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

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

On 9/9/26 6:09 PM, Kiryl Shutsemau wrote:
> On Wed, Sep 09, 2026 at 03:59:37PM +0800, Baolin Wang wrote:
>>
>>
>> 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,
> 
> They are the same for the PMD contexts, but not for mthp_khugepaged:
> mthp_khugepaged_collapse() passes len = hpage_pmd_size, the range scanned,
> while nr_hpages is the number of folios asked for.  collapse_single_mthp()
> asks for one order-N folio in a whole PMD.
> 
> That matters on arm64 with 64K pages, where the PMD is 512M: with len the
> single-mTHP case would wait up to 7 seconds for one folio, with
> nr_hpages * hpage_size it gets the 3 second floor.  The budget should
> follow what gets built, not what gets scanned, so I would keep it.

OK. Got it. Thanks.

> Does the Reviewed-by stand with that?

Yes. Please keep my reviewed tag.