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=-17.4 required=3.0 tests=DKIMWL_WL_MED,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH, MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,USER_AGENT_GIT, USER_IN_DEF_DKIM_WL 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 DFFA7C35247 for ; Wed, 5 Feb 2020 19:57:14 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id AD524218AC for ; Wed, 5 Feb 2020 19:57:14 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="eTwYibTw" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727471AbgBET5L (ORCPT ); Wed, 5 Feb 2020 14:57:11 -0500 Received: from mail-pl1-f201.google.com ([209.85.214.201]:43597 "EHLO mail-pl1-f201.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727450AbgBET5L (ORCPT ); Wed, 5 Feb 2020 14:57:11 -0500 Received: by mail-pl1-f201.google.com with SMTP id n17so1723339plp.10 for ; Wed, 05 Feb 2020 11:57:10 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=date:in-reply-to:message-id:mime-version:references:subject:from:to :cc; bh=PuVF7evpyL1IfNp/BZc5IBmfpPrjFc9Sd5OoulZaPYY=; b=eTwYibTwJtvWYNlkl13M5V8guR9zMPOD9T267/OIZ1NRxDX/uVQvaSYKr6qCBjJOpn cOCgteAVwIo8qqZDhkC3hO3TFluych2w2PZZBRRujjIkwfclnsSdZTM85wLMZXew2f3e FDzJO75SbTf2LiEW7KVKNGqoqQfVNHyy40tXd1ZgvdiwYY7mA8jLkVGKKmvkNMK1SCFZ AleB4yGkVlkwWtJxxTJiBOiecc8q9SCs9H0dFu6NUKXe0V39N+CGx0hO8neQQjb3u8/D Btpm++u3CICW7/2KmTg6sFEhotsEzvdbEdv/14MCDtF9qupOKSsHpIzy0de6MbF/qG1r ZXrw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:in-reply-to:message-id:mime-version :references:subject:from:to:cc; bh=PuVF7evpyL1IfNp/BZc5IBmfpPrjFc9Sd5OoulZaPYY=; b=KrqvVcvhtT8elv43+tu896p0z6SmZx0prCWFRt+kfuGPpwKtBYXa8qLwjFVjHxsm+v KydF0yvga9dlh7TLTc+mCAvknN/0LplnGuWux0BCNINNgIw+QIVAJPYNyN56huukTTzu DFfs+uFXfvkZbN//noh6jXIAr0MnpDy0YdprKhV8bZdahv9dCC0Z8lAaiiKhX/zSFv8f 8HAFc2BoBC0ozX/LvwpMN0dkSatQH1ICVkM/NTe1S4C07H2WD/F6uaDpgQNfho/fzFtJ lhZ4vgoOBVauX2dwlcEtuoow1iq5i3t2rbOQ5jJmpEMICwDlB4JRL33vZPrsgd30i4Ko 7QiA== X-Gm-Message-State: APjAAAVrFE7Fc5wZmuO40SDFZWq04GW1dDLDUvf67SAC/ldAe7K2oVUK YqpGXNYOOoo3UZ539tvAmCC6DAA/ZQ== X-Google-Smtp-Source: APXvYqx/wnxdg8HvcO2bBozrEKRdWQqLV39Jj/9pBkAo8UqtLRdiRmlINailKqpXmI4LBA4uyEU/+ZAIqA== X-Received: by 2002:a63:b047:: with SMTP id z7mr16462018pgo.431.1580932630181; Wed, 05 Feb 2020 11:57:10 -0800 (PST) Date: Wed, 5 Feb 2020 11:57:06 -0800 In-Reply-To: <20200205045526.GA15286@ming.t460p> Message-Id: <20200205195706.49438-1-sqazi@google.com> Mime-Version: 1.0 References: <20200205045526.GA15286@ming.t460p> X-Mailer: git-send-email 2.25.0.341.g760bfbb309-goog Subject: [PATCH] block: Limit number of items taken from the I/O scheduler in one go From: Salman Qazi To: Jens Axboe , Ming Lei , Bart Van Assche , linux-block@vger.kernel.org, linux-kernel@vger.kernel.org Cc: Jesse Barnes , Gwendal Grignou , Hannes Reinecke , Christoph Hellwig , Salman Qazi Content-Type: text/plain; charset="UTF-8" Sender: linux-block-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org Flushes bypass the I/O scheduler and get added to hctx->dispatch in blk_mq_sched_bypass_insert. This can happen while a kworker is running hctx->run_work work item and is past the point in blk_mq_sched_dispatch_requests where hctx->dispatch is checked. The blk_mq_do_dispatch_sched call is not guaranteed to end in bounded time, because the I/O scheduler can feed an arbitrary number of commands. Since we have only one hctx->run_work, the commands waiting in hctx->dispatch will wait an arbitrary length of time for run_work to be rerun. A similar phenomenon exists with dispatches from the software queue. The solution is to poll hctx->dispatch in blk_mq_do_dispatch_sched and blk_mq_do_dispatch_ctx and return from the run_work handler and let it rerun. Signed-off-by: Salman Qazi --- block/blk-mq-sched.c | 37 +++++++++++++++++++++++++++++++------ 1 file changed, 31 insertions(+), 6 deletions(-) diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c index ca22afd47b3d..52249fddeb66 100644 --- a/block/blk-mq-sched.c +++ b/block/blk-mq-sched.c @@ -84,12 +84,16 @@ void blk_mq_sched_restart(struct blk_mq_hw_ctx *hctx) * Only SCSI implements .get_budget and .put_budget, and SCSI restarts * its queue by itself in its completion handler, so we don't need to * restart queue if .get_budget() returns BLK_STS_NO_RESOURCE. + * + * Returns true if hctx->dispatch was found non-empty and + * run_work has to be run again. */ -static void blk_mq_do_dispatch_sched(struct blk_mq_hw_ctx *hctx) +static bool blk_mq_do_dispatch_sched(struct blk_mq_hw_ctx *hctx) { struct request_queue *q = hctx->queue; struct elevator_queue *e = q->elevator; LIST_HEAD(rq_list); + bool ret = false; do { struct request *rq; @@ -97,6 +101,11 @@ static void blk_mq_do_dispatch_sched(struct blk_mq_hw_ctx *hctx) if (e->type->ops.has_work && !e->type->ops.has_work(hctx)) break; + if (!list_empty_careful(&hctx->dispatch)) { + ret = true; + break; + } + if (!blk_mq_get_dispatch_budget(hctx)) break; @@ -113,6 +122,8 @@ static void blk_mq_do_dispatch_sched(struct blk_mq_hw_ctx *hctx) */ list_add(&rq->queuelist, &rq_list); } while (blk_mq_dispatch_rq_list(q, &rq_list, true)); + + return ret; } static struct blk_mq_ctx *blk_mq_next_ctx(struct blk_mq_hw_ctx *hctx, @@ -130,16 +141,25 @@ static struct blk_mq_ctx *blk_mq_next_ctx(struct blk_mq_hw_ctx *hctx, * Only SCSI implements .get_budget and .put_budget, and SCSI restarts * its queue by itself in its completion handler, so we don't need to * restart queue if .get_budget() returns BLK_STS_NO_RESOURCE. + * + * Returns true if hctx->dispatch was found non-empty and + * run_work has to be run again. */ -static void blk_mq_do_dispatch_ctx(struct blk_mq_hw_ctx *hctx) +static bool blk_mq_do_dispatch_ctx(struct blk_mq_hw_ctx *hctx) { struct request_queue *q = hctx->queue; LIST_HEAD(rq_list); struct blk_mq_ctx *ctx = READ_ONCE(hctx->dispatch_from); + bool ret = false; do { struct request *rq; + if (!list_empty_careful(&hctx->dispatch)) { + ret = true; + break; + } + if (!sbitmap_any_bit_set(&hctx->ctx_map)) break; @@ -165,6 +185,7 @@ static void blk_mq_do_dispatch_ctx(struct blk_mq_hw_ctx *hctx) } while (blk_mq_dispatch_rq_list(q, &rq_list, true)); WRITE_ONCE(hctx->dispatch_from, ctx); + return ret; } void blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx *hctx) @@ -172,6 +193,7 @@ void blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx *hctx) struct request_queue *q = hctx->queue; struct elevator_queue *e = q->elevator; const bool has_sched_dispatch = e && e->type->ops.dispatch_request; + bool run_again = false; LIST_HEAD(rq_list); /* RCU or SRCU read lock is needed before checking quiesced flag */ @@ -208,19 +230,22 @@ void blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx *hctx) blk_mq_sched_mark_restart_hctx(hctx); if (blk_mq_dispatch_rq_list(q, &rq_list, false)) { if (has_sched_dispatch) - blk_mq_do_dispatch_sched(hctx); + run_again = blk_mq_do_dispatch_sched(hctx); else - blk_mq_do_dispatch_ctx(hctx); + run_again = blk_mq_do_dispatch_ctx(hctx); } } else if (has_sched_dispatch) { - blk_mq_do_dispatch_sched(hctx); + run_again = blk_mq_do_dispatch_sched(hctx); } else if (hctx->dispatch_busy) { /* dequeue request one by one from sw queue if queue is busy */ - blk_mq_do_dispatch_ctx(hctx); + run_again = blk_mq_do_dispatch_ctx(hctx); } else { blk_mq_flush_busy_ctxs(hctx, &rq_list); blk_mq_dispatch_rq_list(q, &rq_list, false); } + + if (run_again) + blk_mq_run_hw_queue(hctx, true); } bool blk_mq_sched_try_merge(struct request_queue *q, struct bio *bio, -- 2.25.0.341.g760bfbb309-goog