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 F1328C43334 for ; Thu, 14 Jul 2022 15:14:54 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S238391AbiGNPOy (ORCPT ); Thu, 14 Jul 2022 11:14:54 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:51256 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232191AbiGNPOx (ORCPT ); Thu, 14 Jul 2022 11:14:53 -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 ESMTP id 816334507F for ; Thu, 14 Jul 2022 08:14:52 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1657811691; 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=OeF6usJeTyAcTdKe5UBwqQy7jEW4hkpRgeYYoNc1qmc=; b=KIFkiLckc39SYnjHrymz5JKb1GASAtimkW2ppnmVmkNwnAQ6ZPrcrKWMttgUCH/aagm6bR Q8l1w/y00w47nXjrFGGokUlgYfZ7hJtqxtYlxMJacRnbl+042mYMVH11Wcc00iZCmwQpJm jKvv+fHVsHIEQuDpBVjQ7BqCjplI+jo= 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-324-cF3MWFlaO3-_NJpZ-c4sCw-1; Thu, 14 Jul 2022 11:14:46 -0400 X-MC-Unique: cF3MWFlaO3-_NJpZ-c4sCw-1 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.rdu2.redhat.com [10.11.54.7]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 9E7C718F025F; Thu, 14 Jul 2022 15:14:45 +0000 (UTC) Received: from T590 (ovpn-8-16.pek2.redhat.com [10.72.8.16]) by smtp.corp.redhat.com (Postfix) with ESMTPS id B2B05140EBE3; Thu, 14 Jul 2022 15:14:37 +0000 (UTC) Date: Thu, 14 Jul 2022 23:14:32 +0800 From: Ming Lei To: Kanchan Joshi Cc: Sagi Grimberg , hch@lst.de, kbusch@kernel.org, axboe@kernel.dk, io-uring@vger.kernel.org, linux-nvme@lists.infradead.org, linux-block@vger.kernel.org, asml.silence@gmail.com, joshiiitr@gmail.com, anuj20.g@samsung.com, gost.dev@samsung.com, ming.lei@redhat.com Subject: Re: [PATCH for-next 4/4] nvme-multipath: add multipathing for uring-passthrough commands Message-ID: References: <20220711110155.649153-1-joshi.k@samsung.com> <20220711110155.649153-5-joshi.k@samsung.com> <3fc68482-fb24-1f39-5428-faa3a8db9ecb@grimberg.me> <20220711183746.GA20562@test-zns> <5f30c7de-03b1-768a-d44f-594ed2d1dc75@grimberg.me> <20220712042332.GA14780@test-zns> <3a2b281b-793b-b8ad-6a27-138c89a46fac@grimberg.me> <20220713053757.GA15022@test-zns> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20220713053757.GA15022@test-zns> X-Scanned-By: MIMEDefang 2.85 on 10.11.54.7 Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On Wed, Jul 13, 2022 at 11:07:57AM +0530, Kanchan Joshi wrote: > > > > > > The way I would do this that in nvme_ioucmd_failover_req (or in the > > > > > > retry driven from command retriable failure) I would do the above, > > > > > > requeue it and kick the requeue work, to go over the requeue_list and > > > > > > just execute them again. Not sure why you even need an explicit retry > > > > > > code. > > > > > During retry we need passthrough command. But passthrough command is not > > > > > stable (i.e. valid only during first submission). We can make it stable > > > > > either by: > > > > > (a) allocating in nvme (b) return -EAGAIN to io_uring, and > > > > > it will do allocate + deferral > > > > > Both add a cost. And since any command can potentially fail, that > > > > > means taking that cost for every IO that we issue on mpath node. Even if > > > > > no failure (initial or subsquent after IO) occcured. > > > > > > > > As mentioned, I think that if a driver consumes a command as queued, > > > > it needs a stable copy for a later reformation of the request for > > > > failover purposes. > > > > > > So what do you propose to make that stable? > > > As I mentioned earlier, stable copy requires allocating/copying in fast > > > path. And for a condition (failover) that may not even occur. > > > I really think currrent solution is much better as it does not try to make > > > it stable. Rather it assembles pieces of passthrough command if retry > > > (which is rare) happens. > > > > Well, I can understand that io_uring_cmd is space constrained, otherwise > > we wouldn't be having this discussion. > > Indeed. If we had space for keeping passthrough command stable for > retry, that would really have simplified the plumbing. Retry logic would > be same as first submission. > > > However io_kiocb is less > > constrained, and could be used as a context to hold such a space. > > > > Even if it is undesired to have io_kiocb be passed to uring_cmd(), it > > can still hold a driver specific space paired with a helper to obtain it > > (i.e. something like io_uring_cmd_to_driver_ctx(ioucmd) ). Then if the > > space is pre-allocated it is only a small memory copy for a stable copy > > that would allow a saner failover design. > > I am thinking along the same lines, but it's not about few bytes of > space rather we need 80 (72 to be precise). Will think more, but > these 72 bytes really stand tall in front of my optimism. > > Do you see anything is possible in nvme-side? > Now also passthrough command (although in a modified form) gets copied > into this preallocated space i.e. nvme_req(req)->cmd. This part - I understand it can't be allocated in nvme request which is freed during retry, and looks the extra space has to be bound with io_uring_cmd. thanks, Ming