoQGlE1l6bGaDnnMLcYu+Asp3kDT0w4zYGsx 5r6XQVRH4+5N6eHZiaeYtFOujp5n+pjBaQK7wUUjDilPQ5QMzIuCL4YjVoylWiBNknvQWBXS lQCWmavOT9sttGQXdPCC5ynI+1ymZC1ORZKANLnRAb0NH/UCzcsstw2TAkFnMEbo9Zu9w7Kv AxBQXWeXhJI9XQssfrf4Gusdqx8nPEpfOqCtbbwJMATbHyqLt7/oz/5deGuwxgb65pWIzufa N7eop7uh+6bezi+rugUI+w6DABEBAAGJAiUEGAECAA8FAlXLn5ECGwwFCQlmAYAACgkQTd4Q 9wD/g1qA6w/+M+ggFv+JdVsz5+ZIc6MSyGUozASX+bmIuPeIecc9UsFRatc91LuJCKMkD9Uv GOcWSeFpLrSGRQ1Z7EMzFVU//qVs6uzhsNk0RYMyS0B6oloW3FpyQ+zOVylFWQCzoyyf227y GW8HnXunJSC+4PtlL2AY4yZjAVAPLK2l6mhgClVXTQ/S7cBoTQKP+jvVJOoYkpnFxWE9pn4t H5QIFk7Ip8TKr5k3fXVWk4lnUi9MTF/5L/mWqdyIO1s7cjharQCstfWCzWrVeVctpVoDfJWp 4LwTuQ5yEM2KcPeElLg5fR7WB2zH97oI6/Ko2DlovmfQqXh9xWozQt0iGy5tWzh6I0JrlcxJ ileZWLccC4XKD1037Hy2FLAjzfoWgwBLA6ULu0exOOdIa58H4PsXtkFPrUF980EEibUp0zFz GotRVekFAceUaRvAj7dh76cToeZkfsjAvBVb4COXuhgX6N4pofgNkW2AtgYu1nUsPAo+NftU CxrhjHtLn4QEBpkbErnXQyMjHpIatlYGutVMS91XTQXYydCh5crMPs7hYVsvnmGHIaB9ZMfB njnuI31KBiLUks+paRkHQlFcgS2N3gkRBzH7xSZ+t7Re3jvXdXEzKBbQ+dC3lpJB0wPnyMcX FOTT3aZT7IgePkt5iC/BKBk3hqKteTnJFeVIT7EC+a6YUFg= Organization: Red Hat GmbH Message-ID: <626d344e-8243-c161-cd07-ed1276eba73d@redhat.com> Date: Wed, 22 Jan 2020 19:15:47 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.3.1 MIME-Version: 1.0 In-Reply-To: <20200122164618.GY29276@dhcp22.suse.cz> Content-Type: text/plain; charset=windows-1252 Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 Content-Transfer-Encoding: quoted-printable X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 22.01.20 17:46, Michal Hocko wrote: > On Wed 22-01-20 12:58:16, David Hildenbrand wrote: >> On 22.01.20 11:54, David Hildenbrand wrote: >>> On 22.01.20 11:42, Michal Hocko wrote: >>>> On Wed 22-01-20 11:39:08, David Hildenbrand wrote: >>>>>>>> Really, the interface is flawed and should have never been merge= d in the >>>>>>>> first place. We cannot simply remove it altogether I am afraid s= o let's >>>>>>>> at least remove the bogus code and pretend that the world is a b= etter >>>>>>>> place where everything is removable except the reality sucks... >>>>>>> >>>>>>> As I expressed already, the interface works as designed/documente= d and >>>>>>> has been used like that for years. >>>>>> >>>>>> It seems we do differ in the usefulness though. Using a crappy int= erface >>>>>> for years doesn't make it less crappy. I do realize we cannot remo= ve the >>>>>> interface but we can remove issues with the implementation and I d= are to >>>>>> say that most existing users wouldn't really notice. >>>>> >>>>> Well, at least powerpc-utils (why this interface was introduced) wi= ll >>>>> notice a) performance wise and b) because more logging output will = be >>>>> generated (obviously non-offlineable blocks will be tried to offlin= e). >>>> >>>> I would really appreciate some specific example for a real usecase. = I am >>>> not familiar with powerpc-utils worklflows myself. >>>> >>> >>> Not an expert myself: >>> >>> https://github.com/ibm-power-utilities/powerpc-utils >>> >>> -> src/drmgr/drslot_chrp_mem.c >>> >>> On request to remove some memory it will >>> >>> a) Read "->removable" of all memory blocks ("lmb") >>> b) Check if the request can be fulfilled using the removable blocks >>> c) Try to offline the memory blocks by trying to offline it. If that >>> succeeded, trigger removeal of it using some hypervisor hooks. >>> >>> Interestingly, with "AMS ballooning", it will already consider the >>> "removable" information useless (most probably, because of >>> non-migratable balloon pages that can be offlined - I assume the powe= rpc >>> code that I converted to proper balloon compaction just recently). a) >>> and b) is skipped. >>> >>> Returning "yes" on all blocks will make them handle it just like if "= AMS >>> ballooning" is active. So any memory block will be tried. Should work >>> but will be slower if no ballooning is active. >>> >> >> On lsmem: >> >> https://www.ibm.com/support/knowledgecenter/linuxonibm/com.ibm.linux.z= .lgdd/lgdd_r_lsmem_cmd.html >> >> " >> Removable >> yes if the memory range can be set offline, no if it cannot be set >> offline. A dash (-) means that the range is already offline. The kerne= l >> method that identifies removable memory ranges is heuristic and not >> exact. Occasionally, memory ranges are falsely reported as removable o= r >> falsely reported as not removable. >> " >> >> Usage of lsmem paird with chmem: >> >> https://access.redhat.com/solutions/3937181 >> >> >> Especially interesting for IBM z Systems, whereby memory >> onlining/offlining will trigger the actual population of memory in the >> hypervisor. So if an admin wants to offline some memory (to give it ba= ck >> to the hypervisor), it would use lsmem to identify such blocks first, >> instead of trying random blocks until one offlining request succeeds. >=20 > I am sorry for being dense here but I still do not understand why s390 It's good that we talk about it :) It's hard to reconstruct actual use cases from tools and some documentation only ... Side note (just FYI): One difference on s390x compared to other architectures (AFAIKS) is that once memory is offline, you might not be allowed (by the hypervisor) to online it again - because it was effectively unplugged. Such memory is not removed via remove_memory(), it's simply kept offline. > and the way how it does the hotremove matters here. Afterall there are > no arch specific operations done until the memory is offlined. Also > randomly checking memory blocks and then hoping that the offline will > succeed is not way much different from just trying the offline the > block. Both have to crawl through the pfn range and bail out on the > unmovable memory. I think in general we have to approaches to memory unplugging. 1. Know explicitly what you want to unplug (e.g., a DIMM spanning multiple memory blocks). 2. Find random memory blocks you can offline/unplug. For 1, I think we both agree that we don't need this. Just try to offline and you know if it worked. Now of course, for 2 you can try random blocks until you succeeded. From a sysadmin point of view that's very inefficient. From a powerpc-utils point of view, that's inefficient. I learned just now, "chmem"[1] has a mode where you can specify a "size" and not only a range. So a sysadmin can still control onlining/offlining for this use case with a few commands. In other tools (e.g., powerpc-utils), well, you have to try to offline random memory blocks (just like chmem does). AFAIK, once we turn /sys/.../removable useless, I can see the following changes: 1. Trying to offline a certain amount of memory blocks gets slower/takes longer/is less efficient. Might be tolerable. The tools seem to keep working. 2. You can no longer make a rough estimate how much memory you could offline - before you actually try to offline it. I can only imagine that something like this makes sense in a virtual environment (e.g., IBM z) to balance memory between virtual machines, but I am not aware of a real user of something like that. So what I can do is a) Come up with a patch that rips that stuff out (well I have that already lying around) b) Describe the existing users + changes we will see c) CC relevant people I identify (lsmem/chmem/powerpc-utils/etc.) on the patch to see if we are missing other use cases/users/implications. Sounds like a plan? [1] https://git.kernel.org/pub/scm/utils/util-linux/util-linux.git/tree/sys-u= tils/chmem.c --=20 Thanks, David / dhildenb