linux-kernel - recordmcount commutes with "ld -r"

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Message-ID: <4A8040D8.8020504@bitwagon.com>
Date:	Mon, 10 Aug 2009 08:46:32 -0700
From:	John Reiser <jreiser@...wagon.com>
To:	Steven Rostedt <srostedt@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>
CC:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: recordmcount commutes with "ld -r"

Executing recordmcount.pl for each *.o is adding minutes to the duration
of my full kernel builds.  Here is a way to recoup most of those minutes.

recordmcount commutes with "ld -r".  Run "ld -r" on the outputs from
running recordmcount on each *.o, or run recordmcount on the output from
aggregating the original *.o using "ld -r".  Either way, the final
__mcount_loc section contains a list of locations of calls to mcount.
The ELF32_R_SYM (ELF64_R_SYM) of the relocations may be different, but
they will be equivalent.  Subsequent static binding (ld without -r)
will produce identical results.  Instead of running recordmcount on each
*.o input file that is part of built-in.o or <module>.ko,  then
just run recordmcount on built-in.o or <module>.ko that is constructed
from the original compiler-generated *.o.

There is a special case for building vmlinux, namely the archive
libraries lib/lib.a and arch/$ARCH/lib/lib.a.  recordmcount must be run
on each member individually.  Alternately, recordmcount could be run
on vmlinux.o (exactly once per build; not on any built-in.o)
if vmlinux.o is then used to build vmlinux.

I noticed another property.  Logically, recordmcount could modify a
.o file in place.  Both /bin/ld and the kernel module loader ignore
bytes that are not designated by the ElfXX_Shdr[].  The __mcount_loc
section and its relocations can be appended to the original file, then
"activated" by rewriting the ElfXX_Ehdr fields .e_shnum and .e_shoff.
This avoids some file operations as well as several fork+exec that are
performed by recordmcount.pl.  recordmcount becomes very fast.
The bytes for the old ElfXX_Shdr[] remain as uncollected "garbage",
typically a few kilobytes in each built-in.o or <module>.ko.
If desired then the garbage may be excised quickly by running "ld -r".

I have written recordmcount.c which does such modify-in-place for all
architectures supported by recordcmount, and tested it successfully on
i686, x86_64, and 32-bit PowerPC, including cross-platform processing
of *.o from any architecture.  The differing data structures between
Elf32 and Elf64 require parallel code in many places, so the C file is
900 lines.  That might be too long for a mailing list, so I will defer
posting the file.

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