On 18/02/2025 03:13, Andrew Morton wrote:
On Mon, 17 Feb 2025 18:00:22 +0000 "Colin King (gmail)" <colin.i.king@xxxxxxxxx> wrote:fOn 17/02/2025 17:58, Matthew Wilcox wrote:On Mon, Feb 17, 2025 at 05:09:34PM +0000, Colin Ian King wrote:Adding an unlikely() hint on the masked start comparison error return path improves run-time performance of the mincore system call. Benchmarking on an i9-12900 shows an improvement of 7ns on mincore calls on a 256KB mmap'd region where 50% of the pages we resident. Results based on running 20 tests with turbo disabled (to reduce clock freq turbo changes), with 10 second run per test and comparing the number of mincores calls per second. The % standard deviation of the 20 tests was ~0.10%, so results are reliable.I think you've elided _just_ enough information here that nobody can judge whether your stats skills are any good ;-) You've told us 7ns (per call, presumably) and you've told us 0.10% standard deviation, but you haven't told us how long the syscall takes, so nobody can tell whether 7ns is within 0.10% or not ;-)Ugh, my bad. Improvement was from ~970 down to 963 ns, so small ~0.7% improvement.It actually doesn't change the generated code:
I've compare the generated x86 object code using gcc 14.2.1 20240912 (Fedora 41) and 14.2.0 (Debian 14.2.0-17), 14.2.1 20250211 (Clear Linux) and I get differences in the generated object code comparing old and new, and the improvement on ClearLinux is more significant too because it uses -O3. So I'm confident the change is generating improved object code.
hp2:/usr/src/25> diff -u mm/mincore.lst.old mm/mincore.lst --- mm/mincore.lst.old 2025-02-17 19:11:34.093727411 -0800 +++ mm/mincore.lst 2025-02-17 19:12:59.797009056 -0800 @@ -1563,7 +1563,7 @@ start = untagged_addr(start);/* Check the start address: needs to be page-aligned.. */- if (start & ~PAGE_MASK) + if (unlikely(start & ~PAGE_MASK)) b27: 31 ff xor %edi,%edi asm (ALTERNATIVE("", b29: 90 nop
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