netdev - Re: [PATCH v7 0/7] netdev: intel: Eliminate duplicate barriers on weakly-ordered archs

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Date:   Tue, 27 Mar 2018 13:33:48 -0400
From:   Sinan Kaya <okaya@...eaurora.org>
To:     jeffrey.t.kirsher@...el.com
Cc:     Alexander Duyck <alexander.duyck@...il.com>,
        intel-wired-lan <intel-wired-lan@...ts.osuosl.org>,
        Netdev <netdev@...r.kernel.org>,
        Timur Tabi <timur@...eaurora.org>, sulrich@...eaurora.org,
        linux-arm-msm@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v7 0/7] netdev: intel: Eliminate duplicate barriers on
 weakly-ordered archs

On 3/27/2018 12:54 PM, Jeff Kirsher wrote:
> On Tue, 2018-03-27 at 08:42 -0400, Sinan Kaya wrote:
>> On 3/23/2018 10:34 PM, okaya@...eaurora.org wrote:
>>> On 2018-03-23 19:58, Jeff Kirsher wrote:
>>>> On Fri, 2018-03-23 at 14:53 -0700, Alexander Duyck wrote:
>>>>> On Fri, Mar 23, 2018 at 11:52 AM, Sinan Kaya <okaya@...eaurora.
>>>>> org>
>>>>> wrote:
>>>>>> Code includes wmb() followed by writel() in multiple places.
>>>>>> writel()
>>>>>> already has a barrier on some architectures like arm64.
>>>>>>
>>>>>> This ends up CPU observing two barriers back to back before
>>>>>> executing
>>>>>> the
>>>>>> register write.
>>>>>>
>>>>>> Since code already has an explicit barrier call, changing
>>>>>> writel() to
>>>>>> writel_relaxed().
>>>>>>
>>>>>> I did a regex search for wmb() followed by writel() in each
>>>>>> drivers
>>>>>> directory.
>>>>>> I scrubbed the ones I care about in this series.
>>>>>>
>>>>>> I considered "ease of change", "popular usage" and
>>>>>> "performance
>>>>>> critical
>>>>>> path" as the determining criteria for my filtering.
>>>>>>
>>>>>> We used relaxed API heavily on ARM for a long time but
>>>>>> it did not exist on other architectures. For this reason,
>>>>>> relaxed
>>>>>> architectures have been paying double penalty in order to use
>>>>>> the
>>>>>> common
>>>>>> drivers.
>>>>>>
>>>>>> Now that relaxed API is present on all architectures, we can
>>>>>> go and
>>>>>> scrub
>>>>>> all drivers to see what needs to change and what can remain.
>>>>>>
>>>>>> We start with mostly used ones and hope to increase the
>>>>>> coverage over
>>>>>> time.
>>>>>> It will take a while to cover all drivers.
>>>>>>
>>>>>> Feel free to apply patches individually.
>>>>>
>>>>> I looked over the set and they seem good.
>>>>>
>>>>> Reviewed-by: Alexander Duyck <alexander.h.duyck@...el.com>
>>>>
>>>> Grrr, patch 1 does not apply cleanly to my next-queue tree (dev-
>>>> queue
>>>> branch).  I will deal with this series in a day or two, after I
>>>> have dealt
>>>> with my driver pull requests.
>>>
>>> Sorry, you will have to replace the ones you took from me.
>>
>> Double sorry now.
>>
>> I don't know if you have been following "RFC on writel and
>> writel_relaxed" thread
>> or not but there are some new developments about wmb() requirement. 
>>
>> Basically, wmb() should never be used before writel() as writel()
>> seem to
>> provide coherency and observability guarantee.
>>
>> wmb()+writel_relaxed() is slower on some architectures than plain
>> writel()
>>
>> I'll have to rework these patches to have writel() only. 
>>
>> Are you able to drop the applied ones so that I can post V8 or is it
>> too late?
> 
> Currently I do not have any of your patches applied to my next-queue
> tree (dev-queue branch).  So feel free to do any revisions you need to
> do and to re-submit to intel-wired-lan@...ts.osuosl.org (IWL) mailing
> list.
> 

Thanks, Good to know. 

I'm waiting for the discussion to settle. I'll update as soon as I get
a clear direction.

-- 
Sinan Kaya
Qualcomm Datacenter Technologies, Inc. as an affiliate of Qualcomm Technologies, Inc.
Qualcomm Technologies, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project.