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=-7.0 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH, MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS autolearn=unavailable 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 BF743C2BB54 for ; Wed, 8 Apr 2020 02:09:59 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 8A2B420748 for ; Wed, 8 Apr 2020 02:09:59 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="UyB/0s7C" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726437AbgDHCJ6 (ORCPT ); Tue, 7 Apr 2020 22:09:58 -0400 Received: from us-smtp-2.mimecast.com ([207.211.31.81]:43193 "EHLO us-smtp-delivery-1.mimecast.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726428AbgDHCJ6 (ORCPT ); Tue, 7 Apr 2020 22:09:58 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1586311796; 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=6ychpKtsLvzMcxDHmIed9+i4bZtaWbCb6SkiRe01J5s=; b=UyB/0s7CY6jJrU2FzQQCtw9VzzeKdc2s++uUKY9DcV6pw0VCMCSOc8qJ84VoDGSFGlThzc WdtuBEARWdZ7R7++h718YmWgSu8nm7LWZ8cyCjXHTJOsNgJmlzc8HwYN0E/j9OIX9zZklT Frfao+xMtGz2Wy+S4+Dmm/GVTyvGJ8A= 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-197-rJgHgpGAPw2OXF3jAERDPA-1; Tue, 07 Apr 2020 22:09:52 -0400 X-MC-Unique: rJgHgpGAPw2OXF3jAERDPA-1 Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.phx2.redhat.com [10.5.11.16]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 5B3FC107ACC4; Wed, 8 Apr 2020 02:09:50 +0000 (UTC) Received: from localhost.localdomain (ovpn-8-28.pek2.redhat.com [10.72.8.28]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 49B365C1C5; Wed, 8 Apr 2020 02:09:40 +0000 (UTC) Date: Wed, 8 Apr 2020 10:09:36 +0800 From: Ming Lei To: Douglas Anderson Cc: axboe@kernel.dk, jejb@linux.ibm.com, martin.petersen@oracle.com, linux-scsi@vger.kernel.org, sqazi@google.com, Gwendal Grignou , groeck@chromium.org, linux-block@vger.kernel.org, paolo.valente@linaro.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v3 3/4] blk-mq: Rerun dispatching in the case of budget contention Message-ID: <20200408020936.GB337494@localhost.localdomain> References: <20200407220005.119540-1-dianders@chromium.org> <20200407145906.v3.3.I28278ef8ea27afc0ec7e597752a6d4e58c16176f@changeid> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200407145906.v3.3.I28278ef8ea27afc0ec7e597752a6d4e58c16176f@changeid> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Apr 07, 2020 at 03:00:04PM -0700, Douglas Anderson wrote: > If ever a thread running blk-mq code tries to get budget and fails it > immediately stops doing work and assumes that whenever budget is freed > up that queues will be kicked and whatever work the thread was trying > to do will be tried again. > > One path where budget is freed and queues are kicked in the normal > case can be seen in scsi_finish_command(). Specifically: > - scsi_finish_command() > - scsi_device_unbusy() > - # Decrement "device_busy", AKA release budget > - scsi_io_completion() > - scsi_end_request() > - blk_mq_run_hw_queues() > > The above is all well and good. The problem comes up when a thread > claims the budget but then releases it without actually dispatching > any work. Since we didn't schedule any work we'll never run the path > of finishing work / kicking the queues. > > This isn't often actually a problem which is why this issue has > existed for a while and nobody noticed. Specifically we only get into > this situation when we unexpectedly found that we weren't going to do > any work. Code that later receives new work kicks the queues. All > good, right? > > The problem shows up, however, if timing is just wrong and we hit a > race. To see this race let's think about the case where we only have > a budget of 1 (only one thread can hold budget). Now imagine that a > thread got budget and then decided not to dispatch work. It's about > to call put_budget() but then the thread gets context switched out for > a long, long time. While in this state, any and all kicks of the > queue (like the when we received new work) will be no-ops because > nobody can get budget. Finally the thread holding budget gets to run > again and returns. All the normal kicks will have been no-ops and we > have an I/O stall. > > As you can see from the above, you need just the right timing to see > the race. To start with, the only case it happens if we thought we > had work, actually managed to get the budget, but then actually didn't > have work. That's pretty rare to start with. Even then, there's > usually a very small amount of time between realizing that there's no > work and putting the budget. During this small amount of time new > work has to come in and the queue kick has to make it all the way to > trying to get the budget and fail. It's pretty unlikely. > > One case where this could have failed is illustrated by an example of > threads running blk_mq_do_dispatch_sched(): > > * Threads A and B both run has_work() at the same time with the same > "hctx". Imagine has_work() is exact. There's no lock, so it's OK > if Thread A and B both get back true. > * Thread B gets interrupted for a long time right after it decides > that there is work. Maybe its CPU gets an interrupt and the > interrupt handler is slow. > * Thread A runs, get budget, dispatches work. > * Thread A's work finishes and budget is released. > * Thread B finally runs again and gets budget. > * Since Thread A already took care of the work and no new work has > come in, Thread B will get NULL from dispatch_request(). I believe > this is specifically why dispatch_request() is allowed to return > NULL in the first place if has_work() must be exact. > * Thread B will now be holding the budget and is about to call > put_budget(), but hasn't called it yet. > * Thread B gets interrupted for a long time (again). Dang interrupts. > * Now Thread C (maybe with a different "hctx" but the same queue) > comes along and runs blk_mq_do_dispatch_sched(). > * Thread C won't do anything because it can't get budget. Thread C will re-run queue in this case: Just thought scsi_mq_get_budget() does handle the case via re-run queue: if (atomic_read(&sdev->device_busy) == 0 && !scsi_device_blocked(sdev)) blk_mq_delay_run_hw_queue(hctx, SCSI_QUEUE_DELAY); So looks no such race. > * Finally Thread B will run again and put the budget without kicking > any queues. > > Even though the example above is with blk_mq_do_dispatch_sched() I > believe the race is possible any time someone is holding budget but > doesn't do work. > > Unfortunately, the unlikely has become more likely if you happen to be > using the BFQ I/O scheduler. BFQ, by design, sometimes returns "true" > for has_work() but then NULL for dispatch_request() and stays in this > state for a while (currently up to 9 ms). Suddenly you only need one > race to hit, not two races in a row. With my current setup this is > easy to reproduce in reboot tests and traces have actually shown that > we hit a race similar to the one describe above. > > In theory we could choose to just fix blk_mq_do_dispatch_sched() to > kick the queues when it puts budget. That would fix the BFQ case and > one could argue that all the other cases are just theoretical. While > that is true, for all the other cases it should be very uncommon to > run into the case where we need put_budget(). Having an extra queue > kick for safety there shouldn't affect much and keeps the race at bay. > > One last note is that (at least in the SCSI case) budget is shared by > all "hctx"s that have the same queue. Thus we need to make sure to > kick the whole queue, not just re-run dispatching on a single "hctx". > > Signed-off-by: Douglas Anderson > --- > > Changes in v3: > - Always kick when putting the budget. > - Delay blk_mq_do_dispatch_sched() kick by 3 ms for inexact has_work(). > - Totally rewrote commit message. > > Changes in v2: > - Replace ("scsi: core: Fix stall...") w/ ("blk-mq: Rerun dispatch...") > > block/blk-mq.h | 14 +++++++++++++- > 1 file changed, 13 insertions(+), 1 deletion(-) > > diff --git a/block/blk-mq.h b/block/blk-mq.h > index 10bfdfb494fa..1270505367ab 100644 > --- a/block/blk-mq.h > +++ b/block/blk-mq.h > @@ -180,12 +180,24 @@ unsigned int blk_mq_in_flight(struct request_queue *q, struct hd_struct *part); > void blk_mq_in_flight_rw(struct request_queue *q, struct hd_struct *part, > unsigned int inflight[2]); > > +#define BLK_MQ_BUDGET_DELAY 3 /* ms units */ > + > static inline void blk_mq_put_dispatch_budget(struct blk_mq_hw_ctx *hctx) > { > struct request_queue *q = hctx->queue; > > - if (q->mq_ops->put_budget) > + if (q->mq_ops->put_budget) { > q->mq_ops->put_budget(hctx); > + > + /* > + * The only time we call blk_mq_put_dispatch_budget() is if > + * we released the budget without dispatching. Holding the > + * budget could have blocked any "hctx"s with the same queue > + * and if we didn't dispatch then there's no guarantee anyone > + * will kick the queue. Kick it ourselves. > + */ > + blk_mq_delay_run_hw_queues(q, BLK_MQ_BUDGET_DELAY); No, please don't do that un-conditionally we just need to re-run queue when there has work to do. Thanks, Ming