RE: [RFC PATCH v2] iov_iter: Convert iterate*() to inline funcs

From: David Howells

Sent: 14 August 2023 22:40

quoted

        _copy_from_iter                          inc 0x36e -> 0x395 +0x27

Here a disassembly of _copy_from_iter() from unpatched and patched, marked up for
the different iterator-type branches.  To summarise:

		UNPATCHED	PATCHED
		START	LEN	START	LEN
		=======	=======	=======	=======
Prologue	0	77	0	76
UBUF		77	36	76	36
IOVEC		113	148	112	105
BVEC		261	159	217	163
KVEC		420	125	380	116
XARRAY		545	286	496	374
DISCARD/Epi	831	42	870	42
Return		873	-	912	-


The overall change in the entire file, according to size, is:
   19855     744       0   20599    5077 build3/lib/iov_iter.o -- before
   19739     864       0   20603    507b build3/lib/iov_iter.o -- after

It is harder to compare because of some of the random name changes.
The version of the source I found seems to pass priv2 to functions
that don't use it?

Since the functions aren't inlined you get the cost of passing
the parameters.
This seems to affect the common cases.
Is that all left over from a version that passed function pointers
(with the hope they'd be inlined?).
Just directly inlining the simple copies should help.

I rather hope the should_fail_usercopy() and instrument_copy_xxx()
calls are usually either absent or, at most, nops.

This all seems to have a lot fewer options than last time I looked.
Is it worth optimising the KVEC case with a single buffer?

	David

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