Re: [PATCH v2 net-next] net: Implement fast csum_partial for x86_64

Hi,

On 05.01.2016 19:41, Tom Herbert wrote:

Implement assembly routine for csum_partial for 64 bit x86. This
primarily speeds up checksum calculation for smaller lengths such as
those that are present when doing skb_postpull_rcsum when getting
CHECKSUM_COMPLETE from device or after CHECKSUM_UNNECESSARY
conversion.

This implementation is similar to csum_partial implemented in
checksum_32.S, however since we are dealing with 8 bytes at a time
there are more cases for small lengths-- for that we employ a jump
table. Also, we don't do anything special for alignment, unaligned
accesses on x86 do not appear to be a performance issue.

Testing:

Verified correctness by testing arbitrary length buffer filled with
random data. For each buffer I compared the computed checksum
using the original algorithm for each possible alignment (0-7 bytes).

Checksum performance:

Isolating old and new implementation for some common cases:

                         Old      New
Case                    nsecs    nsecs     Improvement
---------------------+--------+--------+-----------------------------
1400 bytes (0 align)    194.5    174.3     10%    (Big packet)
40 bytes (0 align)      13.8     5.8       57%    (Ipv6 hdr common case)
8 bytes (4 align)       8.4      2.9       65%    (UDP, VXLAN in IPv4)
14 bytes (0 align)      10.6     5.8       45%    (Eth hdr)
14 bytes (4 align)      10.8     5.8       46%    (Eth hdr in IPv4)

Signed-off-by: Tom Herbert <redacted>

Also,

Acked-by: Hannes Frederic Sowa <redacted>

Tested with the same test cases as the old patch and showed no problems 
and same improvements.

Tom, did you have a look if it makes sense to add a second carry 
addition train with the adcx instruction, which does not signal carry 
via the carry flag but with the overflow flag? This instruction should 
not have any dependencies with the adc instructions and could help the 
CPU to parallelize the code even more (increased instructions per cycle).

Thanks,
Hannes