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=-2.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,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 29795C43141 for ; Fri, 29 Jun 2018 14:29:55 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id D847527F8D for ; Fri, 29 Jun 2018 14:29:54 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org D847527F8D Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=surriel.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S934832AbeF2O3w (ORCPT ); Fri, 29 Jun 2018 10:29:52 -0400 Received: from shelob.surriel.com ([96.67.55.147]:34296 "EHLO shelob.surriel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933566AbeF2O3n (ORCPT ); Fri, 29 Jun 2018 10:29:43 -0400 Received: from imladris.surriel.com ([96.67.55.152]) by shelob.surriel.com with esmtpsa (TLSv1.2:ECDHE-RSA-AES256-GCM-SHA384:256) (Exim 4.90_1) (envelope-from ) id 1fYuOx-0000Zu-Ta; Fri, 29 Jun 2018 10:29:31 -0400 From: Rik van Riel To: linux-kernel@vger.kernel.org Cc: x86@kernel.org, luto@kernel.org, dave.hansen@linux.intel.com, mingo@kernel.org, kernel-team@fb.com, tglx@linutronix.de, efault@gmx.de, songliubraving@fb.com, hpa@zytor.com, Rik van Riel Subject: [PATCH 7/7] x86,switch_mm: skip atomic operations for init_mm Date: Fri, 29 Jun 2018 10:29:18 -0400 Message-Id: <20180629142918.26601-8-riel@surriel.com> X-Mailer: git-send-email 2.14.4 In-Reply-To: <20180629142918.26601-1-riel@surriel.com> References: <20180629142918.26601-1-riel@surriel.com> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Song noticed switch_mm_irqs_off taking a lot of CPU time in recent kernels,using 1.8% of a 48 CPU system during a netperf to localhost run. Digging into the profile, we noticed that cpumask_clear_cpu and cpumask_set_cpu together take about half of the CPU time taken by switch_mm_irqs_off. However, the CPUs running netperf end up switching back and forth between netperf and the idle task, which does not require changes to the mm_cpumask. Furthermore, the init_mm cpumask ends up being the most heavily contended one in the system. Simply skipping changes to mm_cpumask(&init_mm) reduces overhead. Signed-off-by: Rik van Riel Reported-and-tested-by: Song Liu --- arch/x86/mm/tlb.c | 11 +++++++---- 1 file changed, 7 insertions(+), 4 deletions(-) diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c index 5a01fcb22a7e..b55e6b7df7c9 100644 --- a/arch/x86/mm/tlb.c +++ b/arch/x86/mm/tlb.c @@ -311,14 +311,17 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next, } /* Stop remote flushes for the previous mm */ - VM_WARN_ON_ONCE(!cpumask_test_cpu(cpu, mm_cpumask(real_prev)) && - real_prev != &init_mm); - cpumask_clear_cpu(cpu, mm_cpumask(real_prev)); + if (real_prev != &init_mm) { + VM_WARN_ON_ONCE(!cpumask_test_cpu(cpu, + mm_cpumask(real_prev))); + cpumask_clear_cpu(cpu, mm_cpumask(real_prev)); + } /* * Start remote flushes and then read tlb_gen. */ - cpumask_set_cpu(cpu, mm_cpumask(next)); + if (next != &init_mm) + cpumask_set_cpu(cpu, mm_cpumask(next)); next_tlb_gen = atomic64_read(&next->context.tlb_gen); choose_new_asid(next, next_tlb_gen, &new_asid, &need_flush); -- 2.14.4