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 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 D91BDC433E0 for ; Fri, 22 May 2020 01:58:02 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id A8A61207F7 for ; Fri, 22 May 2020 01:58:02 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="Jq1hVcNF" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726335AbgEVB5k (ORCPT ); Thu, 21 May 2020 21:57:40 -0400 Received: from us-smtp-2.mimecast.com ([207.211.31.81]:29825 "EHLO us-smtp-delivery-1.mimecast.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726860AbgEVB5j (ORCPT ); Thu, 21 May 2020 21:57:39 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1590112658; 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=+wDdNyvkCFMdN0Oe8Nl6A+nCz6R/E6o7nQrNSLAaN6Y=; b=Jq1hVcNFqC/jps1reMom+hgCSSEMytcEfBMEx8YjL45JwhfCm9oel2bSxYRVW5MteL4/Xv jY8n5SpRfXcH9FR1DYbrZEsQ/U5Fd8cjNyOwPF+4o4B6brr5y0mRNyqgUHq5fA7DL9qh/u 39I96B6dgqwlIcKVY8MfbgQZYE8MhR8= 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-389-93omiG20O--1Yrc9dX2vLA-1; Thu, 21 May 2020 21:57:34 -0400 X-MC-Unique: 93omiG20O--1Yrc9dX2vLA-1 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id A36C41005510; Fri, 22 May 2020 01:57:32 +0000 (UTC) Received: from T590 (ovpn-13-78.pek2.redhat.com [10.72.13.78]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 0BBDD795A9; Fri, 22 May 2020 01:57:23 +0000 (UTC) Date: Fri, 22 May 2020 09:57:19 +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: <20200522015719.GB755458@T590> References: <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> <87pnaxt9nv.fsf@nanos.tec.linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87pnaxt9nv.fsf@nanos.tec.linutronix.de> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 Sender: linux-block-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On Thu, May 21, 2020 at 08:39:16PM +0200, Thomas Gleixner wrote: > 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. As Jens replied, you mixed the ctx from io uring and blk-mq, both are in two worlds. Any wait in this percpu kthread should just wait for generic resource, not directly related with blk-mq's inactive hctx. Once this thread is migrated to other online cpu, it will move on. Thanks, Ming