Re: [PATCH][next] mm/mincore: improve performance by adding an unlikely hint

[Andrew Morton <akpm@linux-foundation.org>]

On Mon, 17 Feb 2025 18:00:22 +0000 "Colin King (gmail)" <colin.i.king@gmail.com> 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:

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