From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx1.redhat.com ([209.132.183.28]:38630 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751032AbdKAEIQ (ORCPT ); Wed, 1 Nov 2017 00:08:16 -0400 Date: Wed, 1 Nov 2017 12:08:01 +0800 From: Ming Lei To: Bart Van Assche Cc: "hch@infradead.org" , "roman.penyaev@profitbricks.com" , "linux-block@vger.kernel.org" , "osandov@fb.com" , "axboe@fb.com" , "hare@suse.com" Subject: Re: [PATCH V2 0/2] block: remove unnecessary RESTART Message-ID: <20171101040755.GB9527@ming.t460p> References: <20171027044330.11921-1-ming.lei@redhat.com> <1509079996.3244.3.camel@wdc.com> <20171027053834.GA12858@ming.t460p> <1509395096.27259.15.camel@wdc.com> <20171031014728.GA12070@ming.t460p> <1509508448.2604.4.camel@wdc.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: <1509508448.2604.4.camel@wdc.com> Sender: linux-block-owner@vger.kernel.org List-Id: linux-block@vger.kernel.org On Wed, Nov 01, 2017 at 03:54:09AM +0000, Bart Van Assche wrote: > On Tue, 2017-10-31 at 09:47 +0800, Ming Lei wrote: > > On Mon, Oct 30, 2017 at 08:24:57PM +0000, Bart Van Assche wrote: > > > On Fri, 2017-10-27 at 13:38 +0800, Ming Lei wrote: > > > > On Fri, Oct 27, 2017 at 04:53:18AM +0000, Bart Van Assche wrote: > > > > > On Fri, 2017-10-27 at 12:43 +0800, Ming Lei wrote: > > > > > > The 1st patch removes the RESTART for TAG-SHARED because SCSI handles it > > > > > > by itself, and not necessary to waste CPU to do the expensive RESTART. > > > > > > And Roman Pen reported that this RESTART cuts half of IOPS in his case. > > > > > > > > > > > > The 2nd patch removes the RESTART when .get_budget returns BLK_STS_RESOURCE, > > > > > > and this RESTART is handled by SCSI's RESTART(scsi_end_request()) too. > > > > > > > > > > There are more block drivers than the SCSI core that share tags. If the > > > > > > > > Could you share us what the other in-tree driver which share tags is? > > > > > > I think the following in-tree drivers support shared tags (in alphabetical > > > order): > > > * null_blk. See also the shared_tags kernel module parameter. > > > * nvme. See also nvme_alloc_ns(). > > > * scsi-mq. > > > > For both null_blk and nvme, we don't need to deal with cross-queue RESTART, > > because BLK_MQ_S_TAG_WAITING has handled it already. > > blk_mq_dispatch_wait_add() returns immediately if BLK_MQ_S_TAG_WAITING is > already set. Are you really sure that removing the restart mechanism doesn't If this flag is set, that means blk_mq_dispatch_wake() will be run when driver tag is available, so the hw queue will be run at that time. > break the NVMe driver if there are multiple namespaces? Did your commit 6d8c6c0f97ad ("blk-mq: Restart a single queue if tag sets are shared") suppose to fix NVMe? Please take a look at the cover letter: https://marc.info/?t=149158911500010&r=1&w=2 -- Ming