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=-5.4 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=ham 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 23598C33CA2 for ; Fri, 10 Jan 2020 09:32:14 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id CDF722077C for ; Fri, 10 Jan 2020 09:32:13 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="e0sOJP5r" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727257AbgAJJcM (ORCPT ); Fri, 10 Jan 2020 04:32:12 -0500 Received: from us-smtp-1.mimecast.com ([207.211.31.81]:41232 "EHLO us-smtp-delivery-1.mimecast.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726877AbgAJJcM (ORCPT ); Fri, 10 Jan 2020 04:32:12 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1578648729; 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=iKMkOoDRxkVplffIq8jmwKYmNRWBeyWiU140IOAbbFo=; b=e0sOJP5rrpvaWwZtWyMs4pd5+RtPWsQ0N4HmfKjv8TEtBuG0nKMzua3HJzEdnk4jlqGQ6Q VXv5ls/YM3LNWUqBosH6AE91bCc/Igq5/VOZIDl/lTRF3fS22UJ+lq9TVeVsGjLgfF3X1o FoOtsHCctS7HXL0Myjf42FsY5YS0M0w= 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-381-vQ6vLDNuPwqyg9Wh-ur3eQ-1; Fri, 10 Jan 2020 04:32:06 -0500 X-MC-Unique: vQ6vLDNuPwqyg9Wh-ur3eQ-1 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.phx2.redhat.com [10.5.11.15]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 8F41710054E3; Fri, 10 Jan 2020 09:32:04 +0000 (UTC) Received: from [10.36.117.139] (ovpn-117-139.ams2.redhat.com [10.36.117.139]) by smtp.corp.redhat.com (Postfix) with ESMTP id ABC6F7C3F5; Fri, 10 Jan 2020 09:32:02 +0000 (UTC) Subject: Re: [PATCH v4] drivers/base/memory.c: cache blocks in radix tree to accelerate lookup To: Andrew Morton Cc: Scott Cheloha , linux-kernel@vger.kernel.org, "Rafael J. Wysocki" , Greg Kroah-Hartman , nathanl@linux.ibm.com, ricklind@linux.vnet.ibm.com, mhocko@suse.com, Scott Cheloha References: <20200109142758.659c1545cb8df2d05f299a4a@linux-foundation.org> 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: Date: Fri, 10 Jan 2020 10:32:01 +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: Content-Type: text/plain; charset=utf-8 Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 Content-Transfer-Encoding: quoted-printable Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 09.01.20 23:35, David Hildenbrand wrote: >> Am 09.01.2020 um 23:28 schrieb Andrew Morton : >> >> =EF=BB=BFOn Thu, 9 Jan 2020 23:17:09 +0100 David Hildenbrand wrote: >> >>> >>> >>>>> Am 09.01.2020 um 23:00 schrieb Andrew Morton : >>>> >>>> =EF=BB=BFOn Thu, 9 Jan 2020 15:25:16 -0600 Scott Cheloha wrote: >>>> >>>>> Searching for a particular memory block by id is an O(n) operation >>>>> because each memory block's underlying device is kept in an unsorte= d >>>>> linked list on the subsystem bus. >>>>> >>>>> We can cut the lookup cost to O(log n) if we cache the memory block= s in >>>>> a radix tree. With a radix tree cache in place both memory subsyst= em >>>>> initialization and memory hotplug run palpably faster on systems wi= th a >>>>> large number of memory blocks. >>>>> >>>>> ... >>>>> >>>>> @@ -56,6 +57,13 @@ static struct bus_type memory_subsys =3D { >>>>> .offline =3D memory_subsys_offline, >>>>> }; >>>>> >>>>> +/* >>>>> + * Memory blocks are cached in a local radix tree to avoid >>>>> + * a costly linear search for the corresponding device on >>>>> + * the subsystem bus. >>>>> + */ >>>>> +static RADIX_TREE(memory_blocks, GFP_KERNEL); >>>> >>>> What protects this tree from racy accesses? >>> >>> I think the device hotplug lock currently (except during boot where n= o races can happen). >>> >> >> So this? >> >> --- a/drivers/base/memory.c~drivers-base-memoryc-cache-blocks-in-radix= -tree-to-accelerate-lookup-fix >> +++ a/drivers/base/memory.c >> @@ -61,6 +61,9 @@ static struct bus_type memory_subsys =3D { >> * Memory blocks are cached in a local radix tree to avoid >> * a costly linear search for the corresponding device on >> * the subsystem bus. >> + * >> + * Protected by mem_hotplug_lock in mem_hotplug_begin(), and by the g= uaranteed >> + * single-threadness at boot time. >> */ >> static RADIX_TREE(memory_blocks, GFP_KERNEL); >> >> >> But are we sure this is all true? >=20 > I think the device hotplug lock, not the memory hotplug lock. Will doub= le check later. So all writers either hold the device_hotplug_lock or run during boot. Documented e.g., for memory_dev_init(), create_memory_block_devices(), remove_memory_block_devices(). The readers are mainly - find_memory_block() -> called via online_pages()/offline_pages() where we hold the device_hotplug_lock -> called from arch/powerpc/platforms/pseries/hotplug-memory.c, where we always hold the device_hotplug_lock - walk_memory_blocks() -> Callers in drivers/acpi/acpi_memhotplug.c either hold the device_hotplug_lock or are called early during boot -> Callers in mm/memory_hotplug.c either hold the device_hotplug_lock or are called early during boot -> link_mem_sections() is called either early during boot or via add_memory_resource() (whereby all callers either hold the device_hotplug_lock or are called early during boot) -> Callers in arch/powerpc/platforms/powernv/memtrace.c hold the device_hotplug_lock So we are fine. I suggest we document that expected behavior via diff --git a/drivers/base/memory.c b/drivers/base/memory.c index 799b43191dea..8c8dc081597e 100644 --- a/drivers/base/memory.c +++ b/drivers/base/memory.c @@ -585,6 +585,8 @@ static struct memory_block *find_memory_block_by_id(unsigned long block_id) * tree or something here. * * This could be made generic for all device subsystems. + * + * Called under device_hotplug_lock. */ struct memory_block *find_memory_block(struct mem_section *section) { @@ -837,6 +839,8 @@ void __init memory_dev_init(void) * * In case func() returns an error, walking is aborted and the error is * returned. + * + * Called under device_hotplug_lock. */ int walk_memory_blocks(unsigned long start, unsigned long size, void *arg, walk_memory_blocks_func_t func) Please note that the memory hotplug lock is not safe on the reader side. But also not on the writer side after https://lkml.kernel.org/r/157863061737.2230556.3959730620803366776.stgit@= dwillia2-desk3.amr.corp.intel.com --=20 Thanks, David / dhildenb