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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 062FEC77B7F for ; Fri, 12 May 2023 15:56:30 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S241598AbjELP42 (ORCPT ); Fri, 12 May 2023 11:56:28 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:38074 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S241492AbjELP42 (ORCPT ); Fri, 12 May 2023 11:56:28 -0400 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 9550B40F7 for ; Fri, 12 May 2023 08:55:37 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1683906936; 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=vLa5Ytb7cb+okn92pAHPyxyxinKAdhQDfykU4pJW4/g=; b=KeO5WfktKrDKINNa9puaZ1C3BLaDhQlAfotxTi6nl4zXtl+W5bzfGtBFfMmPF9PKv7Ssbf qi4L5Sugca8Tfqr/e25xtRgEUiHGARg4IPZeSvjPdrRZXb72A+ESjuTulkYaWFTDTi6Ioo HIDWb97c9RywPy38+XH9PfBWbOdAATU= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-606--5sh_lCeOgK6dFZQJZdtLw-1; Fri, 12 May 2023 11:55:35 -0400 X-MC-Unique: -5sh_lCeOgK6dFZQJZdtLw-1 Received: from smtp.corp.redhat.com (int-mx08.intmail.prod.int.rdu2.redhat.com [10.11.54.8]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id F38BD867941; Fri, 12 May 2023 15:55:34 +0000 (UTC) Received: from ovpn-8-16.pek2.redhat.com (ovpn-8-16.pek2.redhat.com [10.72.8.16]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 5C682C15BA0; Fri, 12 May 2023 15:55:30 +0000 (UTC) Date: Fri, 12 May 2023 23:55:26 +0800 From: Ming Lei To: Jens Axboe Cc: linux-block@vger.kernel.org, Guangwu Zhang , Yu Kuai , ming.lei@redhat.com Subject: Re: [PATCH] blk-mq: don't queue passthrough request into scheduler Message-ID: References: <20230512150328.192908-1-ming.lei@redhat.com> <70478f95-2852-9bf1-f8f7-630c74641c0f@kernel.dk> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Scanned-By: MIMEDefang 3.1 on 10.11.54.8 Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On Fri, May 12, 2023 at 09:43:32AM -0600, Jens Axboe wrote: > On 5/12/23 9:34?AM, Ming Lei wrote: > > On Fri, May 12, 2023 at 09:25:18AM -0600, Jens Axboe wrote: > >> On 5/12/23 9:19?AM, Ming Lei wrote: > >>> On Fri, May 12, 2023 at 09:08:54AM -0600, Jens Axboe wrote: > >>>> On 5/12/23 9:03?AM, Ming Lei wrote: > >>>>> Passthrough(pt) request shouldn't be queued to scheduler, especially some > >>>>> schedulers(such as bfq) supposes that req->bio is always available and > >>>>> blk-cgroup can be retrieved via bio. > >>>>> > >>>>> Sometimes pt request could be part of error handling, so it is better to always > >>>>> queue it into hctx->dispatch directly. > >>>>> > >>>>> Fix this issue by queuing pt request from plug list to hctx->dispatch > >>>>> directly. > >>>> > >>>> Why not just add the check to the BFQ insertion? That would be a lot > >>>> more trivial and would not be poluting the core with this stuff. > >>> > >>> pt request is supposed to be issued to device directly, and we never > >>> queue it to scheduler before 1c2d2fff6dc0 ("block: wire-up support for > >>> passthrough plugging"). > >>> > >>> some pt request might be part of error handling, and adding it to > >>> scheduler could cause io hang. > >> > >> I'm not suggesting adding it to the scheduler, just having the bypass > >> "add to dispatch" in a different spot. > > > > Originally it is added to dispatch in blk_execute_rq_nowait() for each > > request, but now we support plug for pt request, that is why I add the > > bypass in blk_mq_dispatch_plug_list(), and just grab lock for each batch > > given now blk_execute_rq_nowait() is fast path for nvme uring pt io feature. > > We really have two types of passthrough - normal kind of IO, and > potential error recovery etc IO. The former can plug just fine, and I > don't think we should treat it differently. Might make more sense to > just bypass plugging for error handling type of IO, or pt that doesn't > transfer any data to avoid having a NULL bio inserted into the > scheduler. So far, I guess we can't distinguish the two types. The simplest change could be the previous one[1] by not plugging request of !rq->bio, but pt request with user IO still can be added to scheduler, so the question is if pt request with user IO should be queued to scheduler? [1] https://lore.kernel.org/linux-block/CAGS2=Yob_Ud9A-aTu5hQt8+kW4cyrLX12hNJTrRkJYigFT-AmA@mail.gmail.com/T/#m8ac66a4294707cf1ba301ef2bd2a70f9a0e2c8cb Thanks, Ming