Re: [PATCH v5 03/19] selftests/mm: scale khugepaged's collapse wait with the PMD size

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 9/8/26 8:50 PM, 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, and the case has reported a failure for a collapse that
> was still going.
> 
> The timeout is a ceiling on a poll loop, not a sleep: the loop stops as
> soon as ops->check_huge() sees the collapse, or as soon as full_scans has
> advanced by two.  Raising it costs a passing case nothing.  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.
> 
> Keep three seconds as the floor and add a second per 128M collapsed.  A 2M
> PMD is unchanged, so x86-64 is too; a 512M PMD gets 19 seconds.
> 
> On arm64 with 64K pages a passing ./khugepaged all:anon takes 49 seconds
> under TCG before and after this change.
> 
> Assisted-by: LLM
> Acked-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
> 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>
> ---
>   tools/testing/selftests/mm/khugepaged.c | 7 +++++--
>   1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/tools/testing/selftests/mm/khugepaged.c b/tools/testing/selftests/mm/khugepaged.c
> index 1ca7c6978571..48e0040d53b4 100644
> --- a/tools/testing/selftests/mm/khugepaged.c
> +++ b/tools/testing/selftests/mm/khugepaged.c
> @@ -556,8 +556,11 @@ 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;
> -	int full_scans;
> -	int timeout = 6; /* 3 seconds */
> +	unsigned long bytes = (unsigned long)nr_hpages * hpage_size;

We already pass in the 'len' parameter, and its size is also 'nr_hpages 
* hpage_size", so you can drop the 'bytes' variable. With that,

Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>

> +	int timeout, full_scans;
> +
> +	/* Half-second ticks: three seconds floor, plus a second per 128M */
> +	timeout = 6 + 2 * (bytes / (128UL << 20));
>   
>   	/* Sanity check */
>   	if (!ops->check_huge(p, len, 0, hpage_size))