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=-15.7 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED 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 79D57C433E9 for ; Wed, 24 Feb 2021 20:03:29 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 4485B64F26 for ; Wed, 24 Feb 2021 20:03:29 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235276AbhBXUDQ (ORCPT ); Wed, 24 Feb 2021 15:03:16 -0500 Received: from mail.kernel.org ([198.145.29.99]:55902 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235186AbhBXUCe (ORCPT ); Wed, 24 Feb 2021 15:02:34 -0500 Received: by mail.kernel.org (Postfix) with ESMTPSA id 57F9664F23; Wed, 24 Feb 2021 20:01:16 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linux-foundation.org; s=korg; t=1614196876; bh=sUsO3EhZIzAc5iCkRXCgeaT0vg9qFPJbgpzN4SykNpk=; h=Date:From:To:Subject:In-Reply-To:From; b=ik8LH3+0Gw/ORMtmV4I3olNvPpSs++/WKVjHKEApDG7NypUQTcGFqCBb9+xQxOFAG aspsFWyZyCQ3pmw71jyHId4lMWycIzm1TNE83WBGCUbjAY0PnkxeZjrOAgdiiq2lkD ZjGp3DQxWdjGotePAtLRxYJyF9k1+MjCLnfFiPWo= Date: Wed, 24 Feb 2021 12:01:15 -0800 From: Andrew Morton To: akpm@linux-foundation.org, cai@redhat.com, cl@linux.com, david@redhat.com, iamjoonsoo.kim@lge.com, linux-mm@kvack.org, mhocko@kernel.org, mm-commits@vger.kernel.org, penberg@kernel.org, rientjes@google.com, torvalds@linux-foundation.org, vbabka@suse.cz, vdavydov.dev@gmail.com Subject: [patch 020/173] mm, slab, slub: stop taking cpu hotplug lock Message-ID: <20210224200115.Sn4XTbiPC%akpm@linux-foundation.org> In-Reply-To: <20210224115824.1e289a6895087f10c41dd8d6@linux-foundation.org> User-Agent: s-nail v14.8.16 Precedence: bulk Reply-To: linux-kernel@vger.kernel.org List-ID: X-Mailing-List: mm-commits@vger.kernel.org From: Vlastimil Babka Subject: mm, slab, slub: stop taking cpu hotplug lock 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. Link: https://lkml.kernel.org/r/20210113131634.3671-4-vbabka@suse.cz Signed-off-by: Vlastimil Babka Cc: Christoph Lameter Cc: David Hildenbrand Cc: David Rientjes Cc: Joonsoo Kim Cc: Michal Hocko Cc: Pekka Enberg Cc: Qian Cai Cc: Vladimir Davydov Signed-off-by: Andrew Morton --- mm/slab_common.c | 10 ---------- 1 file changed, 10 deletions(-) --- a/mm/slab_common.c~mm-slab-slub-stop-taking-cpu-hotplug-lock +++ a/mm/slab_common.c @@ -309,8 +309,6 @@ kmem_cache_create_usercopy(const char *n const char *cache_name; int err; - get_online_cpus(); - mutex_lock(&slab_mutex); err = kmem_cache_sanity_check(name, size); @@ -359,8 +357,6 @@ kmem_cache_create_usercopy(const char *n 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", @@ -484,8 +480,6 @@ void kmem_cache_destroy(struct kmem_cach if (unlikely(!s)) return; - get_online_cpus(); - mutex_lock(&slab_mutex); s->refcount--; @@ -500,8 +494,6 @@ void kmem_cache_destroy(struct kmem_cach } out_unlock: mutex_unlock(&slab_mutex); - - put_online_cpus(); } EXPORT_SYMBOL(kmem_cache_destroy); @@ -518,12 +510,10 @@ int kmem_cache_shrink(struct kmem_cache { int ret; - get_online_cpus(); kasan_cache_shrink(cachep); ret = __kmem_cache_shrink(cachep); - put_online_cpus(); return ret; } EXPORT_SYMBOL(kmem_cache_shrink); _