From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 061244A11 for ; Thu, 12 Jun 2025 03:16:37 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=170.10.129.124 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1749698200; cv=none; b=BZoMLQ1LqOtEgUJ2JhyHe8W+j1V9cNMR2qgBt+FJtYZeGVwqbKfUDT1JvOzqJ2+2S224jqQBQ2v3NbCsEklZDJYkQxFgZ0lyPRe/Rz8vz3VXso64KJq0zPL2eHxhcnkGQlitvNNse2uZpJVmjwv9EDhb+mTdB1D7sWLqq+rnTl4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1749698200; c=relaxed/simple; bh=O6YMSS+BC9emiQhoNQe/CBW89b2r5r3b1yGttczaS70=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=ZiHBAleDIJ8p/3X5jN5/N2TuQcz6vSsrwriU7ddiZ9bIBDtnigyZY9W9jrCnAKzT1rG4ttK1mBQEyMNfvCBXgNNIpCxXUSYXbpQvqBzEGeyWZbstjvNPxFu3wZEvvSJ/2zonH1TkQgcHAxTqVfkaEHTn/Qomx0JsT0iVGbaSxBQ= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com; spf=pass smtp.mailfrom=redhat.com; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b=M0Ya999B; arc=none smtp.client-ip=170.10.129.124 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=redhat.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="M0Ya999B" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1749698196; 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: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=xADfTy0SIBAHn67YyeojEKOuM6wfSoiwVdGOv86vpeE=; b=M0Ya999BNywVnz3movbZYeSpOMJslSBRoTSkpsolEcxcSUkIAqDeqGLbLkZEpVej1x0VOn oKRljr83lyrhKhrvAmrsDXbd1A5+HplCrYk22QbRW9ouCv7EcPHAXM9n8NnjGUHj/guIS4 UpIsLJmTx3P7Z2NXShs6bzPQv0yKmfU= Received: from mx-prod-mc-05.mail-002.prod.us-west-2.aws.redhat.com (ec2-54-186-198-63.us-west-2.compute.amazonaws.com [54.186.198.63]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-85-60Wd21jVM9qEaVX6pRhWGQ-1; Wed, 11 Jun 2025 23:16:33 -0400 X-MC-Unique: 60Wd21jVM9qEaVX6pRhWGQ-1 X-Mimecast-MFC-AGG-ID: 60Wd21jVM9qEaVX6pRhWGQ_1749698192 Received: from mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.4]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-05.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id D89091956095; Thu, 12 Jun 2025 03:16:31 +0000 (UTC) Received: from fedora (unknown [10.72.116.109]) by mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 61E6C30002C3; Thu, 12 Jun 2025 03:16:27 +0000 (UTC) Date: Thu, 12 Jun 2025 11:16:23 +0800 From: Ming Lei To: Caleb Sander Mateos Cc: Jens Axboe , linux-block@vger.kernel.org, Uday Shankar Subject: Re: [PATCH] ublk: document auto buffer registration(UBLK_F_AUTO_BUF_REG) Message-ID: References: <20250609121426.1997271-1-ming.lei@redhat.com> Precedence: bulk X-Mailing-List: linux-block@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.4 On Wed, Jun 11, 2025 at 08:54:53AM -0700, Caleb Sander Mateos wrote: > On Mon, Jun 9, 2025 at 7:07 PM Ming Lei wrote: > > > > On Mon, Jun 09, 2025 at 03:29:34PM -0700, Caleb Sander Mateos wrote: > > > On Mon, Jun 9, 2025 at 5:14 AM Ming Lei wrote: > > > > > > > > Document recently merged feature auto buffer registration(UBLK_F_AUTO_BUF_REG). > > > > > > > > Signed-off-by: Ming Lei > > > > > > Thanks, this is a nice explanation. Just a few suggestions. > > > > > > > --- > > > > Documentation/block/ublk.rst | 67 ++++++++++++++++++++++++++++++++++++ > > > > 1 file changed, 67 insertions(+) > > > > > > > > diff --git a/Documentation/block/ublk.rst b/Documentation/block/ublk.rst > > > > index c368e1081b41..16ffca54eed4 100644 > > > > --- a/Documentation/block/ublk.rst > > > > +++ b/Documentation/block/ublk.rst > > > > @@ -352,6 +352,73 @@ For reaching best IO performance, ublk server should align its segment > > > > parameter of `struct ublk_param_segment` with backend for avoiding > > > > unnecessary IO split, which usually hurts io_uring performance. > > > > > > > > +Auto Buffer Registration > > > > +------------------------ > > > > + > > > > +The ``UBLK_F_AUTO_BUF_REG`` feature automatically handles buffer registration > > > > +and unregistration for I/O requests, which simplifies the buffer management > > > > +process and reduces overhead in the ublk server implementation. > > > > + > > > > +This is another feature flag for using zero copy, and it is compatible with > > > > +``UBLK_F_SUPPORT_ZERO_COPY``. > > > > + > > > > +Feature Overview > > > > +~~~~~~~~~~~~~~~~ > > > > + > > > > +This feature automatically registers request buffers to the io_uring context > > > > +before delivering I/O commands to the ublk server and unregisters them when > > > > +completing I/O commands. This eliminates the need for manual buffer > > > > +registration/unregistration via ``UBLK_IO_REGISTER_IO_BUF`` and > > > > +``UBLK_IO_UNREGISTER_IO_BUF`` commands, then IO handling in ublk server > > > > +can avoid dependency on the two uring_cmd operations. > > > > + > > > > +This way not only simplifies ublk server implementation, but also makes > > > > +concurrent IO handling becomes possible. > > > > > > I'm not sure what "concurrent IO handling" refers to. Any ublk server > > > can handle incoming I/O requests concurrently, regardless of what > > > features it has enabled. Do you mean it avoids the need for linked > > > io_uring requests to properly order