netdev - Re: Add NAPI support to ll

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Date:	Wed, 20 Apr 2011 13:06:44 +0200
From:	Michal Simek <monstr@...str.eu>
To:	Ben Hutchings <bhutchings@...arflare.com>
CC:	Eric Dumazet <eric.dumazet@...il.com>, netdev@...r.kernel.org
Subject: Re: Add NAPI support to ll_temac driver

Hi,

Ben Hutchings wrote:
> On Tue, 2011-04-19 at 14:48 +0200, Michal Simek wrote:
>> Ben Hutchings wrote:
>>> On Tue, 2011-04-19 at 12:43 +0200, Eric Dumazet wrote:
>>> [...]
>>>> One possible way to get better performance is to change driver to
>>>> allocate skbs only right before calling netif_rx(), so that you dont
>>>> have to access cold sk_buff data twice (once when allocating skb and put
>>>> it in ring buffer, a second time when receiving frame)
>>>>
>>>> drivers/net/niu.c is a good example for this (NAPI + netdev_alloc_skb()
>>>> just in time + pull in skbhead only first cache line of packet)
>>> [...]
>>>
>>> If the hardware can do RX checksumming (it's not clear) then the driver
>>> should pass the paged buffers into GRO and that will take care of skb
>>> allocation as necessary.
>> Hardware supports RX and TX partial checksumming. I can enable it. The driver 
>> has also this option and from my tests there is of course some performance 
>> improvemetn.
>>
>> Just for sure - here are links on documentation.
>> http://www.xilinx.com/support/documentation/ip_documentation/xps_ll_temac.pdf
>> or
>> http://www.xilinx.com/support/documentation/ip_documentation/axi_ethernet/v2_01_a/ds759_axi_ethernet.pdf
> 
> I'm not going to read those.  Just providing brief advice.
> 
>> About SKB allocation. I fixed our non mainline driver to allocate skb based on 
>> current mtu size. Mainline driver allocate max mtu (9k). This has also impact on 
>> performance because Microblaze works with smaller SKBs.
>>
>> Can you please be more specific about passing the paged buffers into GRO?
>> Or point me to any documentation or code which can help me to understand what 
>> that means.
> 
> You would use napi_get_frags() to get a new or recycled skb, fill in
> skb->frags, then call napi_gro_frags() to pass it into GRO.  The benet,
> cxgb3 and sfc drivers do this.

I have measured TX path and I have found that driver design is not so good.
It is always create one BD for one SKB and it starts DMA to copy packet to 
controller and send it. On 66MHz cpu it takes approximately 800 cpu cycles (not 
800 instructions) for sending (1.5k packet).
Current driver also enable irq for TX and when the packet is send interrupt is 
generated and skb is freed.
I see that it takes more time to handle the IRQ than busy waiting when DMA is 
done. I looked at sfc driver and there is any TX queue and any notifier. Hos 
does it work? Is it required to have any hw support?

Thanks,
Michal



-- 
Michal Simek, Ing. (M.Eng)
w: www.monstr.eu p: +42-0-721842854
Maintainer of Linux kernel 2.6 Microblaze Linux - http://www.monstr.eu/fdt/
Microblaze U-BOOT custodian
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