netdev - Re: [PATCH net-next 1/2] net: Take common prefetch code structure into a function

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Date:   Wed, 8 May 2019 07:48:00 +0000
From:   Tariq Toukan <tariqt@...lanox.com>
To:     Jesper Dangaard Brouer <brouer@...hat.com>,
        Jakub Kicinski <jakub.kicinski@...ronome.com>
CC:     "David S. Miller" <davem@...emloft.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Eran Ben Elisha <eranbe@...lanox.com>,
        Alexander Duyck <alexander.duyck@...il.com>
Subject: Re: [PATCH net-next 1/2] net: Take common prefetch code structure
 into a function



On 5/7/2019 1:08 PM, Jesper Dangaard Brouer wrote:
> On Mon, 6 May 2019 16:51:57 -0700
> Jakub Kicinski <jakub.kicinski@...ronome.com> wrote:
> 
>> On Sun,  5 May 2019 13:36:06 +0300, Tariq Toukan wrote:
>>> Many device drivers use the same prefetch code structure to
>>> deal with small L1 cacheline size.
>>> Take this code into a function and call it from the drivers.
>>>
>>> Suggested-by: Jakub Kicinski <jakub.kicinski@...ronome.com>
>>> Signed-off-by: Tariq Toukan <tariqt@...lanox.com>
>>> Reviewed-by: Saeed Mahameed <saeedm@...lanox.com>
>>> Cc: Jesper Dangaard Brouer <brouer@...hat.com>
>>
>> We could bike shed on the name a little - net_prefetch_headers() ?
>> but at least a short kdoc explanation for the purpose of this helper
>> would be good IMHO.
> 
> I would at least improve the commit message.  As Alexander so nicely
> explained[1], this prefetch purpose: "the 2 prefetches are needed for x86
> if you want a full TCP or IPv6 header pulled into the L1 cache for
> instance."  Although, this is not true for a minimum TCP-packet
> Eth(14)+IP(20)+TCP(20)=54 bytes. An I missing an alignment in my calc?
> 
> [1] https://lore.kernel.org/netdev/CAKgT0UeEL3W42eDqSt97xnn3tXDtWMf4sdPByAtvbx=Z7Sx7hQ@mail.gmail.com/
> 
> The name net_prefetch_headers() suggested by Jakub makes sense, as this
> indicate that this should be used for prefetching packet headers.
> 
> As Alexander also explained, I was wrong in thinking the HW DCU (Data
> Cache Unit) prefetcher will fetch two cache-lines automatically.  As
> the DCU prefetcher is a streaming prefetcher, and doesn't see our
> access pattern, which is why we need this.
> 

Thanks all for your comments.
I will fix and re-spin once the window re-opens.

Tariq