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=-4.1 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no 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 4B6B0C433E0 for ; Sat, 8 Aug 2020 19:05:29 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id E1E9220678 for ; Sat, 8 Aug 2020 19:05:28 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=broadcom.com header.i=@broadcom.com header.b="W7+tfcNT" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726531AbgHHTF1 (ORCPT ); Sat, 8 Aug 2020 15:05:27 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:53858 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726307AbgHHTF0 (ORCPT ); Sat, 8 Aug 2020 15:05:26 -0400 Received: from mail-qt1-x830.google.com (mail-qt1-x830.google.com [IPv6:2607:f8b0:4864:20::830]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 7FEB3C061756 for ; Sat, 8 Aug 2020 12:05:26 -0700 (PDT) Received: by mail-qt1-x830.google.com with SMTP id s23so3833639qtq.12 for ; Sat, 08 Aug 2020 12:05:26 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=broadcom.com; s=google; h=from:references:in-reply-to:mime-version:thread-index:date :message-id:subject:to:cc; bh=yL2/UZwpI1fKUgMiVTUcG0n2gTeUCe4ep7Qh3IOPQe4=; b=W7+tfcNTadux9ZruydDOwvn25cK3jUiSZFc16ccW/MZfXQpxkZu851dMozMIgBCpXo SmNST2s9nIRlDyg7rGsxUPTOfuPq2hE2FLOc0fQsRwH9eHM67asHmjHVQ0T9eap+rZ33 w/F/xIOUjLN1FFkegvRY5W5xjA2N/wZWaQt5E= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:from:references:in-reply-to:mime-version :thread-index:date:message-id:subject:to:cc; bh=yL2/UZwpI1fKUgMiVTUcG0n2gTeUCe4ep7Qh3IOPQe4=; b=k3+yVFpVbLpfzp6brliYU8GCYn2zUMIvQTeEO/V3IyCHAHZ/R5faYzN+RpW2McxRqU 4zTe3UG6LLCvIa0oG6PW8RVfnECX/GDYwztQCQcLrlyYt42G9TLQZ9Mp5hz1DXTIXy0q 79VXt0qrkv9/+eRX6ZD/UTHfyC1ptSScS3z/7o1usutCW0Hm8LWap3Tg7qbTdnBPWerr xiDUCG5z/ccJRp2aOzfVPNezXCp+uUPtpK/D64vOYuPBlOcPiE94A50Et+zOad3IQD/b 7O6T8f5uq8u+HUlIVdZIuuqwVN7swGckmfktYsPcntLPHRSeBrhe7TaVX875i5bIP56O LArA== X-Gm-Message-State: AOAM532h82QnYGUCkF+Hgq+2cJ6wg86g6Mx2g+c8+mnT+s4GXNHUvbso lIZxA/Q2FJSEMoCTMBbGy8P+mWrw32eUB8cFFo/Tqg== X-Google-Smtp-Source: ABdhPJxSDSvoxmQ/18LDhxUUacOJwztXKNmzsKQZ9/amd5uvd5KoQ8N7La375Ji8nJ7hS9NO5RxU4crYPZSEmZV/AoU= X-Received: by 2002:ac8:faf:: with SMTP id b44mr20343687qtk.190.1596913524420; Sat, 08 Aug 2020 12:05:24 -0700 (PDT) From: Kashyap Desai References: <20200728084511.GA1326626@T590> <965cf22eea98c00618570da8424d0d94@mail.gmail.com> <20200729153648.GA1698748@T590> <7f94eaf2318cc26ceb64bde88d59d5e2@mail.gmail.com> <20200804083625.GA1958244@T590> <20200805084031.GA1995289@T590> <5adffdf805179428bdd0dd6c293a4f7d@mail.gmail.com> <20200806133819.GA2046861@T590> <20200806152939.GA2062348@T590> In-Reply-To: <20200806152939.GA2062348@T590> MIME-Version: 1.0 X-Mailer: Microsoft Outlook 15.0 Thread-Index: AQMjI78N3hI4nYPl7m+8RPNdz72u+AIQ6BN/Af8DR5wB98/f/wIl0wtMAyOnIMMBUiS2bQGsIvrnAdpdHIkCtpqGHQEvo/4KpfSfM2A= Date: Sun, 9 Aug 2020 00:35:21 +0530 Message-ID: <3f35b0f67c73c8c4996fdad80eb6d963@mail.gmail.com> Subject: RE: [PATCH RFC v7 10/12] megaraid_sas: switch fusion adapters to MQ To: Ming Lei Cc: John Garry , axboe@kernel.dk, jejb@linux.ibm.com, martin.petersen@oracle.com, don.brace@microsemi.com, Sumit Saxena , bvanassche@acm.org, hare@suse.com, hch@lst.de, Shivasharan Srikanteshwara , linux-block@vger.kernel.org, linux-scsi@vger.kernel.org, esc.storagedev@microsemi.com, chenxiang66@hisilicon.com, "PDL,MEGARAIDLINUX" 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 > On Thu, Aug 06, 2020 at 08:07:38PM +0530, Kashyap Desai wrote: > > > > Hi Ming - > > > > > > > > There is still some race which is not handled. Take a case of IO > > > > is not able to get budget and it has already marked flag. > > > > flag will be seen non-zero in completion path and > > > > completion path will attempt h/w queue run. (But this particular > > > > IO is still not in s/w queue.). > > > > Attempt of running h/w queue from completion path will not flush > > > > any IO since there is no IO in s/w queue. > > > > > > Then where is the IO to be submitted in case of running out of budget? > > > > Typical race in your latest patch is - (Lets consider command A,B and > > C) Command A did not receive budget. Command B completed (which was > > already > > Command A doesn't get budget, and A is still in sw/scheduler queue because > we try to acquire budget before dequeuing request from sw/scheduler queue, > see __blk_mq_do_dispatch_sched() and blk_mq_do_dispatch_ctx(). > > Not consider direct issue, because the hw queue will be run explicitly when > not getting budget, see __blk_mq_try_issue_directly. > > Not consider command A being added to hctx->dispatch too, because blk-mq > will re-run the queue, see blk_mq_dispatch_rq_list(). Ming - After going through your comment (I noted your comment and thanks for correcting my understanding.) and block layer code, I realize that it is a different race condition. My previous explanation was not accurate. I debug further and figure out what is actually happening - Consider below scenario/sequence - Thread -1 - Detected budget contention. Set restarts = 1. Thread -2 - old restarts = 1. start hw queue. Thread -3 - old restarts = 1. start hw queue. Thread -2 - move restarts = 0. In my testing, I noticed that both thread-2 and thread-3 started h/w queue but there was no work for them to do. It is possible because some other context of h/w queue run might have done that job. It means, IO of thread-1 is already posted. Thread -4 - Detected budget contention. Set restart = 1 (because thread-2 has move restarts=0). Thread -3 - move restarts = 0 (because this thread see old value = 1 but that is actually updated one more time by thread-4 and theread-4 actually wanted to run h/w queues). IO of Thread-4 will not be scheduled. We have to make sure that completion IO path do atomic_cmpxchng of restarts flag before running the h/w queue. Below code change - (main fix is sequence of atomic_cmpxchg and blk_mq_run_hw_queues) fix the issue. --- a/drivers/scsi/scsi_lib.c +++ b/drivers/scsi/scsi_lib.c @@ -594,8 +594,27 @@ static bool scsi_end_request(struct request *req, blk_status_t error, if (scsi_target(sdev)->single_lun || !list_empty(&sdev->host->starved_list)) kblockd_schedule_work(&sdev->requeue_work); - else - blk_mq_run_hw_queues(q, true); + else { + /* + * smp_mb() implied in either rq->end_io or blk_mq_free_request + * is for ordering writing .device_busy in scsi_device_unbusy() + * and reading sdev->restarts. + */ + int old = atomic_read(&sdev->restarts); + + if (old) { + /* + * ->restarts has to be kept as non-zero if there is + * new budget contention comes. + */ + atomic_cmpxchg(&sdev->restarts, old, 0); + + /* run the queue after restarts flag is updated + * to avoid race condition with .get_budget + */ + blk_mq_run_hw_queues(sdev->request_queue, true); + } + } percpu_ref_put(&q->q_usage_counter); return false; Kashyap