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 97A92C433E0 for ; Thu, 21 May 2020 18:39:37 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 7677A20738 for ; Thu, 21 May 2020 18:39:37 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729294AbgEUSjg (ORCPT ); Thu, 21 May 2020 14:39:36 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:37768 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728240AbgEUSjg (ORCPT ); Thu, 21 May 2020 14:39:36 -0400 Received: from Galois.linutronix.de (Galois.linutronix.de [IPv6:2a0a:51c0:0:12e:550::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 3DDB0C061A0E; Thu, 21 May 2020 11:39:36 -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 1jbq6A-0006og-PX; Thu, 21 May 2020 20:39:19 +0200 Received: by nanos.tec.linutronix.de (Postfix, from userid 1000) id E386A100C2D; Thu, 21 May 2020 20:39:16 +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: <20200521092340.GA751297@T590> References: <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> <87367tvh6g.fsf@nanos.tec.linutronix.de> <20200521092340.GA751297@T590> Date: Thu, 21 May 2020 20:39:16 +0200 Message-ID: <87pnaxt9nv.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, Ming Lei writes: > On Thu, May 21, 2020 at 10:13:59AM +0200, Thomas Gleixner wrote: >> Ming Lei writes: >> > On Thu, May 21, 2020 at 12:14:18AM +0200, Thomas Gleixner wrote: >> > - 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? > > Given it is unlikely path, we can add msleep() before retrying when INACTIVE bit > is observed by current thread, and this way can avoid spinning and should work > for other schedulers. That should work, but pretty is something else >> >> 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? > > drain_io() releases all tag of this hctx, then wait_on_ctx() will be waken up > after any tag is released. drain_io() is already done ... So looking at that thread function: static int io_sq_thread(void *data) { struct io_ring_ctx *ctx = data; while (...) { .... to_submit = io_sqring_entries(ctx); --> preemption hotplug runs mq_offline() set_ctx_inactive(); drain_io(); finished(); --> thread runs again mutex_lock(&ctx->uring_lock); ret = io_submit_sqes(ctx, to_submit, NULL, -1, true); mutex_unlock(&ctx->uring_lock); .... if (!to_submit || ret == -EBUSY) ... wait_on_ctx(); Can this happen or did drain_io() already take care of the 'to_submit' items and the call to io_submit_sqes() turns into a zero action ? If the above happens then nothing will wake it up because the context draining is done and finished. Thanks, tglx