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=-8.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 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 223AAC0650F for ; Tue, 30 Jul 2019 13:59:56 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 0098F206E0 for ; Tue, 30 Jul 2019 13:59:55 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729110AbfG3N7y (ORCPT ); Tue, 30 Jul 2019 09:59:54 -0400 Received: from mx1.redhat.com ([209.132.183.28]:38748 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726351AbfG3N7y (ORCPT ); Tue, 30 Jul 2019 09:59:54 -0400 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 0BE5D81F0E; Tue, 30 Jul 2019 13:59:54 +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 208BA60856; Tue, 30 Jul 2019 13:59:53 +0000 (UTC) Subject: Re: [PATCH v3] sched/core: Don't use dying mm as active_mm of kthreads To: Peter Zijlstra Cc: Ingo Molnar , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Andrew Morton , Phil Auld , Michal Hocko , Rik van Riel References: <20190729210728.21634-1-longman@redhat.com> <20190730084321.GL31381@hirez.programming.kicks-ass.net> From: Waiman Long Organization: Red Hat Message-ID: <396ac6c6-6c99-3cb8-6ff7-106c82df29ab@redhat.com> Date: Tue, 30 Jul 2019 09:59:52 -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: <20190730084321.GL31381@hirez.programming.kicks-ass.net> 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 [10.5.110.27]); Tue, 30 Jul 2019 13:59:54 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 7/30/19 4:43 AM, Peter Zijlstra wrote: > On Mon, Jul 29, 2019 at 05:07:28PM -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. >> >> Fix that by forcing the kernel threads to use init_mm as the active_mm >> during a kernel thread to kernel thread transition if the previous >> active_mm is dying (!mm_users). This will allows the freeing of resources >> associated with the dying mm ASAP. >> >> The presence of a kernel-to-kernel thread transition indicates that >> the cpu is probably idling with no higher priority user task to run. >> So the overhead of loading the mm_users cacheline should not really >> matter in this case. >> >> My testing on an x86 system showed that the mm_struct was freed within >> seconds after the task exited instead of staying alive for minutes or >> even longer on a mostly idle system before this patch. >> >> Signed-off-by: Waiman Long >> --- >> kernel/sched/core.c | 21 +++++++++++++++++++-- >> 1 file changed, 19 insertions(+), 2 deletions(-) >> >> diff --git a/kernel/sched/core.c b/kernel/sched/core.c >> index 795077af4f1a..41997e676251 100644 >> --- a/kernel/sched/core.c >> +++ b/kernel/sched/core.c >> @@ -3214,6 +3214,8 @@ static __always_inline struct rq * >> context_switch(struct rq *rq, struct task_struct *prev, >> struct task_struct *next, struct rq_flags *rf) >> { >> + struct mm_struct *next_mm = next->mm; >> + >> prepare_task_switch(rq, prev, next); >> >> /* >> @@ -3229,8 +3231,22 @@ context_switch(struct rq *rq, struct task_struct *prev, >> * >> * kernel -> user switch + mmdrop() active >> * user -> user switch >> + * >> + * kernel -> kernel and !prev->active_mm->mm_users: >> + * switch to init_mm + mmgrab() + mmdrop() >> */ >> - if (!next->mm) { // to kernel >> + if (!next_mm) { // to kernel >> + /* >> + * Checking is only done on kernel -> kernel transition >> + * to avoid any performance overhead while user tasks >> + * are running. >> + */ >> + if (unlikely(!prev->mm && >> + !atomic_read(&prev->active_mm->mm_users))) { >> + next_mm = next->active_mm = &init_mm; >> + mmgrab(next_mm); >> + goto mm_switch; >> + } >> enter_lazy_tlb(prev->active_mm, next); >> >> next->active_mm = prev->active_mm; > So I _really_ hate this complication. I'm thinking if you really care > about this the time is much better spend getting rid of the active_mm > tracking for x86 entirely. > That is fine. I won't pursue further. I will take a look at your suggestion when I have time, but it will probably be a while :-) Cheers, Longman