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 3C733C433E1 for ; Wed, 20 May 2020 08:04:07 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 16D9520708 for ; Wed, 20 May 2020 08:04:07 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726486AbgETIED (ORCPT ); Wed, 20 May 2020 04:04:03 -0400 Received: from verein.lst.de ([213.95.11.211]:48303 "EHLO verein.lst.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726224AbgETIED (ORCPT ); Wed, 20 May 2020 04:04:03 -0400 Received: by verein.lst.de (Postfix, from userid 2407) id AE88368C65; Wed, 20 May 2020 10:03:58 +0200 (CEST) Date: Wed, 20 May 2020 10:03:57 +0200 From: Christoph Hellwig To: Jens Axboe , Ming Lei Cc: Christoph Hellwig , linux-kernel@vger.kernel.org, Thomas Gleixner , linux-block@vger.kernel.org, John Garry , Bart Van Assche , Hannes Reinecke , io-uring@vger.kernel.org Subject: 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: <20200520080357.GA4197@lst.de> References: <20200518093155.GB35380@T590> <87imgty15d.fsf@nanos.tec.linutronix.de> <20200518115454.GA46364@T590> <20200518131634.GA645@lst.de> <20200518141107.GA50374@T590> <20200518165619.GA17465@lst.de> <20200519015420.GA70957@T590> <20200519153000.GB22286@lst.de> <20200520011823.GA415158@T590> <20200520030424.GI416136@T590> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200520030424.GI416136@T590> User-Agent: Mutt/1.5.17 (2007-11-01) Sender: io-uring-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org On Wed, May 20, 2020 at 11:04:24AM +0800, Ming Lei wrote: > On Wed, May 20, 2020 at 09:18:23AM +0800, Ming Lei wrote: > > On Tue, May 19, 2020 at 05:30:00PM +0200, Christoph Hellwig wrote: > > > On Tue, May 19, 2020 at 09:54:20AM +0800, Ming Lei wrote: > > > > As Thomas clarified, workqueue hasn't such issue any more, and only other > > > > per CPU kthreads can run until the CPU clears the online bit. > > > > > > > > So the question is if IO can be submitted from such kernel context? > > > > > > What other per-CPU kthreads even exist? > > > > I don't know, so expose to wider audiences. > > One user is io uring with IORING_SETUP_SQPOLL & IORING_SETUP_SQ_AFF, see > io_sq_offload_start(), and it is a IO submission kthread. As far as I can tell that code is buggy, as it still needs to migrate the thread away when the cpu is offlined. This isn't a per-cpu kthread in the sene of having one for each CPU. Jens?