From patchwork Tue Apr 7 22:00:04 2020 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Doug Anderson X-Patchwork-Id: 11479021 Return-Path: Received: from mail.kernel.org (pdx-korg-mail-1.web.codeaurora.org [172.30.200.123]) by pdx-korg-patchwork-2.web.codeaurora.org (Postfix) with ESMTP id 137D5913 for ; Tue, 7 Apr 2020 22:00:50 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id DB39020747 for ; Tue, 7 Apr 2020 22:00:49 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=chromium.org header.i=@chromium.org header.b="DoMYYcsn" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726598AbgDGWAs (ORCPT ); Tue, 7 Apr 2020 18:00:48 -0400 Received: from mail-pg1-f195.google.com ([209.85.215.195]:37227 "EHLO mail-pg1-f195.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726550AbgDGWAo (ORCPT ); Tue, 7 Apr 2020 18:00:44 -0400 Received: by mail-pg1-f195.google.com with SMTP id r4so2371164pgg.4 for ; Tue, 07 Apr 2020 15:00:42 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=chromium.org; s=google; h=from:to:cc:subject:date:message-id:in-reply-to:references :mime-version:content-transfer-encoding; bh=Me9YjeaVzWgazxTUthRuKJI46sDGdqwI3du7HWGuOE8=; b=DoMYYcsntVvHTK2oXwnp3vDDbf3nT3otp0Pm+CFLtccf3kJadBX/hZRhcI+IUM7OHu 9GtnDCkyi0neXGAopex9ODL02nb2SkLNI9F9jFhOpGDWQH8JSCPaQyp2PFwBCyKzwP31 RESBufGdSgg25ZKaFUfV1vNerMXA6d58coYtU= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:from:to:cc:subject:date:message-id:in-reply-to :references:mime-version:content-transfer-encoding; bh=Me9YjeaVzWgazxTUthRuKJI46sDGdqwI3du7HWGuOE8=; b=gkXkrcqiGiChkQcsU1xtX4TRTHLe6KLlTsPTUD+C+29nbZSfkp2IYsh7yAzH3FAD0m LJ6kJEnNRMGGgKZNO0lW5Zd3YMPBjkT5jqmswBFWXxKBgX5ovDY2aObowN/lTpO16GQl mIcCeH7hioaNAWA+9PBdCKoyU4FFJemb+0xL7qX5dfSSFheqjunNGzCLTYttcsrenDMz gJ7BrJCBaTLkEWbzRNVz/Xx+McaO+ayVydNYm9k6aGE8zOE2LhkdXYJHoJbbWKwpN2ty yw033E52Fk7G5WbnEZCBlJEA4E+MDZNCt6/E+V6y1yA0woEAWvHtG6tEg/Q7sKuTEG2R w+yw== X-Gm-Message-State: AGi0PuZ5LO56BUP/mRRPGiAt7HltKVkp8xMSITrpazyKXQdZW+h+gD1h DuesJx+suvdegprEv1qmhJi3zA== X-Google-Smtp-Source: APiQypIc2emGqDVPItIPl9ujfmle5KLS4GyLNUOKdjuBUMmxWIOMYhRxDK6Fup63PD2P+ZhrpVonLQ== X-Received: by 2002:a63:c06:: with SMTP id b6mr4046203pgl.218.1586296841645; Tue, 07 Apr 2020 15:00:41 -0700 (PDT) Received: from tictac2.mtv.corp.google.com ([2620:15c:202:1:24fa:e766:52c9:e3b2]) by smtp.gmail.com with ESMTPSA id g3sm880112pgd.64.2020.04.07.15.00.40 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 07 Apr 2020 15:00:41 -0700 (PDT) From: Douglas Anderson To: axboe@kernel.dk, jejb@linux.ibm.com, martin.petersen@oracle.com Cc: Ming Lei , linux-scsi@vger.kernel.org, sqazi@google.com, Gwendal Grignou , groeck@chromium.org, linux-block@vger.kernel.org, paolo.valente@linaro.org, Douglas Anderson , linux-kernel@vger.kernel.org Subject: [PATCH v3 3/4] blk-mq: Rerun dispatching in the case of budget contention Date: Tue, 7 Apr 2020 15:00:04 -0700 Message-Id: <20200407145906.v3.3.I28278ef8ea27afc0ec7e597752a6d4e58c16176f@changeid> X-Mailer: git-send-email 2.26.0.292.g33ef6b2f38-goog In-Reply-To: <20200407220005.119540-1-dianders@chromium.org> References: <20200407220005.119540-1-dianders@chromium.org> MIME-Version: 1.0 Sender: linux-block-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org 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. * 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); + } } static inline bool blk_mq_get_dispatch_budget(struct blk_mq_hw_ctx *hctx)