netdev - Re: [PATCH 2/7] sh_eth: RX checksum offload support

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Message-ID: <d95fff4f-7750-87c4-3664-eb5efb593b75@cogentembedded.com>
Date:   Mon, 28 Jan 2019 18:45:26 +0300
From:   Sergei Shtylyov <sergei.shtylyov@...entembedded.com>
To:     Simon Horman <horms@...ge.net.au>
Cc:     netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
        linux-renesas-soc@...r.kernel.org, linux-sh@...r.kernel.org
Subject: Re: [PATCH 2/7] sh_eth: RX checksum offload support

Hello!

On 01/28/2019 03:18 PM, Simon Horman wrote:

>> Add support for the RX checksum offload. This is enabled by default and
>> may be disabled and re-enabled using 'ethtool':
>>
>> # ethtool -K eth0 rx {on|off}
>>
>> Some Ether MACs provide a simple checksumming scheme which appears to be
>> completely compatible with CHECKSUM_COMPLETE: sum of all packet data after
>> the L2 header is appended to packet data; this may be trivially read by
>> the driver and used to update the skb accordingly. The same checksumming
>> scheme is implemented in the EtherAVB MACs and now supported by tha 'ravb'
>> driver.
>>
>> In terms of performance, throughput is close to gigabit line rate with the
>> RX checksum offload both enabled and disabled.  The 'perf' output, however,
>> appears to indicate that significantly less time is spent in do_csum() --
>> this is as expected.
> 
> Nice.
> 
> FYI, this seems similar to what I observed for RAVB, perhaps on H3 I don't
> exactly recall. On E3, which has less CPU power, I recently observed that
> with rx-csum enabled I can achieve gigabit line rate, but with rx-csum
> disabled throughput is significantly lower. I.e. on that system throughput
> is CPU bound with 1500 byte packets unless rx-csum enabled.

   Unfortunately, we can't teset these patches on the other gen3 boards. ISTR
you have RZ/A1H board... if it's still with you, I'd appreciate testing.

> Next point:
> 
> 2da64300fbc ("ravb: expand rx descriptor data to accommodate hw checksum")
> is fresh in my mind and I wonder if mdp->rx_buf_sz needs to grow to ensure
> that there is always enough space for the csum.

   Well, if you look at sh_eth_ring_init(), you'll see that the driver reserves
plenty of space at the end the RX buffers.

> In particular, have you
> tested this with MTU-size frames with VLANs. (My test is to run iperf3 over
> a VLAN netdev, netperf over a VLAN netdev would likely work just as well.)

   Could you refresh me on how to bring up a VLAN on a given interface?

[...]
>> The above results collected on the R-Car V3H Starter Kit board.
>>
>> Based on the commit 4d86d3818627 ("ravb: RX checksum offload")...
>>
>> Signed-off-by: Sergei Shtylyov <sergei.shtylyov@...entembedded.com>
[...]

MBR, Sergei