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[209.132.183.28]) by mx.google.com with ESMTPS id a11si134798uah.111.2019.07.31.08.49.22 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 31 Jul 2019 08:49:23 -0700 (PDT) Received-SPF: pass (google.com: domain of longman@redhat.com designates 209.132.183.28 as permitted sender) client-ip=209.132.183.28; Authentication-Results: mx.google.com; spf=pass (google.com: domain of longman@redhat.com designates 209.132.183.28 as permitted sender) smtp.mailfrom=longman@redhat.com; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=redhat.com Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.phx2.redhat.com [10.5.11.13]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 5434130C26DA; Wed, 31 Jul 2019 15:49:22 +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 68AA160852; Wed, 31 Jul 2019 15:49:20 +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> <01125822-c883-18ce-42e4-942a4f28c128@redhat.com> <76dbc397e21d64da75cd07d90b3ca15ca50d6fbb.camel@surriel.com> From: Waiman Long Organization: Red Hat Message-ID: <3307e8f7-4a68-95fc-a5dd-925fd3a5f8d7@redhat.com> Date: Wed, 31 Jul 2019 11:49:19 -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: <76dbc397e21d64da75cd07d90b3ca15ca50d6fbb.camel@surriel.com> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.47]); Wed, 31 Jul 2019 15:49:22 +0000 (UTC) X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 7/31/19 11:07 AM, Rik van Riel wrote: > On Wed, 2019-07-31 at 10:15 -0400, Waiman Long wrote: >> 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. > Would it be possible to work around that by scheduling > a work item that drops the active_mm? > > After all, a work item runs in a kernel thread, so by > the time the work item is run, either the kernel will > still be running the mm you want to get rid of as > active_mm, or it will have already gotten rid of it > earlier. Yes, that may work. Thanks, Longman