From patchwork Mon Feb 8 20:33:13 2021 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Andrew Morton X-Patchwork-Id: 379098 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=-15.8 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=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 AE8E7C433E0 for ; Mon, 8 Feb 2021 20:35:24 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 7D03364E54 for ; Mon, 8 Feb 2021 20:35:24 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231478AbhBHUfA (ORCPT ); Mon, 8 Feb 2021 15:35:00 -0500 Received: from mail.kernel.org ([198.145.29.99]:54264 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231616AbhBHUd5 (ORCPT ); Mon, 8 Feb 2021 15:33:57 -0500 Received: by mail.kernel.org (Postfix) with ESMTPSA id 394B364DE1; Mon, 8 Feb 2021 20:33:14 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linux-foundation.org; s=korg; t=1612816395; bh=TCPAJkVC2RFPxk43uOrn3kV0jgeOkUjfIxSZt6rXyGE=; h=Date:From:To:Subject:From; b=sKK/oh5KixQS5oQyxWKtv57nX6iphZ8sd5pW32rS3KQMj0qIPW7eHwA0NJ3uLOdHr uOI5358/1kQZMJJRJw5Kodxflk6xt53ZfJtKmvuNCvu4QivH0ozgTQLU72uA9ri+UX eb9ZYmMCQtM/y9aMb6aAg/9BMkkBy1T9HiyN6XRI= Date: Mon, 08 Feb 2021 12:33:13 -0800 From: akpm@linux-foundation.org To: aneesh.kumar@linux.ibm.com, bharata@linux.ibm.com, catalin.marinas@arm.com, cl@linux.com, guro@fb.com, hannes@cmpxchg.org, iamjoonsoo.kim@lge.com, jannh@google.com, mgorman@techsingularity.net, mhocko@kernel.org, mm-commits@vger.kernel.org, rientjes@google.com, shakeelb@google.com, stable@vger.kernel.org, vbabka@suse.cz, vincent.guittot@linaro.org, will@kernel.org Subject: + mm-slub-better-heuristic-for-number-of-cpus-when-calculating-slab-order.patch added to -mm tree Message-ID: <20210208203313.ZEFK8sg7I%akpm@linux-foundation.org> User-Agent: s-nail v14.8.16 MIME-Version: 1.0 Precedence: bulk List-ID: X-Mailing-List: stable@vger.kernel.org The patch titled Subject: mm, slub: better heuristic for number of cpus when calculating slab order has been added to the -mm tree. Its filename is mm-slub-better-heuristic-for-number-of-cpus-when-calculating-slab-order.patch This patch should soon appear at https://ozlabs.org/~akpm/mmots/broken-out/mm-slub-better-heuristic-for-number-of-cpus-when-calculating-slab-order.patch and later at https://ozlabs.org/~akpm/mmotm/broken-out/mm-slub-better-heuristic-for-number-of-cpus-when-calculating-slab-order.patch Before you just go and hit "reply", please: a) Consider who else should be cc'ed b) Prefer to cc a suitable mailing list as well c) Ideally: find the original patch on the mailing list and do a reply-to-all to that, adding suitable additional cc's *** Remember to use Documentation/process/submit-checklist.rst when testing your code *** The -mm tree is included into linux-next and is updated there every 3-4 working days ------------------------------------------------------ From: Vlastimil Babka Subject: mm, slub: better heuristic for number of cpus when calculating slab order When creating a new kmem cache, SLUB determines how large the slab pages will based on number of inputs, including the number of CPUs in the system. Larger slab pages mean that more objects can be allocated/free from per-cpu slabs before accessing shared structures, but also potentially more memory can be wasted due to low slab usage and fragmentation. The rough idea of using number of CPUs is that larger systems will be more likely to benefit from reduced contention, and also should have enough memory to spare. Number of CPUs used to be determined as nr_cpu_ids, which is number of possible cpus, but on some systems many will never be onlined, thus commit 045ab8c9487b ("mm/slub: let number of online CPUs determine the slub page order") changed it to nr_online_cpus(). However, for kmem caches created early before CPUs are onlined, this may lead to permamently low slab page sizes. Vincent reports a regression [1] of hackbench on arm64 systems: > I'm facing significant performances regression on a large arm64 server > system (224 CPUs). Regressions is also present on small arm64 system > (8 CPUs) but in a far smaller order of magnitude > On 224 CPUs system : 9 iterations