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 4EBC0C32751 for ; Wed, 31 Jul 2019 14:15:34 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 2ED14206A3 for ; Wed, 31 Jul 2019 14:15:34 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726794AbfGaOPc (ORCPT ); Wed, 31 Jul 2019 10:15:32 -0400 Received: from mx1.redhat.com ([209.132.183.28]:48324 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726308AbfGaOPc (ORCPT ); Wed, 31 Jul 2019 10:15:32 -0400 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.phx2.redhat.com [10.5.11.14]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 019922BF02; Wed, 31 Jul 2019 14:15:32 +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 BD30E5D9C5; Wed, 31 Jul 2019 14:15:17 +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> <3e2ff4c9-c51f-8512-5051-5841131f4acb@redhat.com> <8021be4426fdafdce83517194112f43009fb9f6d.camel@surriel.com> From: Waiman Long Organization: Red Hat Message-ID: <01125822-c883-18ce-42e4-942a4f28c128@redhat.com> Date: Wed, 31 Jul 2019 10:15:17 -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: 7bit Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.39]); Wed, 31 Jul 2019 14:15:32 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 7/31/19 9:48 AM, Rik van Riel wrote: > On Tue, 2019-07-30 at 17:01 -0400, Waiman Long wrote: >> On 7/29/19 8:26 PM, Rik van Riel wrote: >>> On Mon, 2019-07-29 at 17:42 -0400, Waiman Long wrote: >>> >>>> 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. >>> OK. On what kernel are you seeing this? >>> >>> On current upstream, the code in native_flush_tlb_others() >>> will send a TLB flush to every CPU in mm_cpumask() if page >>> table pages have been freed. >>> >>> That should cause the lazy TLB CPUs to switch to init_mm >>> when the exit->zap_page_range path gets to the point where >>> it frees page tables. >>> >> I was using the latest upstream 5.3-rc2 kernel. It may be the case >> that >> the mm has been switched, but the mm_count field of the active_mm of >> the >> kthread is not being decremented until a user task runs on a CPU. > Is that something we could fix from the TLB flushing > code? > > When switching to init_mm, drop the refcount on the > lazy mm? > > That way that overhead is not added to the context > switching code. I have thought about that. That will require changing the active_mm of the current task to point to init_mm, for example. Since TLB flush is done in interrupt context, proper coordination between interrupt and process context will require some atomic instruction which will defect the purpose. Cheers, Longman