From mboxrd@z Thu Jan  1 00:00:00 1970
From: Ferruh Yigit <ferruh.yigit@intel.com>
Subject: Re: [PATCH v3 3/4] bonding: take queue spinlock in rx/tx
 burst functions
Date: Wed, 15 Feb 2017 18:01:45 +0000
Message-ID: <d95e6d02-6564-0c49-3be1-d7f9db378ff3@intel.com>
References: <1464280727-25752-2-git-send-email-bernard.iremonger@intel.com>
 <5839452.IzJ3v2K0cK@xps13>
 <8CEF83825BEC744B83065625E567D7C21A03A6D7@IRSMSX108.ger.corp.intel.com>
 <1632739.trvk2NaClS@xps13>
Mime-Version: 1.0
Content-Type: text/plain; charset=windows-1252
Content-Transfer-Encoding: 8bit
Cc: DPDK <dev@dpdk.org>
To: Thomas Monjalon <thomas.monjalon@6wind.com>,
 Bernard Iremonger <bernard.iremonger@intel.com>,
 Bruce Richardson <bruce.richardson@intel.com>,
 "Ananyev, Konstantin" <konstantin.ananyev@intel.com>,
 Declan Doherty <declan.doherty@intel.com>
Return-path: <dev-bounces@dpdk.org>
Received: from mga03.intel.com (mga03.intel.com [134.134.136.65])
 by dpdk.org (Postfix) with ESMTP id B7F1898
 for <dev@dpdk.org>; Wed, 15 Feb 2017 19:01:49 +0100 (CET)
In-Reply-To: <1632739.trvk2NaClS@xps13>
List-Id: DPDK patches and discussions <dev.dpdk.org>
List-Unsubscribe: <http://dpdk.org/ml/options/dev>,
 <mailto:dev-request@dpdk.org?subject=unsubscribe>
List-Archive: <http://dpdk.org/ml/archives/dev/>
List-Post: <mailto:dev@dpdk.org>
List-Help: <mailto:dev-request@dpdk.org?subject=help>
List-Subscribe: <http://dpdk.org/ml/listinfo/dev>,
 <mailto:dev-request@dpdk.org?subject=subscribe>
Errors-To: dev-bounces@dpdk.org
Sender: "dev" <dev-bounces@dpdk.org>

On 6/16/2016 7:38 PM, thomas.monjalon at 6wind.com (Thomas Monjalon) wrote:
> 2016-06-16 16:41, Iremonger, Bernard:
>> Hi Thomas,
>> <snip>
>>> 2016-06-16 15:32, Bruce Richardson:
>>>> On Mon, Jun 13, 2016 at 01:28:08PM +0100, Iremonger, Bernard wrote:
>>>>>> Why does this particular PMD need spinlocks when doing RX and TX,
>>>>>> while other device types do not? How is adding/removing devices
>>>>>> from a bonded device different to other control operations that
>>>>>> can be done on physical PMDs? Is this not similar to say bringing
>>>>>> down or hotplugging out a physical port just before an RX or TX
>>> operation takes place?
>>>>>> For all other PMDs we rely on the app to synchronise control and
>>>>>> data plane operation - why not here?
>>>>>>
>>>>>> /Bruce
>>>>>
>>>>> This issue arose during VM live migration testing.
>>>>> For VM live migration it is necessary (while traffic is running) to be able to
>>> remove a bonded slave device, stop it, close it and detach it.
>>>>> It a slave device is removed from a bonded device while traffic is running
>>> a segmentation fault may occur in the rx/tx burst function. The spinlock has
>>> been added to prevent this occurring.
>>>>>
>>>>> The bonding device already uses a spinlock to synchronise between the
>>> add and remove functionality and the slave_link_status_change_monitor
>>> code.
>>>>>
>>>>> Previously testpmd did not allow, stop, close or detach of PMD while
>>>>> traffic was running. Testpmd has been modified with the following
>>>>> patchset
>>>>>
>>>>> http://dpdk.org/dev/patchwork/patch/13472/
>>>>>
>>>>> It now allows stop, close and detach of a PMD provided in it is not
>>> forwarding and is not a slave of bonded PMD.
>>>>>
>>>> I will admit to not being fully convinced, but if nobody else has any
>>>> serious objections, and since this patch has been reviewed and acked,
>>>> I'm ok to merge it in. I'll do so shortly.
>>>
>>> Please hold on.
>>> Seeing locks introduced in the Rx/Tx path is an alert.
>>> We clearly need a design document to explain where locks can be used and
>>> what are the responsibility of the control plane.
>>> If everybody agrees in this document that DPDK can have some locks in the
>>> fast path, then OK to merge it.
>>>
>>> So I would say NACK for 16.07 and maybe postpone to 16.11.
>>
>> Looking at the documentation for the bonding PMD.
>>
>> http://dpdk.org/doc/guides/prog_guide/link_bonding_poll_mode_drv_lib.html
>>
>> In section 10.2 it states the following:
>>
>> Bonded devices support the dynamical addition and removal of slave devices using the rte_eth_bond_slave_add / rte_eth_bond_slave_remove APIs.
>>
>> If a slave device is added or removed while traffic is running, there is the possibility of a segmentation fault in the rx/tx burst functions. This is most likely to occur in the round robin bonding mode.
>>
>> This patch set fixes what appears to be a bug in the bonding PMD.
> 
> It can be fixed by removing this statement in the doc.
> 
> One of the design principle of DPDK is to avoid locks.
> 
>> Performance measurements have been made with this patch set applied and without the patches applied using 64 byte packets. 
>>
>> With the patches applied the following drop in performance was observed:
>>
>> % drop for fwd+io:	0.16%
>> % drop for fwd+mac:	0.39%
>>
>> This patch set has been reviewed and ack'ed, so I think it should be applied in 16.07
> 
> I understand your point of view and I gave mine.
> Now we need more opinions from others.
> 

Hi,

These patches are sitting in the patchwork for a long time. Discussion
never concluded and patches kept deferred each release.

I think we should give a decision about them:

1- We can merge them in this release, they are fixing a valid problem,
and patches are already acked.

2- We can reject them, if not having them for more than six months not
caused a problem, perhaps they are not really that required. And if
somebody needs them in the future, we can resurrect them from patchwork.

I vote for option 2, any comments?

Thanks,
ferruh