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On RX an SKB is allocated and the received buffer is copied into it.
But on some architectures, the memcpy() needs the source and destination
buffers to have the same alignment to be efficient.
This is not our case, because SKB data pointer is misaligned by two bytes
to compensate the ethernet header.
Align the RX buffer the same way as the SKB one, so the copy is faster.
An iperf3 RX test gives a decent improvement on a RISC-V machine:
before:
[ ID] Interval Transfer Bitrate Retr
[ 5] 0.00-10.00 sec 733 MBytes 615 Mbits/sec 88 sender
[ 5] 0.00-10.01 sec 730 MBytes 612 Mbits/sec receiver
after:
[ ID] Interval Transfer Bitrate Retr
[ 5] 0.00-10.00 sec 1.10 GBytes 942 Mbits/sec 0 sender
[ 5] 0.00-10.00 sec 1.09 GBytes 940 Mbits/sec receiver
And the memcpy() overhead during the RX drops dramatically.
before:
Overhead Shared O Symbol
43.35% [kernel] [k] memcpy
33.77% [kernel] [k] __asm_copy_to_user
3.64% [kernel] [k] sifive_l2_flush64_range
after:
Overhead Shared O Symbol
45.40% [kernel] [k] __asm_copy_to_user
28.09% [kernel] [k] memcpy
4.27% [kernel] [k] sifive_l2_flush64_range
Signed-off-by: Matteo Croce <mcroce@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
a955318fe6