Re: [PATCH RESEND 1/3] initramfs: move unnecessary memcmp from hot path
From: David Disseldorp <hidden>
Date: 2021-08-04 09:31:32
Also in:
linux-fsdevel
Ping, any feedback on this change?
I think it's a no brainer, but for kicks I ran a few unrealistic micro
benchmarks on my laptop. Extraction time for a cpio image with 1M+
directories improved by 5ms (pre: 14.614s, post: 14.609s), when averaged
across 20 runs of:
qemu-system-x86_64 -machine accel=kvm -smp cpus=1 -m 10240 \
-kernel ~/linux/arch/x86/boot/bzImage \
-initrd ./initrds/gen_cpio.out \
-append "initramfs_async=0 console=ttyS0 panic=0" -nographic \
| awk '/Trying to unpack rootfs/ {start_ts = $2};
/Freeing initrd memory/ {end_ts = $2}
END {printf "%f\n", end_ts - start_ts}'
Cheers, David
On Wed, 21 Jul 2021 13:51:51 +0200, David Disseldorp wrote:
quoted hunk ↗ jump to hunk
do_header() is called for each cpio entry and first checks for "newc" magic before parsing further. The magic check includes a special case error message if POSIX.1 ASCII (cpio -H odc) magic is detected. This special case POSIX.1 check needn't be done in the hot path, so move it under the non-newc-magic error path. Signed-off-by: David Disseldorp <redacted> --- init/initramfs.c | 9 ++++----- 1 file changed, 4 insertions(+), 5 deletions(-)diff --git a/init/initramfs.c b/init/initramfs.c index af27abc59643..f01590cefa2d 100644 --- a/init/initramfs.c +++ b/init/initramfs.c@@ -256,12 +256,11 @@ static int __init do_collect(void) static int __init do_header(void) { - if (memcmp(collected, "070707", 6)==0) { - error("incorrect cpio method used: use -H newc option"); - return 1; - } if (memcmp(collected, "070701", 6)) { - error("no cpio magic"); + if (memcmp(collected, "070707", 6) == 0) + error("incorrect cpio method used: use -H newc option"); + else + error("no cpio magic"); return 1; } parse_header(collected);