netdev - Re: [PATCH] net: Make skb_segment not to compute checksum if network controller supports checksumming

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Message-ID: <CABGOaVRF9D21--aFi6VJ9MWMn0GxR-s8PssXnzbEjSneafbh5A@mail.gmail.com>
Date:   Fri, 28 Feb 2020 10:54:41 +0530
From:   Yadu Kishore <kyk.segfault@...il.com>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc:     Network Development <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>
Subject: Re: [PATCH] net: Make skb_segment not to compute checksum if network
 controller supports checksumming

> Did you measure a cycle efficiency improvement? As discussed in the
> referred email thread, the kernel uses checksum_and_copy because it is
> generally not significantly more expensive than copy alone
> skb_segment already is a very complex function. New code needs to
> offer a tangible benefit.

I ran iperf TCP Tx traffic of 1000 megabytes and captured the cpu cycle
utilization using perf:
"perf record -e cycles -a iperf \
-c 192.168.2.53 -p 5002 -fm -n 1048576000 -i 2  -l 8k -w 8m"

I see the following are the top consumers of cpu cycles:

Function                                   %cpu cycles
=======                                   =========
skb_mac_gso_segment            0.02
inet_gso_segment                     0.26
tcp4_gso_segment                    0.02
tcp_gso_segment                      0.19
skb_segment                             0.52
skb_copy_and_csum_bits         0.64
do_csum                                    7.25
memcpy                                     3.71
__alloc_skb                                0.91
==========                              ====
SUM                                           13.52

The measurement was done on an arm64 hikey960 platform running android with
linux kernel ver 4.19.23.
I see that 7.25% of the cpu cycles is spent computing the checksum against the
total of 13.52% of cpu cycles.
Which means around 52.9% of the total cycles is spent doing checksum.
Hence the attempt to try to offload checksum in the case of GSO also.

> Is this not already handled by __copy_skb_header above? If ip_summed
> has to be initialized, so have csum_start and csum_offset. That call
> should have initialized all three.

Thanks, I will look into why even though __copy_skb_header is being
called, I am still
seeing skb->ip_summed set to CHECKSUM_NONE in the network driver.