From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Date: Wed, 29 Nov 2017 22:46:51 +0100 From: Christoph Hellwig Subject: Re: [PATCH] [RFC] um: Convert ubd driver to blk-mq Message-ID: <20171129214651.GA5846@lst.de> References: <20171126131053.32300-1-richard@nod.at> MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20171126131053.32300-1-richard@nod.at> Sender: linux-kernel-owner@vger.kernel.org To: Richard Weinberger Cc: user-mode-linux-devel@lists.sourceforge.net, linux-kernel@vger.kernel.org, axboe@fb.com, anton.ivanov@cambridgegreys.com, hch@lst.de, linux-block@vger.kernel.org List-ID: On Sun, Nov 26, 2017 at 02:10:53PM +0100, Richard Weinberger wrote: > MAX_SG is 64, used for blk_queue_max_segments(). This comes from > a0044bdf60c2 ("uml: batch I/O requests"). Is this still a good/sane > value for blk-mq? blk-mq itself doesn't change the tradeoff. > The driver does IO batching, for each request it issues many UML struct > io_thread_req request to the IO thread on the host side. > One io_thread_req per SG page. > Before the conversion the driver used blk_end_request() to indicate that > a part of the request is done. > blk_mq_end_request() does not take a length parameter, therefore we can > only mark the whole request as done. See the new is_last property on the > driver. > Maybe there is a way to partially end requests too in blk-mq? You can, take a look at scsi_end_request which handles this for blk-mq and the legacy layer. That being said I wonder if batching really makes that much sene if you execute each segment separately? > Another obstacle with IO batching is that UML IO thread requests can > fail. Not only due to OOM, also because the pipe between the UML kernel > process and the host IO thread can return EAGAIN. > In this case the driver puts the request into a list and retried later > again when the pipe turns writable. > I’m not sure whether this restart logic makes sense with blk-mq, maybe > there is a way in blk-mq to put back a (partial) request? blk_mq_requeue_request requeues requests that have been partially exectuted (or not at all for that matter).