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=-5.8 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no 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 64BAAC433ED for ; Thu, 15 Apr 2021 10:07:07 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 4277361249 for ; Thu, 15 Apr 2021 10:07:07 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230474AbhDOKH2 (ORCPT ); Thu, 15 Apr 2021 06:07:28 -0400 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:49545 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229481AbhDOKH2 (ORCPT ); Thu, 15 Apr 2021 06:07:28 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1618481224; h=from:from:reply-to:subject:subject: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=5h/tQ++Sviq4eJsAmzDM+cjj0s5ZURGF7O45zwCAayo=; b=GzzIBZ3guySXsfo7KuRW1GZ7QVTqDZ3VRAq7cDBLxXGF4c0EyuVoDDzGCWATN8MF+/sLIe TRNWt0QXfmk/dSsIooEiG55mBmKzNvxlcVKQpgab/ZFvOwA4ISIN56+BEbg79+RpYYIzva B+mrwvCRW2GaujojB/c/iZjb88ktCsQ= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-167-tbnuFREWPLKzhf0e1ljNIw-1; Thu, 15 Apr 2021 06:07:00 -0400 X-MC-Unique: tbnuFREWPLKzhf0e1ljNIw-1 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 8C7991008066; Thu, 15 Apr 2021 10:06:59 +0000 (UTC) Received: from T590 (ovpn-13-200.pek2.redhat.com [10.72.13.200]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 69F336C32D; Thu, 15 Apr 2021 10:06:49 +0000 (UTC) Date: Thu, 15 Apr 2021 18:06:44 +0800 From: Ming Lei To: JeffleXu Cc: Christoph Hellwig , Jens Axboe , linux-block@vger.kernel.org, Mike Snitzer , dm-devel@redhat.com, Hannes Reinecke Subject: Re: [PATCH V5 11/12] block: add poll_capable method to support bio-based IO polling Message-ID: References: <20210401021927.343727-1-ming.lei@redhat.com> <20210401021927.343727-12-ming.lei@redhat.com> <20210412093856.GA978201@infradead.org> <5f30059d-6650-8268-b681-d8567ac1c509@linux.alibaba.com> <0ceb3060-bce4-c39d-26cf-8c715ebbfd51@linux.alibaba.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <0ceb3060-bce4-c39d-26cf-8c715ebbfd51@linux.alibaba.com> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On Thu, Apr 15, 2021 at 05:21:56PM +0800, JeffleXu wrote: > > > On 4/15/21 3:43 PM, Ming Lei wrote: > > On Thu, Apr 15, 2021 at 09:34:36AM +0800, JeffleXu wrote: > >> > >> > >> On 4/14/21 7:24 PM, Ming Lei wrote: > >>> On Wed, Apr 14, 2021 at 04:38:25PM +0800, JeffleXu wrote: > >>>> > >>>> > >>>> On 4/12/21 5:38 PM, Christoph Hellwig wrote: > >>>>> On Thu, Apr 01, 2021 at 10:19:26AM +0800, Ming Lei wrote: > >>>>>> From: Jeffle Xu > >>>>>> > >>>>>> This method can be used to check if bio-based device supports IO polling > >>>>>> or not. For mq devices, checking for hw queue in polling mode is > >>>>>> adequate, while the sanity check shall be implementation specific for > >>>>>> bio-based devices. For example, dm device needs to check if all > >>>>>> underlying devices are capable of IO polling. > >>>>>> > >>>>>> Though bio-based device may have done the sanity check during the > >>>>>> device initialization phase, cacheing the result of this sanity check > >>>>>> (such as by cacheing in the queue_flags) may not work. Because for dm > >>>>>> devices, users could change the state of the underlying devices through > >>>>>> '/sys/block//io_poll', bypassing the dm device above. In this case, > >>>>>> the cached result of the very beginning sanity check could be > >>>>>> out-of-date. Thus the sanity check needs to be done every time 'io_poll' > >>>>>> is to be modified. > >>>>> > >>>>> I really don't think thi should be a method, and I really do dislike > >>>>> how we have all this "if (is_mq)" junk. Why can't we have a flag on > >>>>> the gendisk that signals if the device can support polling that > >>>>> is autoamtically set for blk-mq and as-needed by bio based drivers? > >>>> > >>>> That would consume one more bit of queue->queue_flags. > >>>> > >>>> Besides, DM/MD is somehow special here that when one of the underlying > >>>> devices is disabled polling through '/sys/block//io_poll', > >>>> currently there's no mechanism notifying the above MD/DM to clear the > >>>> previously set queue_flags. Thus the outdated queue_flags still > >>>> indicates this DM/MD is capable of polling, while in fact one of the > >>>> underlying device has been disabled for polling. > >>> > >>> Right, just like there isn't queue limit progagation. > >>> > >>> Another blocker could be that bio based queue doesn't support queue > >>> freezing. > >> > >> Do you mean the queue freezing is called in the following code snippet? > >> > >> ``` > >> static ssize_t queue_poll_store(struct request_queue *q, const char > >> *page, size_t count) > >> { > >> ... > >> if (poll_on) { > >> blk_queue_flag_set(QUEUE_FLAG_POLL, q); > >> } else { > >> blk_mq_freeze_queue(q); > >> blk_queue_flag_clear(QUEUE_FLAG_POLL, q); > >> blk_mq_unfreeze_queue(q); > >> } > >> ``` > > > > Yes, if it is a bio based queue. Or bio queued queue(DM, MD or others) may > > use freeze_queue to do similar thing. > > > >> > >> And I can't understand how bio-based queue doesn't support queue freezing. > >> > >> ``` > >> submit_bio_noacct > >> __submit_bio_noacct > >> bio_queue_enter > >> ``` > >> > >> Every time submitting a bio, bio_queue_enter() will be called, and once > >> the queue has been frozen, bio_queue_enter() will wait there until the > >> queue is unfrozen. > > > > Not like blk-mq, the refcount is just grabbed during submission for bio based > > queue. > > Could you please explain it more detailed .... Please see __submit_bio(), in which the queue ref is dropped. > > > I will research a bit and see if we can extend freeze queue for > > covering bio based queue. One trouble is that bio is ended before > > freeing request. > > > >> > >>> > >>>> > >>>> Mike had ever suggested that we can trust the queue_flag, and clear the > >>>> outdated queue_flags when later the IO submission or polling routine > >>>> finally finds that the device is not capable of polling. Currently > >>>> submit_bio_checks() will silently clear the REQ_HIPRI flag and still > >>>> submit the bio when the device is actually not capable of polling. To > >>>> fix the issue, could we break the submission and return an error code in > >>>> submit_bio_checks() if the device is not capable of polling when > >>>> submitting HIPRI bio? > >>> > >>> I think we may just leave it alone, if underlying queue becomes not pollable, > >>> the bio still can be submitted & completed via IRQ, just not efficient enough. > >> > >> Yes it still works. I agree if there's no better solution... > >> > >> And what about the issue Christoph originally concerned? Do we use one > >> more flag bit indicating if the queue capable of polling, or the > >> poll_capable() method way? > > > > Just wondering why we can't use QUEUE_FLAG_POLL simply? If user wants to > > enable it, let's do it for them. And bio driver can start with default poll > > state by checking underlying queues. > > > > Consider the following scenario: QUEUE_FLAG_POLL is set after > initialization, indicating the device capable of polling; then polling > is turned off by '/sys/block//io_poll', thus QUEUE_FLAG_POLL is > cleared. If the flag is cleared, the bio will be submitted to irq queue, what is the problem? Thanks, Ming