Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752481AbdLEQ2v (ORCPT ); Tue, 5 Dec 2017 11:28:51 -0500 Received: from mx1.redhat.com ([209.132.183.28]:52136 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751325AbdLEQ2t (ORCPT ); Tue, 5 Dec 2017 11:28:49 -0500 Date: Wed, 6 Dec 2017 00:28:31 +0800 From: Ming Lei To: Bart Van Assche Cc: "linux-scsi@vger.kernel.org" , "hch@infradead.org" , "jejb@linux.vnet.ibm.com" , "linux-block@vger.kernel.org" , "axboe@fb.com" , "martin.petersen@oracle.com" , "linux-kernel@vger.kernel.org" , "hare@suse.com" , "holger@applied-asynchrony.com" Subject: Re: [PATCH] SCSI: run queue if SCSI device queue isn't ready and queue is idle Message-ID: <20171205162825.GA23788@ming.t460p> References: <20171205075256.10319-1-ming.lei@redhat.com> <1512490099.2660.6.camel@sandisk.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1512490099.2660.6.camel@sandisk.com> User-Agent: Mutt/1.9.1 (2017-09-22) X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.30]); Tue, 05 Dec 2017 16:28:49 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2231 Lines: 55 On Tue, Dec 05, 2017 at 04:08:20PM +0000, Bart Van Assche wrote: > On Tue, 2017-12-05 at 15:52 +0800, Ming Lei wrote: > > diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c > > index db9556662e27..1816dd8259b3 100644 > > --- a/drivers/scsi/scsi_lib.c > > +++ b/drivers/scsi/scsi_lib.c > > @@ -1967,6 +1967,8 @@ static bool scsi_mq_get_budget(struct blk_mq_hw_ctx *hctx) > > out_put_device: > > put_device(&sdev->sdev_gendev); > > out: > > + if (atomic_read(&sdev->device_busy) == 0 && !scsi_device_blocked(sdev)) > > + blk_mq_delay_run_hw_queue(hctx, SCSI_QUEUE_DELAY); > > return false; > > } > > This cannot work since multiple threads can call scsi_mq_get_budget() That is exactly the way we are handling these cases before 0df21c86bdbf(scsi: implement .get_budget and .put_budget for blk-mq), so if it can't work, that is not fault of commit 0df21c86bdbf. > concurrently and hence it can happen that none of them sees > atomic_read(&sdev->device_busy) == 0. BTW, the above patch is something I If there is concurrent .get_budget(), one of them will see the counter becoming zero finally because each sdev->device_busy is inc/dec atomically. Or scsi_dev_queue_ready() return true. Anyway, we need this patch to avoid possible regression. If you think there are bugs in blk-mq RESTART, just root cause and and fix it. > consider as a workaround. Have you considered to fix the root cause and to > add blk_mq_sched_mark_restart_hctx() where these are missing, e.g. in > blk_mq_sched_dispatch_requests()? The patch below is not a full solution > but resulted in a significant improvement in my tests: > > diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c > index 69e3226e66ca..9d86876ec503 100644 > --- a/block/blk-mq-sched.c > +++ b/block/blk-mq-sched.c > @@ -226,6 +226,7 @@ void blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx *hctx) > * TODO: get more budgets, and dequeue more requests in > * one time. > */ > + blk_mq_sched_mark_restart_hctx(hctx); > blk_mq_do_dispatch_ctx(hctx); > } else { > blk_mq_flush_busy_ctxs(hctx, &rq_list); This is still a workaround for RESTART, see my comment before: https://marc.info/?l=linux-block&m=151217500929341&w=2 -- Ming