From patchwork Wed Jul 27 16:13:33 2016 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Paolo Valente X-Patchwork-Id: 72891 Delivered-To: patch@linaro.org Received: by 10.140.29.52 with SMTP id a49csp400527qga; Wed, 27 Jul 2016 09:18:15 -0700 (PDT) X-Received: by 10.98.92.65 with SMTP id q62mr51053598pfb.70.1469636289828; Wed, 27 Jul 2016 09:18:09 -0700 (PDT) Return-Path: Received: from vger.kernel.org (vger.kernel.org. [209.132.180.67]) by mx.google.com with ESMTP id zd7si7059008pab.115.2016.07.27.09.18.09; Wed, 27 Jul 2016 09:18:09 -0700 (PDT) Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67; Authentication-Results: mx.google.com; spf=pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org; dmarc=fail (p=NONE dis=NONE) header.from=linaro.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757785AbcG0QRz (ORCPT + 29 others); Wed, 27 Jul 2016 12:17:55 -0400 Received: from spostino.sms.unimo.it ([155.185.44.3]:53093 "EHLO spostino.sms.unimo.it" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757723AbcG0QRu (ORCPT ); Wed, 27 Jul 2016 12:17:50 -0400 Received: from [185.14.9.64] (port=43484 helo=localhost.localdomain) by spostino.sms.unimo.it with esmtpsa (TLS1.2:RSA_AES_128_CBC_SHA256:128) (Exim 4.80) (envelope-from ) id 1bSRUK-0005Yz-P7; Wed, 27 Jul 2016 18:15:17 +0200 From: Paolo Valente To: Jens Axboe , Tejun Heo Cc: Fabio Checconi , Arianna Avanzini , linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, ulf.hansson@linaro.org, linus.walleij@linaro.org, broonie@kernel.org, Paolo Valente Subject: [PATCH RFC V8 17/22] block, bfq: reduce latency during request-pool saturation Date: Wed, 27 Jul 2016 18:13:33 +0200 Message-Id: <1469636018-31247-18-git-send-email-paolo.valente@linaro.org> X-Mailer: git-send-email 1.9.1 In-Reply-To: <1469636018-31247-1-git-send-email-paolo.valente@linaro.org> References: <2D83ACA0-2513-4019-9A0D-3166FDDE7485@linaro.org> <1469636018-31247-1-git-send-email-paolo.valente@linaro.org> UNIMORE-X-SA-Score: -2.9 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org This patch introduces an heuristic that reduces latency when the I/O-request pool is saturated. This goal is achieved by disabling device idling, for non-weight-raised queues, when there are weight- raised queues with pending or in-flight requests. In fact, as explained in more detail in the comment on the function bfq_bfqq_may_idle(), this reduces the rate at which processes associated with non-weight-raised queues grab requests from the pool, thereby increasing the probability that processes associated with weight-raised queues get a request immediately (or at least soon) when they need one. Along the same line, if there are weight-raised queues, then this patch halves the service rate of async (write) requests for non-weight-raised queues. Signed-off-by: Paolo Valente Signed-off-by: Arianna Avanzini --- block/cfq-iosched.c | 66 ++++++++++++++++++++++++++++++++++++++++++++++++++--- 1 file changed, 63 insertions(+), 3 deletions(-) -- 1.9.1 diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c index 5207ed8..980d321 100644 --- a/block/cfq-iosched.c +++ b/block/cfq-iosched.c @@ -368,6 +368,8 @@ struct bfq_data { * queue in service, even if it is idling). */ int busy_queues; + /* number of weight-raised busy @bfq_queues */ + int wr_busy_queues; /* number of queued requests */ int queued; /* number of requests dispatched and waiting for completion */ @@ -1980,6 +1982,9 @@ static void bfq_del_bfqq_busy(struct bfq_data *bfqd, struct bfq_queue *bfqq, bfqd->busy_queues--; + if (bfqq->wr_coeff > 1) + bfqd->wr_busy_queues--; + bfqg_stats_update_dequeue(bfqq_group(bfqq)); bfq_deactivate_bfqq(bfqd, bfqq, requeue); @@ -1996,6 +2001,9 @@ static void bfq_add_bfqq_busy(struct bfq_data *bfqd, struct bfq_queue *bfqq) bfq_mark_bfqq_busy(bfqq); bfqd->busy_queues++; + + if (bfqq->wr_coeff > 1) + bfqd->wr_busy_queues++; } #if defined(CONFIG_CFQ_GROUP_IOSCHED) && defined(CONFIG_DEBUG_BLK_CGROUP) @@ -3287,7 +3295,16 @@ static unsigned long bfq_serv_to_charge(struct request *rq, if (bfq_bfqq_sync(bfqq) || bfqq->wr_coeff > 1) return blk_rq_sectors(rq); - return blk_rq_sectors(rq) * bfq_async_charge_factor; + /* + * If there are no weight-raised queues, then amplify service + * by just the async charge factor; otherwise amplify service + * by twice the async charge factor, to further reduce latency + * for weight-raised queues. + */ + if (bfqq->bfqd->wr_busy_queues == 0) + return blk_rq_sectors(rq) * bfq_async_charge_factor; + + return blk_rq_sectors(rq) * 2 * bfq_async_charge_factor; } /** @@ -3749,6 +3766,7 @@ static void bfq_add_request(struct request *rq) bfqq->wr_coeff = bfqd->bfq_wr_coeff; bfqq->wr_cur_max_time = bfq_wr_duration(bfqd); + bfqd->wr_busy_queues++; bfqq->entity.prio_changed = 1; } if (prev != bfqq->next_rq) @@ -3947,6 +3965,8 @@ static void bfq_merged_requests(struct request_queue *q, struct request *rq, /* Must be called with bfqq != NULL */ static void bfq_bfqq_end_wr(struct bfq_queue *bfqq) { + if (bfq_bfqq_busy(bfqq)) + bfqq->bfqd->wr_busy_queues--; bfqq->wr_coeff = 1; bfqq->wr_cur_max_time = 0; /* @@ -4807,7 +4827,8 @@ static bool bfq_may_expire_for_budg_timeout(struct bfq_queue *bfqq) static bool bfq_bfqq_may_idle(struct bfq_queue *bfqq) { struct bfq_data *bfqd = bfqq->bfqd; - bool idling_boosts_thr, asymmetric_scenario; + bool idling_boosts_thr, idling_boosts_thr_without_issues, + asymmetric_scenario; if (bfqd->strict_guarantees) return true; @@ -4830,6 +4851,44 @@ static bool bfq_bfqq_may_idle(struct bfq_queue *bfqq) idling_boosts_thr = !bfqd->hw_tag || bfq_bfqq_IO_bound(bfqq); /* + * The value of the next variable, + * idling_boosts_thr_without_issues, is equal to that of + * idling_boosts_thr, unless a special case holds. In this + * special case, described below, idling may cause problems to + * weight-raised queues. + * + * When the request pool is saturated (e.g., in the presence + * of write hogs), if the processes associated with + * non-weight-raised queues ask for requests at a lower rate, + * then processes associated with weight-raised queues have a + * higher probability to get a request from the pool + * immediately (or at least soon) when they need one. Thus + * they have a higher probability to actually get a fraction + * of the device throughput proportional to their high + * weight. This is especially true with NCQ-capable drives, + * which enqueue several requests in advance, and further + * reorder internally-queued requests. + * + * For this reason, we force to false the value of + * idling_boosts_thr_without_issues if there are weight-raised + * busy queues. In this case, and if bfqq is not weight-raised, + * this guarantees that the device is not idled for bfqq (if, + * instead, bfqq is weight-raised, then idling will be + * guaranteed by another variable, see below). Combined with + * the timestamping rules of BFQ (see [1] for details), this + * behavior causes bfqq, and hence any sync non-weight-raised + * queue, to get a lower number of requests served, and thus + * to ask for a lower number of requests from the request + * pool, before the busy weight-raised queues get served + * again. This often mitigates starvation problems in the + * presence of heavy write workloads and NCQ, thereby + * guaranteeing a higher application and system responsiveness + * in these hostile scenarios. + */ + idling_boosts_thr_without_issues = idling_boosts_thr && + bfqd->wr_busy_queues == 0; + + /* * There is then a case where idling must be performed not for * throughput concerns, but to preserve service guarantees. To * introduce it, we can note that allowing the drive to @@ -4903,7 +4962,7 @@ static bool bfq_bfqq_may_idle(struct bfq_queue *bfqq) * is necessary to preserve service guarantees. */ return bfq_bfqq_sync(bfqq) && - (idling_boosts_thr || asymmetric_scenario); + (idling_boosts_thr_without_issues || asymmetric_scenario); } /* @@ -6026,6 +6085,7 @@ static int bfq_init_queue(struct request_queue *q, struct elevator_type *e) * high-definition compressed * video. */ + bfqd->wr_busy_queues = 0; /* * Begin by assuming, optimistically, that the device is a