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

[Matthew Wilcox]

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 ;-)

> Signed-off-by: Colin Ian King <colin.i.king@xxxxxxxxx>
> ---
>  mm/mincore.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/mincore.c b/mm/mincore.c
> index d6bd19e520fc..832f29f46767 100644
> --- a/mm/mincore.c
> +++ b/mm/mincore.c
> @@ -239,7 +239,7 @@ SYSCALL_DEFINE3(mincore, unsigned long, start, size_t, len,
>  	start = untagged_addr(start);
>  
>  	/* Check the start address: needs to be page-aligned.. */
> -	if (start & ~PAGE_MASK)
> +	if (unlikely(start & ~PAGE_MASK))
>  		return -EINVAL;

We might get even more advantage by moving the EINVAL test before
untagged_addr() since we know that the tags are all in the high bits and
we don't need to have the test be dependent on the previous arithmetic.