buffer registration and > > > unregistration with the I/O operations using the registered buffer? > > > > Yes, if io_uring OPs depends on buffer registering & unregistering, these > > OPs can't be issued concurrently any more, that is one io_uring constraint. > > > > I will add the above words. > > > > > > > > > + > > > > +Usage Requirements > > > > +~~~~~~~~~~~~~~~~~~ > > > > + > > > > +1. The ublk server must create a sparse buffer table on the same ``io_ring_ctx`` > > > > + used for ``UBLK_IO_FETCH_REQ`` and ``UBLK_IO_COMMIT_AND_FETCH_REQ``. > > > > + > > > > +2. If uring_cmd is issued on a different ``io_ring_ctx``, manual buffer > > > > + unregistration is required. > > > > > > nit: don't think this needs to be a separate point, could be combined with (1). > > > > OK. > > > > > > > > > + > > > > +3. Buffer registration data must be passed via uring_cmd's ``sqe->addr`` with the > > > > + following structure:: > > > > > > nit: extra ":" > > > > In reStructuredText (reST), the double colon :: serves as a literal block marker to > > indicate preformatted text. > > > > > > > > > + > > > > + struct ublk_auto_buf_reg { > > > > + __u16 index; /* Buffer index for registration */ > > > > + __u8 flags; /* Registration flags */ > > > > + __u8 reserved0; /* Reserved for future use */ > > > > + __u32 reserved1; /* Reserved for future use */ > > > > + }; > > > > > > Suggest using ublk_auto_buf_reg_to_sqe_addr()? Otherwise, it seems > > > ambiguous how this struct is "passed" in sqe->addr. > > > > OK > > > > > > > > > + > > > > +4. All reserved fields in ``ublk_auto_buf_reg`` must be zeroed. > > > > + > > > > +5. Optional flags can be passed via ``ublk_auto_buf_reg.flags``. > > > > + > > > > +Fallback Behavior > > > > +~~~~~~~~~~~~~~~~~ > > > > + > > > > +When ``UBLK_AUTO_BUF_REG_FALLBACK`` is enabled: > > > > + > > > > +1. If auto buffer registration fails: > > > > > > I would switch these. Both (1) and (2) refer to when auto buffer > > > registration fails. So I would expect something like: > > > > > > If auto buffer registration fails: > > > > > > 1. When ``UBLK_AUTO_BUF_REG_FALLBACK`` is enabled: > > > ... > > > 2. If fallback is not enabled: > > > ... > > > > > > > + - The uring_cmd is completed > > > > > > Maybe add "without registering the request buffer"? > > > > > > > + - ``UBLK_IO_F_NEED_REG_BUF`` is set in ``ublksrv_io_desc.op_flags`` > > > > + - The ublk server must manually register the buffer > > > > > > Only if it wants a registered buffer for the ublk request. Technically > > > the ublk server could decide to fall back on user-copy, for example. > > > > Good catch! > > > > > > > > > + > > > > +2. If fallback is not enabled: > > > > + - The ublk I/O request fails silently > > > > > > "silently" is a bit ambiguous. It's certainly not silent to the > > > application submitting the ublk I/O. Maybe say that the ublk I/O > > > request fails and no uring_cmd is completed to the ublk server? > > > > Yes, but the document focus on ublk side, and the client is generic > > for every driver, so I guess it may be fine. > > > > > > > > > + > > > > +Limitations > > > > +~~~~~~~~~~~ > > > > + > > > > +- Requires same ``io_ring_ctx`` for all operations > > > > > > Another limitation that prevents us from adopting the auto buffer > > > registration feature is the need to reserve a unique buffer table > > > index for every ublk tag on the io_ring_ctx. Since the io_ring_ctx > > > buffer table has a max size of 16K (could potentially be increased to > > > 64K), this limit is easily reached when there are a large number of > > > ublk devices or the ublk queue depth is large. I think we could remove > > > this limitation in the future by adding support for allocating buffer > > > indices on demand, analogous to IORING_FILE_INDEX_ALLOC. > > > > OK. > > > > But I guess it isn't big deal in reality since the task context should > > be saturated easily with so big setting. > > I don't know about your "reality" but it's certainly a big deal for us :) > To reduce contention on the blk-mq queues for the application > submitting I/O to the ublk devices, we want a large number of queues > for each ublk device. But we also want a large queue depth for each > individual queue to avoid the async request allocation path in case > any one application thread issues a lot of concurrent I/O to a single > ublk device. And we have 128 ublk devices, which again all want large > queue depths in case the application sends a lot of I/O to a single > ublk device. The result is that concurrently each ublk server thread > fetches 512K ublk I/Os, which is significantly above the io_ring_ctx > buffer table limit. Yes, you can setup 512K I/Os in single task/io_uring context, but how many can be actively handled during unit time? The number could be much less than 512k or 16K, because it is a single pthread/io_uring/cpu core, which may be saturated easily, so most of these IOs may wait somewhere for cpu or whatever resource. So when you have one nice per-task buf-index allocation algorithm, it may not be one issue given 16K is big enough. Thanks, Ming