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=-8.4 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,USER_AGENT_SANE_1 autolearn=ham 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 C7D2DC35247 for ; Fri, 7 Feb 2020 02:07:44 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 93716214AF for ; Fri, 7 Feb 2020 02:07:44 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="ZCbHpq+n" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727028AbgBGCHn (ORCPT ); Thu, 6 Feb 2020 21:07:43 -0500 Received: from us-smtp-delivery-1.mimecast.com ([207.211.31.120]:56217 "EHLO us-smtp-1.mimecast.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726956AbgBGCHn (ORCPT ); Thu, 6 Feb 2020 21:07:43 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1581041262; 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=4iPZgZxpkB9JtkTPYm/bLkl0k6K+GzxSdaOpeHw5LsE=; b=ZCbHpq+ntC20d/bKTcrNhLxiQBwwEGi8UaU3tc8wGgVKcM/vVcl5sXCXmF330DUgQC36Hb 6IkzQ8DNXAa/jD/deH0lj6d5TD0HXT60PUrbRVPQP5/EVEMeJCN4vKiZD+ZqXz1/tEag2b qCu/+iJaM58dIHtoI7Ipppr0OKryj3M= 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-203-F60I2gHHMbCvXzVDoBvNRg-1; Thu, 06 Feb 2020 21:07:40 -0500 X-MC-Unique: F60I2gHHMbCvXzVDoBvNRg-1 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.phx2.redhat.com [10.5.11.14]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id E726B8010F5; Fri, 7 Feb 2020 02:07:38 +0000 (UTC) Received: from ming.t460p (ovpn-8-25.pek2.redhat.com [10.72.8.25]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 1E9B75DA7E; Fri, 7 Feb 2020 02:07:31 +0000 (UTC) Date: Fri, 7 Feb 2020 10:07:27 +0800 From: Ming Lei To: Salman Qazi Cc: Jens Axboe , Bart Van Assche , linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, Jesse Barnes , Gwendal Grignou , Hannes Reinecke , Christoph Hellwig Subject: Re: [PATCH] block: Limit number of items taken from the I/O scheduler in one go Message-ID: <20200207020727.GA9900@ming.t460p> References: <20200206101833.GA20943@ming.t460p> <20200206211222.83170-1-sqazi@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200206211222.83170-1-sqazi@google.com> User-Agent: Mutt/1.12.1 (2019-06-15) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 Sender: linux-block-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On Thu, Feb 06, 2020 at 01:12:22PM -0800, Salman Qazi wrote: > 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 | 47 ++++++++++++++++++++++++++++++++++++++------ > 1 file changed, 41 insertions(+), 6 deletions(-) > > diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c > index ca22afd47b3d..84dde147f646 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 +193,8 @@ 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; > + bool restarted = false; > LIST_HEAD(rq_list); > > /* RCU or SRCU read lock is needed before checking quiesced flag */ > @@ -180,6 +203,9 @@ void blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx *hctx) > > hctx->run++; > > +again: > + run_again = false; > + > /* > * If we have previous entries on our dispatch list, grab them first for > * more fair dispatch. > @@ -208,19 +234,28 @@ 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) { > + if (!restarted) { > + restarted = true; > + goto 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 > Reviewed-by: Ming Lei -- Ming