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=-17.3 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=ham 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 1EC43C47082 for ; Mon, 7 Jun 2021 11:41:29 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 054046120F for ; Mon, 7 Jun 2021 11:41:29 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231614AbhFGLnT (ORCPT ); Mon, 7 Jun 2021 07:43:19 -0400 Received: from smtp-out1.suse.de ([195.135.220.28]:36668 "EHLO smtp-out1.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230463AbhFGLnS (ORCPT ); Mon, 7 Jun 2021 07:43:18 -0400 Received: from relay2.suse.de (relay2.suse.de [149.44.160.134]) by smtp-out1.suse.de (Postfix) with ESMTP id 39EA521A93; Mon, 7 Jun 2021 11:41:27 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.cz; s=susede2_rsa; t=1623066087; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=Y1p34XXNGh13rgcmV89i0Hv1PANOTcFOPO8k8eP1M4Y=; b=SHkREpPwnPLgvg4Hr1PyJ8+pT6ZSTMXxkT2yuB8OK+6Ut/3Gpn56H8lPU2gcIf5t5pFPS/ 3oZ2ayP4XrTlHYgHO0KftMfpb9LaVNCXzQvkZpCB4DkCrBSVyGD5BDc2sNj9o/czg/lzJw NGwY0paYWx+leJTIGJbB2ytQhFCGcaQ= DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=suse.cz; s=susede2_ed25519; t=1623066087; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=Y1p34XXNGh13rgcmV89i0Hv1PANOTcFOPO8k8eP1M4Y=; b=BNu3OcxPPM6wI9r7VyWpHjeqymUvNelFDLsR21xhARu1mXX/B3RcYq5IEwcspHKuKJKcMQ HIhQluYwS94zOQAw== Received: from quack2.suse.cz (unknown [10.100.200.198]) by relay2.suse.de (Postfix) with ESMTP id 2E3F3A3B8A; Mon, 7 Jun 2021 11:41:27 +0000 (UTC) Received: by quack2.suse.cz (Postfix, from userid 1000) id 0CE5A1F2CA8; Mon, 7 Jun 2021 13:41:27 +0200 (CEST) Date: Mon, 7 Jun 2021 13:41:27 +0200 From: Jan Kara To: Hannes Reinecke Cc: Jan Kara , Jens Axboe , linux-block@vger.kernel.org, Ming Lei , Paolo Valente Subject: Re: [PATCH v2] block: Do not pull requests from the scheduler when we cannot dispatch them Message-ID: <20210607114127.GG30275@quack2.suse.cz> References: <20210603104721.6309-1-jack@suse.cz> <42e2e0f1-acf4-a5eb-2c3e-cb20706430a4@suse.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <42e2e0f1-acf4-a5eb-2c3e-cb20706430a4@suse.de> User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On Mon 07-06-21 12:05:52, Hannes Reinecke wrote: > On 6/3/21 12:47 PM, Jan Kara wrote: > > Provided the device driver does not implement dispatch budget accounting > > (which only SCSI does) the loop in __blk_mq_do_dispatch_sched() pulls > > requests from the IO scheduler as long as it is willing to give out any. > > That defeats scheduling heuristics inside the scheduler by creating > > false impression that the device can take more IO when it in fact > > cannot. > > > > For example with BFQ IO scheduler on top of virtio-blk device setting > > blkio cgroup weight has barely any impact on observed throughput of > > async IO because __blk_mq_do_dispatch_sched() always sucks out all the > > IO queued in BFQ. BFQ first submits IO from higher weight cgroups but > > when that is all dispatched, it will give out IO of lower weight cgroups > > as well. And then we have to wait for all this IO to be dispatched to > > the disk (which means lot of it actually has to complete) before the > > IO scheduler is queried again for dispatching more requests. This > > completely destroys any service differentiation. > > > > So grab request tag for a request pulled out of the IO scheduler already > > in __blk_mq_do_dispatch_sched() and do not pull any more requests if we > > cannot get it because we are unlikely to be able to dispatch it. That > > way only single request is going to wait in the dispatch list for some > > tag to free. > > > > Reviewed-by: Ming Lei > > Signed-off-by: Jan Kara > > --- > > block/blk-mq-sched.c | 12 +++++++++++- > > block/blk-mq.c | 2 +- > > block/blk-mq.h | 2 ++ > > 3 files changed, 14 insertions(+), 2 deletions(-) > > > > Jens, can you please merge the patch? Thanks! > > > > diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c > > index 996a4b2f73aa..714e678f516a 100644 > > --- a/block/blk-mq-sched.c > > +++ b/block/blk-mq-sched.c > > @@ -168,9 +168,19 @@ static int __blk_mq_do_dispatch_sched(struct blk_mq_hw_ctx *hctx) > > * in blk_mq_dispatch_rq_list(). > > */ > > list_add_tail(&rq->queuelist, &rq_list); > > + count++; > > if (rq->mq_hctx != hctx) > > multi_hctxs = true; > > - } while (++count < max_dispatch); > > + > > + /* > > + * If we cannot get tag for the request, stop dequeueing > > + * requests from the IO scheduler. We are unlikely to be able > > + * to submit them anyway and it creates false impression for > > + * scheduling heuristics that the device can take more IO. > > + */ > > + if (!blk_mq_get_driver_tag(rq)) > > + break; > > + } while (count < max_dispatch); > > > > if (!count) { > > if (run_queue) > > Doesn't this lead to a double accounting of the allocated tags? > From what I can see we don't really check if the tag is already > allocated in blk_mq_get_driver_tag() ... I think we do check. blk_mq_get_driver_tag() has: if (rq->tag == BLK_MQ_NO_TAG && !__blk_mq_get_driver_tag(rq)) return false; if ((hctx->flags & BLK_MQ_F_TAG_QUEUE_SHARED) && !(rq->rq_flags & RQF_MQ_INFLIGHT)) { rq->rq_flags |= RQF_MQ_INFLIGHT; __blk_mq_inc_active_requests(hctx); } hctx->tags->rqs[rq->tag] = rq; So once we call it, rq->tag will be != BLK_MQ_NO_TAG and RQF_MQ_INFLIGHT will be set. So neither __blk_mq_get_driver_tag() nor __blk_mq_inc_active_requests() will get repeated. Honza -- Jan Kara SUSE Labs, CR