[PATCH v2 02/17] selftests/mm: scale khugepaged's collapse wait with the PMD size

From: "Kiryl Shutsemau (Meta)" <[email protected]>

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, and the case has 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 to collapse.  A
2M PMD is unchanged.  A 512M PMD gets 19 seconds, which is headroom over
the observed tail rather than a measured requirement.  The budget bounds
how long a real failure takes to report, not how long a passing case
waits: wait_for_scan() returns as soon as the collapse turns up.

arm64/64K: khugepaged all:anon 21 pass/1 fail -> 22 pass/0 fail.  x86-64 is
unchanged.

Assisted-by: Claude-Code:claude-opus-5
Signed-off-by: Kiryl Shutsemau (Meta) <[email protected]>
---
 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 51fc1f04d0a0..0f828bfee31f 100644
--- a/tools/testing/selftests/mm/khugepaged.c
+++ b/tools/testing/selftests/mm/khugepaged.c
@@ -602,8 +602,10 @@ 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