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=-0.9 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED 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 50F86C433E0 for ; Thu, 21 May 2020 02:28:07 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 3049A20748 for ; Thu, 21 May 2020 02:28:07 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="hwpQzUaz" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726943AbgEUC2G (ORCPT ); Wed, 20 May 2020 22:28:06 -0400 Received: from us-smtp-delivery-1.mimecast.com ([205.139.110.120]:20390 "EHLO us-smtp-1.mimecast.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726833AbgEUC2G (ORCPT ); Wed, 20 May 2020 22:28:06 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1590028084; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=HiJAaDRSg5DnM9NEbapqZa54OkdMt62Nfi59/wxsSTk=; b=hwpQzUazhe24rfDk9LqbVJ1q5DQqcyKOgp080qZhlLv4wNSagQG7go5xh8ipDiqqhoUSqy 4tf7LiuaRcCJ+qLuQQtyECtQ4CM6hZd6UGu6JwSSm7EqFeaAXVWT0nN4nrfgy//y50YV5c fvTtwg+bSyx4nmiwMipmjpi/uZRxj+o= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-114-FDsArPCQMPSgD6E4nTNMkg-1; Wed, 20 May 2020 22:28:00 -0400 X-MC-Unique: FDsArPCQMPSgD6E4nTNMkg-1 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 mimecast-mx01.redhat.com (Postfix) with ESMTPS id 3D12F800688; Thu, 21 May 2020 02:27:59 +0000 (UTC) Received: from T590 (ovpn-13-123.pek2.redhat.com [10.72.13.123]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 5350382A24; Thu, 21 May 2020 02:27:50 +0000 (UTC) Date: Thu, 21 May 2020 10:27:46 +0800 From: Ming Lei To: Thomas Gleixner Cc: Jens Axboe , Christoph Hellwig , linux-kernel@vger.kernel.org, linux-block@vger.kernel.org, John Garry , Bart Van Assche , Hannes Reinecke , io-uring@vger.kernel.org, Peter Zijlstra Subject: Re: io_uring vs CPU hotplug, was Re: [PATCH 5/9] blk-mq: don't set data->ctx and data->hctx in blk_mq_alloc_request_hctx Message-ID: <20200521022746.GA730422@T590> References: <20200519015420.GA70957@T590> <20200519153000.GB22286@lst.de> <20200520011823.GA415158@T590> <20200520030424.GI416136@T590> <20200520080357.GA4197@lst.de> <8f893bb8-66a9-d311-ebd8-d5ccd8302a0d@kernel.dk> <448d3660-0d83-889b-001f-a09ea53fa117@kernel.dk> <87tv0av1gu.fsf@nanos.tec.linutronix.de> <2a12a7aa-c339-1e51-de0d-9bc6ced14c64@kernel.dk> <87eereuudh.fsf@nanos.tec.linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87eereuudh.fsf@nanos.tec.linutronix.de> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 Sender: io-uring-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org On Thu, May 21, 2020 at 12:14:18AM +0200, Thomas Gleixner wrote: > Jens Axboe writes: > > > On 5/20/20 1:41 PM, Thomas Gleixner wrote: > >> Jens Axboe writes: > >>> On 5/20/20 8:45 AM, Jens Axboe wrote: > >>>> It just uses kthread_create_on_cpu(), nothing home grown. Pretty sure > >>>> they just break affinity if that CPU goes offline. > >>> > >>> Just checked, and it works fine for me. If I create an SQPOLL ring with > >>> SQ_AFF set and bound to CPU 3, if CPU 3 goes offline, then the kthread > >>> just appears unbound but runs just fine. When CPU 3 comes online again, > >>> the mask appears correct. > >> > >> When exactly during the unplug operation is it unbound? > > > > When the CPU has been fully offlined. I check the affinity mask, it > > reports 0. But it's still being scheduled, and it's processing work. > > Here's an example, PID 420 is the thread in question: > > > > [root@archlinux cpu3]# taskset -p 420 > > pid 420's current affinity mask: 8 > > [root@archlinux cpu3]# echo 0 > online > > [root@archlinux cpu3]# taskset -p 420 > > pid 420's current affinity mask: 0 > > [root@archlinux cpu3]# echo 1 > online > > [root@archlinux cpu3]# taskset -p 420 > > pid 420's current affinity mask: 8 > > > > So as far as I can tell, it's working fine for me with the goals > > I have for that kthread. > > Works for me is not really useful information and does not answer my > question: > > >> When exactly during the unplug operation is it unbound? > > The problem Ming and Christoph are trying to solve requires that the > thread is migrated _before_ the hardware queue is shut down and > drained. That's why I asked for the exact point where this happens. > > When the CPU is finally offlined, i.e. the CPU cleared the online bit in > the online mask is definitely too late simply because it still runs on > that outgoing CPU _after_ the hardware queue is shut down and drained. IMO, the patch in Christoph's blk-mq-hotplug.2 still works for percpu kthread. It is just not optimal in the retrying, but it should be fine. When the percpu kthread is scheduled on the CPU to be offlined: - if the kthread doesn't observe the INACTIVE flag, the allocated request will be drained. - otherwise, the kthread just retries and retries to allocate & release, and sooner or later, its time slice is consumed, and migrated out, and the cpu hotplug handler will get chance to run and move on, then the cpu is shutdown. - After the cpu is shutdown, the percpu kthread becomes unbound, and the allocation from new online cpu will succeed. Thanks, Ming