netdev - Re: [PATCH] mlx4: Use GFP_NOFS calls during the ipoib TX path when creating the QP

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Date:	Wed, 26 Feb 2014 00:48:57 +0200
From:	Or Gerlitz <or.gerlitz@...il.com>
To:	Jiri Kosina <jkosina@...e.cz>
Cc:	Roland Dreier <roland@...nel.org>, Amir Vadai <amirv@...lanox.com>,
	Eli Cohen <eli@....mellanox.co.il>,
	Or Gerlitz <ogerlitz@...lanox.com>,
	Eugenia Emantayev <eugenia@...lanox.com>,
	"David S. Miller" <davem@...emloft.net>,
	Mel Gorman <mgorman@...e.de>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mlx4: Use GFP_NOFS calls during the ipoib TX path when
 creating the QP

On Wed, Feb 26, 2014 at 12:40 AM, Jiri Kosina <jkosina@...e.cz> wrote:
> On Wed, 26 Feb 2014, Or Gerlitz wrote:
>
>>>> And what happens if you use IPoIB datagram mode, is/why the patch is
>>>> needed there?

>>> I admittedly am no infiniband expert, but my understanding is that in
>>> principle Connected/Datagram mode is about MTU and checksum
>>> offloading


>> yes, the differences between the mode relate to these aspects, however

> Thanks for confirming

Still, even if different, I still don't see why not use datagram mode
if the problem hits you only for connected mode. E.g datagram mode
supports LSO/GRO and TX/RX checksum offloads which should cover on the
smaller MTU vs. connected mode


>> > but the TX path is the same. Please correct me if I am wrong.

>> no, note that your patch only touched drivers/infiniband/ulp/ipoib/ipoib_cm.c
>> which is basically compiled out if you set CONFIG_INFINIBAND_IPOIB_CM,
>> so surely the TX path for the datagram vs. connected modes are
>> different.

> Yes, but for datagram mode, the tx_ring is allocated in a completely
> different way (not from kworker), so this might be a non-issue, right? I
> will have to look into it more deeply to be really sure; if you can
> provide your insight, that'd be helpful.


Note that even when operating in connected mode, the ipoib net-device
instance can speak in datagram mode with remote nodes who don't
support connected mor and/or when sending multicast -- specifically
ipoib_dev_init() does the setup of the TX ring. Maybe you can just try
this out and see if it works?
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