of hackbench -l 16000 -g 16 > v5.11-rc4 : 9.135sec (+/- 0.45%) > v5.11-rc4 + revert this patch: 3.173sec (+/- 0.48%) > v5.10: 3.136sec (+/- 0.40%) Mel reports a regression [2] of hackbench on x86_64, with lockstat suggesting page allocator contention: > i.e. the patch incurs a 7% to 32% performance penalty. This bisected > cleanly yesterday when I was looking for the regression and then found > the thread. > Numerous caches change size. For example, kmalloc-512 goes from order-0 > (vanilla) to order-2 with the revert. > So mostly this is down to the number of times SLUB calls into the page > allocator which only caches order-0 pages on a per-cpu basis. Clearly num_online_cpus() doesn't work too early in bootup. We could change the order dynamically in a memory hotplug callback, but runtime order changing for existing kmem caches has been already shown as dangerous, and removed in 32a6f409b693 ("mm, slub: remove runtime allocation order changes"). It could be resurrected in a safe manner with some effort, but to fix the regression we need something simpler. We could use num_present_cpus() that should be the number of physically present CPUs even before they are onlined. That would work for PowerPC [3], which triggered the original commit, but that still doesn't work on arm64 [4] as explained in [5]. So this patch tries to determine the best available value without specific arch knowledge. - num_present_cpus() if the number is larger than 1, as that means the arch is likely setting it properly - nr_cpu_ids otherwise This should fix the reported regressions while also keeping the effect of 045ab8c9487b for PowerPC systems. It's possible there are configurations where num_present_cpus() is 1 during boot while nr_cpu_ids is at the same time bloated, so these (if they exist) would keep the large orders based on nr_cpu_ids as was before 045ab8c9487b. [1] https://lore.kernel.org/linux-mm/CAKfTPtA_JgMf_+zdFbcb_V9rM7JBWNPjAz9irgwFj7Rou=xzZg@mail.gmail.com/ [2] https://lore.kernel.org/linux-mm/20210128134512.GF3592@techsingularity.net/ [3] https://lore.kernel.org/linux-mm/20210123051607.GC2587010@in.ibm.com/ [4] https://lore.kernel.org/linux-mm/CAKfTPtAjyVmS5VYvU6DBxg4-JEo5bdmWbngf-03YsY18cmWv_g@mail.gmail.com/ [5] https://lore.kernel.org/linux-mm/20210126230305.GD30941@willie-the-truck/ Link: https://lkml.kernel.org/r/20210208134108.22286-1-vbabka@suse.cz Fixes: 045ab8c9487b ("mm/slub: let number of online CPUs determine the slub page order") Signed-off-by: Vlastimil Babka Reported-by: Vincent Guittot Reported-by: Mel Gorman Tested-by: Vincent Guittot Cc: Catalin Marinas Cc: Aneesh Kumar K.V Cc: Bharata B Rao Cc: Christoph Lameter Cc: Roman Gushchin Cc: Johannes Weiner Cc: Joonsoo Kim Cc: Jann Horn Cc: Michal Hocko Cc: David Rientjes Cc: Shakeel Butt Cc: Will Deacon Cc: Signed-off-by: Andrew Morton --- mm/slub.c | 18 ++++++++++++++++-- 1 file changed, 16 insertions(+), 2 deletions(-) --- a/mm/slub.c~mm-slub-better-heuristic-for-number-of-cpus-when-calculating-slab-order +++ a/mm/slub.c @@ -3423,6 +3423,7 @@ static inline int calculate_order(unsign unsigned int order; unsigned int min_objects; unsigned int max_objects; + unsigned int nr_cpus; /* * Attempt to find best configuration for a slab. This @@ -3433,8 +3434,21 @@ static inline int calculate_order(unsign * we reduce the minimum objects required in a slab. */ min_objects = slub_min_objects; - if (!min_objects) - min_objects = 4 * (fls(num_online_cpus()) + 1); + if (!min_objects) { + /* + * Some architectures will only update present cpus when + * onlining them, so don't trust the number if it's just 1. But + * we also don't want to use nr_cpu_ids always, as on some other + * architectures, there can be many possible cpus, but never + * onlined. Here we compromise between trying to avoid too high + * order on systems that appear larger than they are, and too + * low order on systems that appear smaller than they are. + */ + nr_cpus = num_present_cpus(); + if (nr_cpus <= 1) + nr_cpus = nr_cpu_ids; + min_objects = 4 * (fls(nr_cpus) + 1); + } max_objects = order_objects(slub_max_order, size); min_objects = min(min_objects, max_objects);