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=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 8482BC433DF for ; Thu, 21 May 2020 08:14:22 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 6242520759 for ; Thu, 21 May 2020 08:14:22 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728436AbgEUIOV (ORCPT ); Thu, 21 May 2020 04:14:21 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:52970 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727122AbgEUIOV (ORCPT ); Thu, 21 May 2020 04:14:21 -0400 Received: from Galois.linutronix.de (Galois.linutronix.de [IPv6:2a0a:51c0:0:12e:550::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 65600C061A0E; Thu, 21 May 2020 01:14:21 -0700 (PDT) Received: from p5de0bf0b.dip0.t-ipconnect.de ([93.224.191.11] helo=nanos.tec.linutronix.de) by Galois.linutronix.de with esmtpsa (TLS1.2:DHE_RSA_AES_256_CBC_SHA256:256) (Exim 4.80) (envelope-from ) id 1jbgL2-0004jF-Gl; Thu, 21 May 2020 10:14:00 +0200 Received: by nanos.tec.linutronix.de (Postfix, from userid 1000) id B6DB5100C2D; Thu, 21 May 2020 10:13:59 +0200 (CEST) From: Thomas Gleixner To: Ming Lei 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 In-Reply-To: <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> <20200521022746.GA730422@T590> Date: Thu, 21 May 2020 10:13:59 +0200 Message-ID: <87367tvh6g.fsf@nanos.tec.linutronix.de> MIME-Version: 1.0 Content-Type: text/plain X-Linutronix-Spam-Score: -1.0 X-Linutronix-Spam-Level: - X-Linutronix-Spam-Status: No , -1.0 points, 5.0 required, ALL_TRUSTED=-1,SHORTCIRCUIT=-0.0001 Sender: linux-block-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org Ming Lei writes: > On Thu, May 21, 2020 at 12:14:18AM +0200, Thomas Gleixner wrote: >> 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. 1) This is based on the assumption that the kthread is in the SCHED_OTHER scheduling class. Is that really a valid assumption? 2) What happens in the following scenario: unplug mq_offline set_ctx_inactive() drain_io() io_kthread() try_queue() wait_on_ctx() Can this happen and if so what will wake up that thread? I'm not familiar enough with that code to answer #2, but this really wants to be properly described and documented. Thanks, tglx