From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755232AbdKMTyU (ORCPT ); Mon, 13 Nov 2017 14:54:20 -0500 Received: from mail.kernel.org ([198.145.29.99]:38014 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755133AbdKMTyQ (ORCPT ); Mon, 13 Nov 2017 14:54:16 -0500 DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 96B5321879 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=kernel.org Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=shli@kernel.org Date: Mon, 13 Nov 2017 11:54:13 -0800 From: Shaohua Li To: Tejun Heo Cc: axboe@kernel.dk, linux-kernel@vger.kernel.org, kernel-team@fb.com, lizefan@huawei.com, hannes@cmpxchg.org, cgroups@vger.kernel.org, guro@fb.com Subject: Re: [PATCH 7/7] blk-throtl: don't throttle the same IO multiple times Message-ID: <20171113195413.b5lzqem2pt2bg4oe@kernel.org> References: <20171112222613.3613362-1-tj@kernel.org> <20171112222613.3613362-8-tj@kernel.org> <20171113040716.kaheegc4qub42n6z@kernel.org> <20171113111348.GF983427@devbig577.frc2.facebook.com> <20171113155745.GI983427@devbig577.frc2.facebook.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20171113155745.GI983427@devbig577.frc2.facebook.com> User-Agent: NeoMutt/20170609 (1.8.3) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Nov 13, 2017 at 07:57:45AM -0800, Tejun Heo wrote: > Hello, > > On Mon, Nov 13, 2017 at 03:13:48AM -0800, Tejun Heo wrote: > > You're right. If we wanna take this approach, we need to keep the > > throttled flag while cloning. The clearing part is still correct tho. > > Without that, I get 1/4 bw limit enforced. Hmm... I'm not quite sure > > where that 1/4 is coming from tho. Will investigate more. > > Okay, this is because when we spiit, the split bio is the first part > which gets issued and then the orignal bio is wound forward and > requeued. So, for the splits, the original bio is the one which gets > trimmed in the front and requeued, so not clearing BIO_THROTTLED is > enough. I think we should still copy BIO_THROTTLED on clones so that > we don't get suprises w/ other bio drivers. I'm not sure how you are going to make this correct. The mechanism is very fragile. So for example, 'q->make_request_fn(q, bio)' could just queue the bio somewhere and handle in other context (both dm and md do this). The bio will be called again with generic_make_request. In this case, the second time shouldn't throttle the bio. The bio could be called again with generic_make_request but with bdev changed. In this case, the second time should throttle the bio (against the new bdev). There are a lot of different usages of bio. I'd rather not depend on generic_make_request dispatches new bio immediately. That's why I add a bdev in my patch. Thanks, Shaohua