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=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 62CB8C54FCB for ; Fri, 24 Apr 2020 15:03:30 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 28C9620706 for ; Fri, 24 Apr 2020 15:03:30 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="g9YIPlk5" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726968AbgDXPD3 (ORCPT ); Fri, 24 Apr 2020 11:03:29 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:47772 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-FAIL-OK-FAIL) by vger.kernel.org with ESMTP id S1726698AbgDXPD2 (ORCPT ); Fri, 24 Apr 2020 11:03:28 -0400 Received: from mail-pg1-x549.google.com (mail-pg1-x549.google.com [IPv6:2607:f8b0:4864:20::549]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 63708C09B046 for ; Fri, 24 Apr 2020 08:03:27 -0700 (PDT) Received: by mail-pg1-x549.google.com with SMTP id x16so8055758pgi.0 for ; Fri, 24 Apr 2020 08:03:27 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=date:message-id:mime-version:subject:from:to:cc; bh=Xkkqj+5WLu8COiZ/TfgDWy2Txx9eTlxx/bJwpEwcEjo=; b=g9YIPlk5dCl7/qwmHnLf22yWze7QUqAwpoSl+oVbHvnY2+hjpeJjnrnRjKcAAEFhSP aBeSs0bhg2EqhXVymdS2LzknKnlCnEMd6oEM0IONu6/hDEP3UNk65ApKCS7bGaz8K/Hj LTHzeHAhYYsmU854WZitshEO7A/UeYTli83jA7Tsw+WsnLbF3p3SEYA86lHBNxmQxAeG YOipft3BY574XW7iYWLsr+6/Sw9242uTZ9OYLyD6t2KUhUsqiIA3bqDAVc/StlOVAexv PBrwqzjpRjqP0UALMzsPPNwtUhp/Qr98pg/VCVcFvghDKLXJtiBj3L3apf2P6p14snbO mamg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:message-id:mime-version:subject:from:to:cc; bh=Xkkqj+5WLu8COiZ/TfgDWy2Txx9eTlxx/bJwpEwcEjo=; b=WhCOhUC/x+9uuEftGcwkxeY/2vbr7d+JPoEaxZNs8P30V8lVJ9azNb1W2J8tLI6xaq iODFwMzD+fb4kO7779eDFR+UFigUPJ7TWGUApK/XsmJvLmFrh+z45MqT9MbA5gBrd00H 4pHqJSnhbcki/f7G65o02RsTgcerOjqmKlLl264SIACAA6DgfuwKDLEpne9kdvzlE/v1 85J7o4KGqQX6RnBvZ6swPLsKKegMAe0w9I7r2QtuL60YKqcN89gbEFi4HakxskAW5Y0h ftSmAUlpMAATKSbJgY9rrPgxZCJX3A7g584d2gAEEbfESPL6ZWfV6rRI3AfPExzN2uoD DQWg== X-Gm-Message-State: AGi0PuZQA9t7L4LgxsyWXmZ6XHDtfsikYkLh2Ksi17h2SvoMQzHiBDvQ MjpCi1rdsYo7vJXTvFXYrzXWCsYU0A== X-Google-Smtp-Source: APiQypIeQr9l6rGMJxKwS/oF3yYj1Ak/i9u0uke+OPUcjKK3YwmwqocKr1AZrI6/LebqOsh7bpRFt+/L5w== X-Received: by 2002:a17:90a:1a46:: with SMTP id 6mr6502072pjl.111.1587740606455; Fri, 24 Apr 2020 08:03:26 -0700 (PDT) Date: Fri, 24 Apr 2020 08:03:21 -0700 Message-Id: <20200424150321.38617-1-sqazi@google.com> Mime-Version: 1.0 X-Mailer: git-send-email 2.26.2.303.gf8c07b1a785-goog Subject: [PATCH v3] 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-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@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 Reviewed-by: Ming Lei --- Changes since v2: * Added helper function __blk_mq_sched_dispatch_requests to remove goto * Replaced bool return value with int, using -EAGAIN to restart * Changes to comment formatting block/blk-mq-sched.c | 64 +++++++++++++++++++++++++++++++++++--------- 1 file changed, 51 insertions(+), 13 deletions(-) diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c index 74cedea56034..a86a34580772 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 -EAGAIN if hctx->dispatch was found non-empty and run_work has to + * be run again. This is necessary to avoid starving flushes. */ -static void blk_mq_do_dispatch_sched(struct blk_mq_hw_ctx *hctx) +static int 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); + int ret = 0; 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 = -EAGAIN; + 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 -EAGAIN if hctx->dispatch was found non-empty and run_work has to + * to be run again. This is necessary to avoid starving flushes. */ -static void blk_mq_do_dispatch_ctx(struct blk_mq_hw_ctx *hctx) +static int 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); + int ret = 0; do { struct request *rq; + if (!list_empty_careful(&hctx->dispatch)) { + ret = -EAGAIN; + break; + } + if (!sbitmap_any_bit_set(&hctx->ctx_map)) break; @@ -165,21 +185,17 @@ 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) +int __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; + int ret = 0; LIST_HEAD(rq_list); - /* RCU or SRCU read lock is needed before checking quiesced flag */ - if (unlikely(blk_mq_hctx_stopped(hctx) || blk_queue_quiesced(q))) - return; - - hctx->run++; - /* * If we have previous entries on our dispatch list, grab them first for * more fair dispatch. @@ -208,19 +224,41 @@ 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); + ret = blk_mq_do_dispatch_sched(hctx); else - blk_mq_do_dispatch_ctx(hctx); + ret = blk_mq_do_dispatch_ctx(hctx); } } else if (has_sched_dispatch) { - blk_mq_do_dispatch_sched(hctx); + ret = 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); + ret = blk_mq_do_dispatch_ctx(hctx); } else { blk_mq_flush_busy_ctxs(hctx, &rq_list); blk_mq_dispatch_rq_list(q, &rq_list, false); } + + return ret; +} + +void blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx *hctx) +{ + struct request_queue *q = hctx->queue; + + /* RCU or SRCU read lock is needed before checking quiesced flag */ + if (unlikely(blk_mq_hctx_stopped(hctx) || blk_queue_quiesced(q))) + return; + + hctx->run++; + + /* + * A return of -EAGAIN is an indication that hctx->dispatch is not + * empty and we must run again in order to avoid starving flushes. + */ + if (__blk_mq_sched_dispatch_requests(hctx) == -EAGAIN) { + if (__blk_mq_sched_dispatch_requests(hctx) == -EAGAIN) + blk_mq_run_hw_queue(hctx, true); + } } bool blk_mq_sched_try_merge(struct request_queue *q, struct bio *bio, -- 2.26.2.303.gf8c07b1a785-goog