From patchwork Mon Jan 24 18:32:52 2022 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Greg Kroah-Hartman X-Patchwork-Id: 535107 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 1A222C35273 for ; Mon, 24 Jan 2022 23:05:00 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1842128AbiAXXA7 (ORCPT ); Mon, 24 Jan 2022 18:00:59 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:41432 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1836493AbiAXWjh (ORCPT ); Mon, 24 Jan 2022 17:39:37 -0500 Received: from dfw.source.kernel.org (dfw.source.kernel.org [IPv6:2604:1380:4641:c500::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id A6000C054874; Mon, 24 Jan 2022 13:01:47 -0800 (PST) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by dfw.source.kernel.org (Postfix) with ESMTPS id 4366B6131F; Mon, 24 Jan 2022 21:01:47 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 183EDC340E5; Mon, 24 Jan 2022 21:01:45 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1643058106; bh=cf3LxnhY71jlrMoqRMBqvHmx+grq7ctd4LTG2H4U8cw=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=V+OUp9QdQ87TnIrHlJBTPwqtIyHFVjw6Rx+qRKBmbv0VwLewBsvhxm89REL+LP5gS f1630gGYwOjWaxRBsPPMoNNU+YxMHt5DKH3AHVqj2ls6wvzgGl6Jhu27XI5n2MXMRL EahyIY0jOhex93gDUUD75KDG+xH93MvOi4i8+sR8= From: Greg Kroah-Hartman To: linux-kernel@vger.kernel.org Cc: Greg Kroah-Hartman , stable@vger.kernel.org, Paolo Valente , Jan Kara , Jens Axboe , Sasha Levin Subject: [PATCH 5.16 0183/1039] bfq: Do not let waker requests skip proper accounting Date: Mon, 24 Jan 2022 19:32:52 +0100 Message-Id: <20220124184131.428737070@linuxfoundation.org> X-Mailer: git-send-email 2.34.1 In-Reply-To: <20220124184125.121143506@linuxfoundation.org> References: <20220124184125.121143506@linuxfoundation.org> User-Agent: quilt/0.66 MIME-Version: 1.0 Precedence: bulk List-ID: X-Mailing-List: stable@vger.kernel.org From: Jan Kara [ Upstream commit c65e6fd460b4df796ecd6ea22e132076ed1f2820 ] Commit 7cc4ffc55564 ("block, bfq: put reqs of waker and woken in dispatch list") added a condition to bfq_insert_request() which added waker's requests directly to dispatch list. The rationale was that completing waker's IO is needed to get more IO for the current queue. Although this rationale is valid, there is a hole in it. The waker does not necessarily serve the IO only for the current queue and maybe it's current IO is not needed for current queue to make progress. Furthermore injecting IO like this completely bypasses any service accounting within bfq and thus we do not properly track how much service is waker's queue getting or that the waker is actually doing any IO. Depending on the conditions this can result in the waker getting too much or too few service. Consider for example the following job file: [global] directory=/mnt/repro/ rw=write size=8g time_based runtime=30 ramp_time=10 blocksize=1m direct=0 ioengine=sync [slowwriter] numjobs=1 prioclass=2 prio=7 fsync=200 [fastwriter] numjobs=1 prioclass=2 prio=0 fsync=200 Despite processes have very different IO priorities, they get the same about of service. The reason is that bfq identifies these processes as having waker-wakee relationship and once that happens, IO from fastwriter gets injected during slowwriter's time slice. As a result bfq is not aware that fastwriter has any IO to do and constantly schedules only slowwriter's queue. Thus fastwriter is forced to compete with slowwriter's IO all the time instead of getting its share of time based on IO priority. Drop the special injection condition from bfq_insert_request(). As a result, requests will be tracked and queued in a normal way and on next dispatch bfq_select_queue() can decide whether the waker's inserted requests should be injected during the current queue's timeslice or not. Fixes: 7cc4ffc55564 ("block, bfq: put reqs of waker and woken in dispatch list") Acked-by: Paolo Valente Signed-off-by: Jan Kara Link: https://lore.kernel.org/r/20211125133645.27483-8-jack@suse.cz Signed-off-by: Jens Axboe Signed-off-by: Sasha Levin --- block/bfq-iosched.c | 44 +------------------------------------------- 1 file changed, 1 insertion(+), 43 deletions(-) diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c index fec18118dc309..30918b0e81c02 100644 --- a/block/bfq-iosched.c +++ b/block/bfq-iosched.c @@ -5991,48 +5991,7 @@ static void bfq_insert_request(struct blk_mq_hw_ctx *hctx, struct request *rq, spin_lock_irq(&bfqd->lock); bfqq = bfq_init_rq(rq); - - /* - * Reqs with at_head or passthrough flags set are to be put - * directly into dispatch list. Additional case for putting rq - * directly into the dispatch queue: the only active - * bfq_queues are bfqq and either its waker bfq_queue or one - * of its woken bfq_queues. The rationale behind this - * additional condition is as follows: - * - consider a bfq_queue, say Q1, detected as a waker of - * another bfq_queue, say Q2 - * - by definition of a waker, Q1 blocks the I/O of Q2, i.e., - * some I/O of Q1 needs to be completed for new I/O of Q2 - * to arrive. A notable example of waker is journald - * - so, Q1 and Q2 are in any respect the queues of two - * cooperating processes (or of two cooperating sets of - * processes): the goal of Q1's I/O is doing what needs to - * be done so that new Q2's I/O can finally be - * issued. Therefore, if the service of Q1's I/O is delayed, - * then Q2's I/O is delayed too. Conversely, if Q2's I/O is - * delayed, the goal of Q1's I/O is hindered. - * - as a consequence, if some I/O of Q1/Q2 arrives while - * Q2/Q1 is the only queue in service, there is absolutely - * no point in delaying the service of such an I/O. The - * only possible result is a throughput loss - * - so, when the above condition holds, the best option is to - * have the new I/O dispatched as soon as possible - * - the most effective and efficient way to attain the above - * goal is to put the new I/O directly in the dispatch - * list - * - as an additional restriction, Q1 and Q2 must be the only - * busy queues for this commit to put the I/O of Q2/Q1 in - * the dispatch list. This is necessary, because, if also - * other queues are waiting for service, then putting new - * I/O directly in the dispatch list may evidently cause a - * violation of service guarantees for the other queues - */ - if (!bfqq || - (bfqq != bfqd->in_service_queue && - bfqd->in_service_queue != NULL && - bfq_tot_busy_queues(bfqd) == 1 + bfq_bfqq_busy(bfqq) && - (bfqq->waker_bfqq == bfqd->in_service_queue || - bfqd->in_service_queue->waker_bfqq == bfqq)) || at_head) { + if (!bfqq || at_head) { if (at_head) list_add(&rq->queuelist, &bfqd->dispatch); else @@ -6059,7 +6018,6 @@ static void bfq_insert_request(struct blk_mq_hw_ctx *hctx, struct request *rq, * merge). */ cmd_flags = rq->cmd_flags; - spin_unlock_irq(&bfqd->lock); bfq_update_insert_stats(q, bfqq, idle_timer_disabled,