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,FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_GIT 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 77A2AC1B0D8 for ; Mon, 14 Dec 2020 07:02:53 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 3EE5A2078E for ; Mon, 14 Dec 2020 07:02:53 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2407195AbgLNHCl (ORCPT ); Mon, 14 Dec 2020 02:02:41 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:60778 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2407132AbgLNHCH (ORCPT ); Mon, 14 Dec 2020 02:02:07 -0500 Received: from mail-pj1-x1042.google.com (mail-pj1-x1042.google.com [IPv6:2607:f8b0:4864:20::1042]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 0AAFCC0613D6; Sun, 13 Dec 2020 23:01:27 -0800 (PST) Received: by mail-pj1-x1042.google.com with SMTP id m5so6060163pjv.5; Sun, 13 Dec 2020 23:01:27 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=from:to:cc:subject:date:message-id:in-reply-to:references :mime-version:content-transfer-encoding; bh=ErbENPowD2FhE1fb1KE1SS0KG3DGxAufvlmv8VFmkMA=; b=JvZByPsEXrHdPtVKtHVHGjkSpux7toHopC3Q55F7zTqqwwZ5PHppHHXxc6aXSO6GKB oyfie9gW09mQYPvCRKvx9HMxCB/osCbn29u7jDRUFzqv5IiRSR77KNbem7qfl7TQ7VUq C9p49cwE2Z6FNPa8ejWQnty9JN4XxkoGCm7S49R9NFlkO1txIrWjl5Z0FzqZO3U9viux 06U/I73Dqc+hEzgAZKfCwyUDsCgR7/YUn5DN4UjjGictkGOGay7x42N+o++kqKFggq9l nEnjpbRT8HjzsjpOspZdyeyToedAZplx5wUK8CINpXTu1fCREsOLVQFWrEo8h2bLGOKx zZ4w== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:from:to:cc:subject:date:message-id:in-reply-to :references:mime-version:content-transfer-encoding; bh=ErbENPowD2FhE1fb1KE1SS0KG3DGxAufvlmv8VFmkMA=; b=ChQnzpZzNIl+t2RtLWzFLXJPl6N5BVY5pzOOpyAm7sEKWXC7ZYYbPVlgEazSLayMnc bz6ebbizveWpUbPwZ2gG6uASB3hXYtDl1Pk4kCwLqReZqFhwiX6yJFiMqRy3ilDoxUrY HUGsdbSu0Bs9YphB6jX8X7E46ocjBmm771PVPxHcEJFqDuKPGWzaUo9fQO8/2em2k8k0 BqkaqnpntHd0Lzv1Klm++/UWkjNqxjt/UrslzpJDzvGZ6dBjPsTuS6K0icWoNS7TrD/h WTpyI6m8+OxMDdBbkrLlhdne2xkEsBYmg2M3VZFhmWWNzQb4HGrL9DVDDEp2ehF4LuhV pfbw== X-Gm-Message-State: AOAM530JYihhvR4pgE0LozcQNIqM6fGhZUSAuhAactqMpONUZWg3rcEH gvmLf28K9QTJRqaIiP9KCw97YflYKig= X-Google-Smtp-Source: ABdhPJxblJXiLKkz13t9MjdDE/WHdnRvcf7rwmiGCqMnrUDtgxWEAmDxtkk08GCN+5bnN8Huo5IieQ== X-Received: by 2002:a17:902:b415:b029:d6:ec35:755b with SMTP id x21-20020a170902b415b02900d6ec35755bmr21304417plr.47.1607929286389; Sun, 13 Dec 2020 23:01:26 -0800 (PST) Received: from bobo.ozlabs.ibm.com ([220.240.228.148]) by smtp.gmail.com with ESMTPSA id 84sm19570018pfy.9.2020.12.13.23.01.23 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sun, 13 Dec 2020 23:01:26 -0800 (PST) From: Nicholas Piggin To: linux-kernel@vger.kernel.org Cc: Nicholas Piggin , linux-arch@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-mm@kvack.org, Anton Blanchard , Andy Lutomirski Subject: [PATCH v2 3/5] lazy tlb: shoot lazies, a non-refcounting lazy tlb option Date: Mon, 14 Dec 2020 16:53:10 +1000 Message-Id: <20201214065312.270062-4-npiggin@gmail.com> X-Mailer: git-send-email 2.23.0 In-Reply-To: <20201214065312.270062-1-npiggin@gmail.com> References: <20201214065312.270062-1-npiggin@gmail.