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 844FCC433ED for ; Thu, 15 Apr 2021 07:43:46 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 421826113B for ; Thu, 15 Apr 2021 07:43:46 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230090AbhDOHoG (ORCPT ); Thu, 15 Apr 2021 03:44:06 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]:48144 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231326AbhDOHoG (ORCPT ); Thu, 15 Apr 2021 03:44:06 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1618472623; 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=Dpy1wqaiNbmbjWwjN/sMSXOWeJqICR8ysUNboTmF7fY=; b=MJ6jLqeh/YnDN5EjZbp7RHvfEeeUuAWHWZv2zK7NAu1a0rME7dQnF67I+ymp2ZWbP8+UEn WzrT0zR0uYOpQdddpaNMvC+3AnZs4/GqC/pcRq0DNKTWhjwFwn4K5Mkf1oFWAdNgh8dqpH fl/NNHueLh7whSuqpe7csGn+rigDys4= 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-448-MNjOzgr7OVW-J-D8-DG_0A-1; Thu, 15 Apr 2021 03:43:39 -0400 X-MC-Unique: MNjOzgr7OVW-J-D8-DG_0A-1 Received: from smtp.corp.redhat.com (int-mx08.intmail.prod.int.phx2.redhat.com [10.5.11.23]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 30320800FEA; Thu, 15 Apr 2021 07:43:38 +0000 (UTC) Received: from T590 (ovpn-13-200.pek2.redhat.com [10.72.13.200]) by smtp.corp.redhat.com (Postfix) with ESMTPS id B613816C70; Thu, 15 Apr 2021 07:43:26 +0000 (UTC) Date: Thu, 15 Apr 2021 15:43:22 +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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <5f30059d-6650-8268-b681-d8567ac1c509@linux.alibaba.com> X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org 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. 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. Thanks, Ming