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=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no 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 03292C76186 for ; Mon, 29 Jul 2019 21:42:25 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id C838320679 for ; Mon, 29 Jul 2019 21:42:24 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729897AbfG2VmX (ORCPT ); Mon, 29 Jul 2019 17:42:23 -0400 Received: from mx1.redhat.com ([209.132.183.28]:60134 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727681AbfG2VmX (ORCPT ); Mon, 29 Jul 2019 17:42:23 -0400 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 0D74D308FC47; Mon, 29 Jul 2019 21:42:23 +0000 (UTC) Received: from llong.remote.csb (dhcp-17-160.bos.redhat.com [10.18.17.160]) by smtp.corp.redhat.com (Postfix) with ESMTP id 7831260167; Mon, 29 Jul 2019 21:42:21 +0000 (UTC) Subject: Re: [PATCH v3] sched/core: Don't use dying mm as active_mm of kthreads To: Rik van Riel , Peter Zijlstra , Ingo Molnar Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, Andrew Morton , Phil Auld , Michal Hocko References: <20190729210728.21634-1-longman@redhat.com> From: Waiman Long Organization: Red Hat Message-ID: <3e2ff4c9-c51f-8512-5051-5841131f4acb@redhat.com> Date: Mon, 29 Jul 2019 17:42:20 -0400 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.7.2 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.43]); Mon, 29 Jul 2019 21:42:23 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 7/29/19 5:21 PM, Rik van Riel wrote: > On Mon, 2019-07-29 at 17:07 -0400, Waiman Long wrote: >> It was found that a dying mm_struct where the owning task has exited >> can stay on as active_mm of kernel threads as long as no other user >> tasks run on those CPUs that use it as active_mm. This prolongs the >> life time of dying mm holding up some resources that cannot be freed >> on a mostly idle system. > On what kernels does this happen? > > Don't we explicitly flush all lazy TLB CPUs at exit > time, when we are about to free page tables? There are still a couple of calls that will be done until mm_count reaches 0: - mm_free_pgd(mm); - destroy_context(mm); - mmu_notifier_mm_destroy(mm); - check_mm(mm); - put_user_ns(mm->user_ns); These are not big items, but holding it off for a long time is still not a good thing. > Does this happen only on the CPU where the task in > question is exiting, or also on other CPUs? What I have found is that a long running process on a mostly idle system with many CPUs is likely to cycle through a lot of the CPUs during its lifetime and leave behind its mm in the active_mm of those CPUs.  My 2-socket test system have 96 logical CPUs. After running the test program for a minute or so, it leaves behind its mm in about half of the CPUs with a mm_count of 45 after exit. So the dying mm will stay until all those 45 CPUs get new user tasks to run. > > If it is only on the CPU where the task is exiting, > would the TASK_DEAD handling in finish_task_switch() > be a better place to handle this? I need to switch the mm off the dying one. mm switching is only done in context_switch(). I don't think finish_task_switch() is the right place. -Longman