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On big systems, the mm refcount can become highly contented when doing a lot of context switching with threaded applications (particularly switching between the idle thread and an application thread). Abandoning lazy tlb slows switching down quite a bit in the important user->idle->user cases, so instead implement a non-refcounted scheme that causes __mmdrop() to IPI all CPUs in the mm_cpumask and shoot down any remaining lazy ones. Shootdown IPIs are some concern, but they have not been observed to be a big problem with this scheme (the powerpc implementation generated 314 additional interrupts on a 144 CPU system during a kernel compile). There are a number of strategies that could be employed to reduce IPIs if they turn out to be a problem for some workload. Signed-off-by: Nicholas Piggin --- arch/Kconfig | 17 +++++++++++++++-- kernel/fork.c | 52 +++++++++++++++++++++++++++++++++++++++++++++++++++ 2 files changed, 67 insertions(+), 2 deletions(-) diff --git a/arch/Kconfig b/arch/Kconfig index 84faaba66364..e69c974369cc 100644 --- a/arch/Kconfig +++ b/arch/Kconfig @@ -443,9 +443,22 @@ config MMU_LAZY_TLB config MMU_LAZY_TLB_REFCOUNT def_bool y depends on MMU_LAZY_TLB + depends on !MMU_LAZY_TLB_SHOOTDOWN help - This must be enabled if MMU_LAZY_TLB is enabled until the next - patch. + This refcounts the mm that is used as the lazy TLB mm when switching + switching to a kernel thread. + +config MMU_LAZY_TLB_SHOOTDOWN + bool + depends on MMU_LAZY_TLB + help + Instead of refcounting the "lazy tlb" mm struct, which can cause + contention with multi-threaded apps on large multiprocessor systems, + this option causes __mmdrop to IPI all CPUs in the mm_cpumask and + switch to init_mm if they were using the to-be-freed mm as the lazy + tlb. To implement this, architectures must use _lazy_tlb variants of + mm refcounting, and mm_cpumask must include at least all possible + CPUs in which mm might be lazy. config ARCH_HAVE_NMI_SAFE_CMPXCHG bool diff --git a/kernel/fork.c b/kernel/fork.c index 6d266388d380..74b972d2d8a9 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -669,6 +669,53 @@ static void check_mm(struct mm_struct *mm) #define allocate_mm() (kmem_cache_alloc(mm_cachep, GFP_KERNEL)) #define free_mm(mm) (kmem_cache_free(mm_cachep, (mm))) +static void do_shoot_lazy_tlb(void *arg) +{ + struct mm_struct *mm = arg; + + if (current->active_mm == mm) { + WARN_ON_ONCE(current->mm); + current->active_mm = &init_mm; + switch_mm(mm, &init_mm, current); + } +} + +static void do_check_lazy_tlb(void *arg) +{ + struct mm_struct *mm = arg; + + WARN_ON_ONCE(current->active_mm == mm); +} + +static void shoot_lazy_tlbs(struct mm_struct *mm) +{ + if (IS_ENABLED(CONFIG_MMU_LAZY_TLB_SHOOTDOWN)) { + /* + * IPI overheads have not found to be expensive, but they could + * be reduced in a number of possible ways, for example (in + * roughly increasing order of complexity): + * - A batch of mms requiring IPIs could be gathered and freed + * at once. + * - CPUs could store their active mm somewhere that can be + * remotely checked without a lock, to filter out + * false-positives in the cpumask. + * - After mm_users or mm_count reaches zero, switching away + * from the mm could clear mm_cpumask to reduce some IPIs + * (some batching or delaying would help). + * - A delayed freeing and RCU-like quiescing sequence based on + * mm switching to avoid IPIs completely. + */ + on_each_cpu_mask(mm_cpumask(mm), do_shoot_lazy_tlb, (void *)mm, 1); + if (IS_ENABLED(CONFIG_DEBUG_VM)) + on_each_cpu(do_check_lazy_tlb, (void *)mm, 1); + } else { + /* + * In this case, lazy tlb mms are refounted and would not reach + * __mmdrop until all CPUs have switched away and mmdrop()ed. + */ + } +} + /* * Called when the last reference to the mm * is dropped: either by a lazy thread or by @@ -678,7 +725,12 @@ void __mmdrop(struct mm_struct *mm) { BUG_ON(mm == &init_mm); WARN_ON_ONCE(mm == current->mm); + + /* Ensure no CPUs are using this as their lazy tlb mm */ + shoot_lazy_tlbs(mm); + WARN_ON_ONCE(mm == current->active_mm); + mm_free_pgd(mm); destroy_context(mm); mmu_notifier_subscriptions_destroy(mm); -- 2.23.0