RE: [PATCH] x86/lib: Remove the special case for odd-aligned buffers in csum_partial.c

From: Eric Dumazet

Sent: 13 December 2021 15:56

On Mon, Dec 13, 2021 at 7:37 AM David Laight [off-list ref] wrote:

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From: Dave Hansen

quoted

Sent: 13 December 2021 15:02

.c

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On 12/13/21 6:43 AM, David Laight wrote:

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There is no need to special case the very unusual odd-aligned buffers.
They are no worse than 4n+2 aligned buffers.

Signed-off-by: David Laight <redacted>
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On an i7-7700 misaligned buffers add 2 or 3 clocks (in 115) to a 512 byte
  checksum.
That is just measuring the main loop with an lfence prior to rdpmc to
read PERF_COUNT_HW_CPU_CYCLES.

I'm a bit confused by this changelog.

Are you saying that the patch causes a (small) performance regression?

Are you also saying that the optimization here is not worth it because
it saves 15 lines of code?  Or that the misalignment checks themselves
add 2 or 3 cycles, and this is an *optimization*?

I'm saying that it can't be worth optimising for a misaligned
buffer because the cost of the buffer being misaligned is so small.
So the test for a misaligned buffer are going to cost more than
and plausible gain.

Not only that the buffer will never be odd aligned at all.

The code is left in from a previous version that did do aligned
word reads - so had to do extra for odd alignment.

Note that code is doing misaligned reads for the more likely 4n+2
aligned ethernet receive buffers.
I doubt that even a test for that would be worthwhile even if you
were checksumming full sized ethernet packets.

So the change is deleting code that is never actually executed
from the hot path.

I think I left this code because I got confused with odd/even case,
but this is handled by upper functions like csum_block_add()

What matters is not if the start of a frag is odd/even, but what
offset it is in the overall ' frame', if a frame is split into multiple
areas (scatter/gather)

Yes odd length fragments are a different problem.

	David

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