Re: [PATCH v4 03/19] selftests/mm: scale khugepaged's collapse wait with the PMD size

["Lorenzo Stoakes (ARM)" <ljs@kernel.org>]

On Sat, Aug 15, 2026 at 02:58:45AM +0100, Kiryl Shutsemau wrote:
> From: "Kiryl Shutsemau (Meta)" <kas@kernel.org>
>
> wait_for_scan() gives every case the same three seconds, whatever the huge
> page costs to build.  collapse_full() asks for four of them: 8M at a 2M
> PMD, but 2G at a 512M PMD -- arm64 with 64K base pages.
>
> Three seconds is thin at that size rather than generous.  Across 80 runs of
> collapse_full() on arm64 with 64K pages the wait was half a second in 73 of
> them, with a tail to two seconds.  The case has also timed out in a full
> matrix run, reporting a failure for a collapse that was still going.
>
> Keep three seconds as the floor and add a second per 128M collapsed.  A 2M
> PMD is unchanged.  A 512M PMD gets 19 seconds.

Yikes that does make this run really slow for 64 KiB page size machines and a
key aim in the selftests is that they don't take too egregiously long to run.

Can this somehow be trimmed some other way?

>
> arm64/64K: khugepaged all:anon 21 pass/1 fail -> 22 pass/0 fail.  x86-64 is
> unchanged.
>
> Assisted-by: Claude-Code:claude-opus-5
> Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
> Tested-by: Muhammad Usama Anjum <usama.anjum@arm.com>
> Signed-off-by: Kiryl Shutsemau (Meta) <kas@kernel.org>

In general though the patch seems reasonable so:

Acked-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>

> ---
>  tools/testing/selftests/mm/khugepaged.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/tools/testing/selftests/mm/khugepaged.c b/tools/testing/selftests/mm/khugepaged.c
> index dd924edd8557..c499804a0ec4 100644
> --- a/tools/testing/selftests/mm/khugepaged.c
> +++ b/tools/testing/selftests/mm/khugepaged.c
> @@ -561,8 +561,10 @@ static bool wait_for_scan(const char *msg, char *p, size_t len,
>  		int nr_hpages, int collap_order, struct mem_ops *ops)
>  {
>  	unsigned long hpage_size = page_size << collap_order;
> +	/* Three seconds as a floor, plus a second per 128M to collapse */
> +	const unsigned long bytes = (unsigned long)nr_hpages * hpage_size;
> +	int timeout = 6 + 2 * (bytes / (128UL << 20));
>  	int full_scans;
> -	int timeout = 6; /* 3 seconds */
>
>  	/* Sanity check */
>  	if (!ops->check_huge(p, len, 0, hpage_size))
> --
> 2.54.0
>

--
Cheers, Lorenzo