From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf0-f175.google.com ([209.85.192.175]:33515 "EHLO mail-pf0-f175.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750955AbdECQqd (ORCPT ); Wed, 3 May 2017 12:46:33 -0400 Received: by mail-pf0-f175.google.com with SMTP id q20so15299925pfg.0 for ; Wed, 03 May 2017 09:46:33 -0700 (PDT) Date: Wed, 3 May 2017 09:46:31 -0700 From: Omar Sandoval To: Ming Lei Cc: Jens Axboe , linux-block@vger.kernel.org, Christoph Hellwig , Omar Sandoval Subject: Re: [PATCH 1/4] blk-mq: introduce BLK_MQ_F_SCHED_USE_HW_TAG Message-ID: <20170503164631.GA10775@vader> References: <20170428151539.25514-1-ming.lei@redhat.com> <20170428151539.25514-2-ming.lei@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: <20170428151539.25514-2-ming.lei@redhat.com> Sender: linux-block-owner@vger.kernel.org List-Id: linux-block@vger.kernel.org On Fri, Apr 28, 2017 at 11:15:36PM +0800, Ming Lei wrote: > When blk-mq I/O scheduler is used, we need two tags for > submitting one request. One is called scheduler tag for > allocating request and scheduling I/O, another one is called > driver tag, which is used for dispatching IO to hardware/driver. > This way introduces one extra per-queue allocation for both tags > and request pool, and may not be as efficient as case of none > scheduler. > > Also currently we put a default per-hctx limit on schedulable > requests, and this limit may be a bottleneck for some devices, > especialy when these devices have a quite big tag space. > > This patch introduces BLK_MQ_F_SCHED_USE_HW_TAG so that we can > allow to use hardware/driver tags directly for IO scheduling if > devices's hardware tag space is big enough. Then we can avoid > the extra resource allocation and make IO submission more > efficient. > > Signed-off-by: Ming Lei > --- > block/blk-mq-sched.c | 10 +++++++++- > block/blk-mq.c | 35 +++++++++++++++++++++++++++++------ > include/linux/blk-mq.h | 1 + > 3 files changed, 39 insertions(+), 7 deletions(-) One more note on this: if we're using the hardware tags directly, then we are no longer limited to q->nr_requests requests in-flight. Instead, we're limited to the hw queue depth. We probably want to maintain the original behavior, so I think we need to resize the hw tags in blk_mq_init_sched() if we're using hardware tags.