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=-2.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,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 A7475C83000 for ; Wed, 29 Apr 2020 17:34:07 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 7BC00208FE for ; Wed, 29 Apr 2020 17:34:07 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1588181647; bh=mgzB4f+p6rmmfScN/90JYMP823FyQiRJhWdBt5NpAyk=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=X/wQnM+P1ZV7Q4DTHx0X5YhHDHErDkiZSjtz5XNbQhSWK8rYR9b3rr7Sh2g+Jnea6 F6xnQcOn11PjKDw5gO40aKpE+c9ksseIcrKdBuIcRjKgfHd4Ye5HVijzaiOq2IBSNm 45BC0/NAcmD8rBn/ZR5zc93wYniyOnrP98e1//k8= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726511AbgD2ReG (ORCPT ); Wed, 29 Apr 2020 13:34:06 -0400 Received: from mail.kernel.org ([198.145.29.99]:47996 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726456AbgD2ReG (ORCPT ); Wed, 29 Apr 2020 13:34:06 -0400 Received: from willie-the-truck (236.31.169.217.in-addr.arpa [217.169.31.236]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 6B55720757; Wed, 29 Apr 2020 17:34:04 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1588181646; bh=mgzB4f+p6rmmfScN/90JYMP823FyQiRJhWdBt5NpAyk=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=cvIfiFq8gK/KE/rDMQfrmjdqPmn4ieLPAX6uNlveg+jddJfzpAn49gY4b6o6FTcNX lPkj+bGK6AE/AyzgBHUy1wNjhoa7W0Ec+lK3uBi7PDpX5G8EVzpo4sRZyZpZiPTVNC je/qS19NzHo3Rwwuje74XUD+Mpw7maKhstWzgj9M= Date: Wed, 29 Apr 2020 18:34:01 +0100 From: Will Deacon To: Ming Lei Cc: Peter Zijlstra , Christoph Hellwig , Jens Axboe , linux-block@vger.kernel.org, John Garry , Bart Van Assche , Hannes Reinecke , Thomas Gleixner , paulmck@kernel.org Subject: Re: [PATCH V8 07/11] blk-mq: stop to handle IO and drain IO before hctx becomes inactive Message-ID: <20200429173400.GC30247@willie-the-truck> References: <20200425083224.GA5634@lst.de> <20200425093437.GA495669@T590> <20200425095351.GC495669@T590> <20200425154832.GA16004@lst.de> <20200428155837.GA16910@hirez.programming.kicks-ass.net> <20200429021612.GD671522@T590> <20200429080728.GB29143@willie-the-truck> <20200429094616.GB700644@T590> <20200429122757.GA30247@willie-the-truck> <20200429134327.GC700644@T590> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200429134327.GC700644@T590> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-block-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On Wed, Apr 29, 2020 at 09:43:27PM +0800, Ming Lei wrote: > On Wed, Apr 29, 2020 at 01:27:57PM +0100, Will Deacon wrote: > > On Wed, Apr 29, 2020 at 05:46:16PM +0800, Ming Lei wrote: > > > On Wed, Apr 29, 2020 at 09:07:29AM +0100, Will Deacon wrote: > > > > On Wed, Apr 29, 2020 at 10:16:12AM +0800, Ming Lei wrote: > > > > > On Tue, Apr 28, 2020 at 05:58:37PM +0200, Peter Zijlstra wrote: > > > > > > On Sat, Apr 25, 2020 at 05:48:32PM +0200, Christoph Hellwig wrote: > > > > > > > atomic_inc(&data.hctx->nr_active); > > > > > > > } > > > > > > > data.hctx->tags->rqs[rq->tag] = rq; > > > > > > > > > > > > > > /* > > > > > > > + * Ensure updates to rq->tag and tags->rqs[] are seen by > > > > > > > + * blk_mq_tags_inflight_rqs. This pairs with the smp_mb__after_atomic > > > > > > > + * in blk_mq_hctx_notify_offline. This only matters in case a process > > > > > > > + * gets migrated to another CPU that is not mapped to this hctx. > > > > > > > */ > > > > > > > + if (rq->mq_ctx->cpu != get_cpu()) > > > > > > > smp_mb(); > > > > > > > + put_cpu(); > > > > > > > > > > > > This looks exceedingly weird; how do you think you can get to another > > > > > > CPU and not have an smp_mb() implied in the migration itself? Also, what > > > > > > > > > > What we need is one smp_mb() between the following two OPs: > > > > > > > > > > 1) > > > > > rq->tag = rq->internal_tag; > > > > > data.hctx->tags->rqs[rq->tag] = rq; > > > > > > > > > > 2) > > > > > if (unlikely(test_bit(BLK_MQ_S_INACTIVE, &rq->mq_hctx->state))) > > > > > > > > > > And the pair of the above barrier is in blk_mq_hctx_notify_offline(). > > > > > > > > I'm struggling with this, so let me explain why. My understanding of the > > > > original patch [1] and your explanation above is that you want *either* of > > > > the following behaviours > > > > > > > > - __blk_mq_get_driver_tag() (i.e. (1) above) and test_bit(BLK_MQ_S_INACTIVE, ...) > > > > run on the same CPU with barrier() between them, or > > > > > > > > - There is a migration and therefore an implied smp_mb() between them > > > > > > > > However, given that most CPUs can speculate loads (and therefore the > > > > test_bit() operation), I don't understand how the "everything runs on the > > > > same CPU" is safe if a barrier() is required. In other words, if the > > > > barrier() is needed to prevent the compiler hoisting the load, then the CPU > > > > can still cause problems. > > > > > > Do you think the speculate loads may return wrong value of > > > BLK_MQ_S_INACTIVE bit in case of single CPU? BTW, writing the bit is > > > done on the same CPU. If yes, this machine may not obey cache consistency, > > > IMO. > > > > If the write is on the same CPU, then the read will of course return the > > value written by that write, otherwise we'd have much bigger problems! > > OK, then it is nothing to with speculate loads. > > > > > But then I'm confused, because you're saying that the write is done on the > > same CPU, but previously you were saying that migration occuring before (1) > > was problematic. Can you explain a bit more about that case, please? What > > is running before (1) that is relevant here? > > Please see the following two code paths: > > [1] code path1: > blk_mq_hctx_notify_offline(): > set_bit(BLK_MQ_S_INACTIVE, &hctx->state); > > smp_mb() or smp_mb_after_atomic() > > blk_mq_hctx_drain_inflight_rqs(): > blk_mq_tags_inflight_rqs() > rq = hctx->tags->rqs[index] > and > READ rq->tag > > [2] code path2: > blk_mq_get_driver_tag(): > > process might be migrated to other CPU here and chance is small, > then the follow code will be run on CPU different with code path1 > > rq->tag = rq->internal_tag; > hctx->tags->rqs[rq->tag] = rq; I /think/ this can be distilled to the SB litmus test: // blk_mq_hctx_notify_offline() blk_mq_get_driver_tag(); Wstate = INACTIVE Wtag smp_mb() smp_mb() Rtag Rstate and you want to make sure that either blk_mq_get_driver_tag() sees the state as INACTIVE and does the cleanup, or it doesn't and blk_mq_hctx_notify_offline() sees the newly written tag and waits for the request to complete (I don't get how that happens, but hey). Is that right? > barrier() in case that code path2 is run on same CPU with code path1 > OR > smp_mb() in case that code path2 is run on different CPU with code path1 because > of process migration > > test_bit(BLK_MQ_S_INACTIVE, &data.hctx->state) Couldn't you just check this at the start of blk_mq_get_driver_tag() as well, and then make the smp_mb() unconditional? Will