Re: [dpdk-dev] [PATCH v3] ethdev: fix ethdev data alignment

[Ferruh Yigit <ferruh.yigit@intel.com>]

On 2/12/2018 2:11 PM, Jerin Jacob wrote:
> -----Original Message-----
>> Date: Mon, 12 Feb 2018 14:02:17 +0000
>> From: Matan Azrad <matan@mellanox.com>
>> To: Jerin Jacob <jerin.jacob@caviumnetworks.com>
>> CC: "dev@dpdk.org" <dev@dpdk.org>, "ferruh.yigit@intel.com"
>>  <ferruh.yigit@intel.com>, "stable@dpdk.org" <stable@dpdk.org>, Thomas
>>  Monjalon <thomas@monjalon.net>, Konstantin Ananyev
>>  <konstantin.ananyev@intel.com>, Pavan Nikhilesh
>>  <pbhagavatula@caviumnetworks.com>
>> Subject: RE: [dpdk-dev] [PATCH v3] ethdev: fix ethdev data alignment
>>
>>
>>
>> From: Jerin Jacob, Sent: Monday, February 12, 2018 3:51 PM
>>> -----Original Message-----
>>>> Date: Mon, 12 Feb 2018 13:44:54 +0000
>>>> From: Matan Azrad <matan@mellanox.com>
>>>> To: Jerin Jacob <jerin.jacob@caviumnetworks.com>, "dev@dpdk.org"
>>>>  <dev@dpdk.org>
>>>> CC: "ferruh.yigit@intel.com" <ferruh.yigit@intel.com>, "stable@dpdk.org"
>>>>  <stable@dpdk.org>, Thomas Monjalon <thomas@monjalon.net>,
>>> Konstantin
>>>> Ananyev <konstantin.ananyev@intel.com>, Pavan Nikhilesh
>>>> <pbhagavatula@caviumnetworks.com>
>>>> Subject: RE: [dpdk-dev] [PATCH v3] ethdev: fix ethdev data alignment
>>>>
>>>> Hi Jerin
>>>>
>>>> From: Jerin Jacob [mailto:jerin.jacob@caviumnetworks.com]
>>>>> The struct rte_eth_dev_data is used in ethdev fastpath routines and
>>>>> it not aligned to cache line size. This patch fixes the ethdev data
>>> alignment.
>>>>>
>>>>> The alignment was broken from the "first public release" changeset
>>>>> where ethdev data address was aligned only to the first port.
>>>>> Remaining ports alignment was defined by the size of the struct
>>>>> (rte_eth_dev_data). This scheme is not guaranteed to be cache line
>>>>> aligned all the time.
>>>>>
>>>>> "ethdev: add port ownership" change set introduced a
>>>>> rte_eth_dev_shared_data container for port ownership change, This
>>>>> resulted in rte_eth_dev->data memory for the first port also as
>>>>> cache unaligned.
>>>>>
>>>>> Added a compiler alignment attribute to make sure rte_eth_dev->data
>>>>> always cache aligned so that CPU/compiler
>>>>> 1) Avoid sharing the element with another cache line
>>>>> 2) Can load/store the elements in struct rte_eth_dev_data as
>>>>> naturally aligned.
>>>>>
>>>>> Some platform like thunderX could see performance regression of 1%
>>>>> at
>>>>> "ethdev: add port ownership" change set with
>>>>> 1 port/1 queue l3fwd application and this patch fixes that regression.
>>>>>
>>>>> example command:
>>>>> sudo ./examples/l3fwd/build/l3fwd -c 0xff00 -- -p 0x1 --config="(0,0,9)"
>>>>>
>>>>> Fixes: af75078fece3 ("first public release")
>>>>> Fixes: 5b7ba31148a8 ("ethdev: add port ownership")
>>>>
>>>> I don't think you need the add the 5b7ba31148a8 fix line.
>>>> Maybe think about it in the next way:
>>>> Is your fix can stay as a same fix before the port ownership feature
>>> addition?
>>>> If yes, You are not fixing it.
>>>
>>> I don't think so. 5b7ba31148a8 breaking the first port alignment case(as
>>> mentioned in the commit log clearly).
>>> First port alignment was correct prior to 5b7ba31148a8 change set
>>>
>>> Do you agree?
>>
>> No.
>> The root cause of the issue this fix is fixing, is that the structure is not aligned, it is not relevant to port ownership.
>> The port ownership just exposes it.
> 
> My point is simple. First port alignment was correct prior to 5b7ba31148a8 change set.
> it is broken from there.

+1 for this logic, adding fixes line makes sense for me.

> 
> It is minor thing. I let, ethdev maintainer to decide what to added in Fixes on apply.
>