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 1A880C433ED for ; Wed, 14 Apr 2021 11:25:40 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id E614E61164 for ; Wed, 14 Apr 2021 11:25:39 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1350423AbhDNLZz (ORCPT ); Wed, 14 Apr 2021 07:25:55 -0400 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:35556 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1350346AbhDNLZ3 (ORCPT ); Wed, 14 Apr 2021 07:25:29 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1618399508; 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=o5AfkVNBiB9daKdhjQ5//WEK3xyfYoRYmBAHAAyv7i8=; b=dPfwwVFC1QHVJnMejPmPAyUJofWso5izz0sgjqRqetdipgtHCMc5uZIHTOxgG0WD3qkUJv /VTdOg9Gp4nIbabRU2L4KZ3e0fpgGYeZO/UTCKwRDKyrQe+ihlTB0ixhIRe/ssLMm7Knyf 6vZx6qKVnfr/c8oaUamsg8c+rrcmRFQ= 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-587-HCbjsELuNau_X6i9fUL_Qw-1; Wed, 14 Apr 2021 07:25:04 -0400 X-MC-Unique: HCbjsELuNau_X6i9fUL_Qw-1 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.phx2.redhat.com [10.5.11.14]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 5EC718026B1; Wed, 14 Apr 2021 11:25:03 +0000 (UTC) Received: from T590 (ovpn-12-91.pek2.redhat.com [10.72.12.91]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 6A1D15D9D0; Wed, 14 Apr 2021 11:24:51 +0000 (UTC) Date: Wed, 14 Apr 2021 19:24:46 +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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org 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. > > 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. Thanks, Ming