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=-16.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_GIT 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 57B54C433DB for ; Wed, 13 Jan 2021 13:17:45 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 148E423433 for ; Wed, 13 Jan 2021 13:17:45 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726634AbhAMNR1 (ORCPT ); Wed, 13 Jan 2021 08:17:27 -0500 Received: from mx2.suse.de ([195.135.220.15]:56652 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725809AbhAMNR0 (ORCPT ); Wed, 13 Jan 2021 08:17:26 -0500 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id DD262B803; Wed, 13 Jan 2021 13:16:44 +0000 (UTC) From: Vlastimil Babka To: Andrew Morton , linux-mm@kvack.org Cc: linux-kernel@vger.kernel.org, Christoph Lameter , Pekka Enberg , David Rientjes , Joonsoo Kim , Vladimir Davydov , Qian Cai , David Hildenbrand , Michal Hocko , Vlastimil Babka Subject: [PATCH 3/3] mm, slab, slub: stop taking cpu hotplug lock Date: Wed, 13 Jan 2021 14:16:34 +0100 Message-Id: <20210113131634.3671-4-vbabka@suse.cz> X-Mailer: git-send-email 2.29.2 In-Reply-To: <20210113131634.3671-1-vbabka@suse.cz> References: <20210113131634.3671-1-vbabka@suse.cz> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org SLAB has been using get/put_online_cpus() around creating, destroying and shrinking kmem caches since 95402b382901 ("cpu-hotplug: replace per-subsystem mutexes with get_online_cpus()") in 2008, which is supposed to be replacing a private mutex (cache_chain_mutex, called slab_mutex today) with system-wide mechanism, but in case of SLAB it's in fact used in addition to the existing mutex, without explanation why. SLUB appears to have avoided the cpu hotplug lock initially, but gained it due to common code unification, such as 20cea9683ecc ("mm, sl[aou]b: Move kmem_cache_create mutex handling to common code"). Regardless of the history, checking if the hotplug lock is actually needed today suggests that it's not, and therefore it's better to avoid this system-wide lock and the ordering this imposes wrt other locks (such as slab_mutex). Specifically, in SLAB we have for_each_online_cpu() in do_tune_cpucache() protected by slab_mutex, and cpu hotplug callbacks that also take the slab_mutex, which is also taken by the common slab function that currently also take the hotplug lock. Thus the slab_mutex protection should be sufficient. Also per-cpu array caches are allocated for each possible cpu, so not affected by their online/offline state. In SLUB we have for_each_online_cpu() in functions that show statistics and are already unprotected today, as racing with hotplug is not harmful. Otherwise SLUB relies on percpu allocator. The slub_cpu_dead() hotplug callback takes the slab_mutex. To sum up, this patch removes get/put_online_cpus() calls from slab as it should be safe without further adjustments. Signed-off-by: Vlastimil Babka --- mm/slab_common.c | 10 ---------- 1 file changed, 10 deletions(-) diff --git a/mm/slab_common.c b/mm/slab_common.c index e040b3820a75..0ca99ebefbf2 100644 --- a/mm/slab_common.c +++ b/mm/slab_common.c @@ -310,8 +310,6 @@ kmem_cache_create_usercopy(const char *name, const char *cache_name; int err; - get_online_cpus(); - mutex_lock(&slab_mutex); err = kmem_cache_sanity_check(name, size); @@ -360,8 +358,6 @@ kmem_cache_create_usercopy(const char *name, out_unlock: mutex_unlock(&slab_mutex); - put_online_cpus(); - if (err) { if (flags & SLAB_PANIC) panic("kmem_cache_create: Failed to create slab '%s'. Error %d\n", @@ -487,8 +483,6 @@ void kmem_cache_destroy(struct kmem_cache *s) if (unlikely(!s)) return; - get_online_cpus(); - mutex_lock(&slab_mutex); s->refcount--; @@ -503,8 +497,6 @@ void kmem_cache_destroy(struct kmem_cache *s) } out_unlock: mutex_unlock(&slab_mutex); - - put_online_cpus(); } EXPORT_SYMBOL(kmem_cache_destroy); @@ -521,12 +513,10 @@ int kmem_cache_shrink(struct kmem_cache *cachep) { int ret; - get_online_cpus(); kasan_cache_shrink(cachep); ret = __kmem_cache_shrink(cachep); - put_online_cpus(); return ret; } EXPORT_SYMBOL(kmem_cache_shrink); -- 2.29.2