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

[Kiryl Shutsemau <kirill@shutemov.name>]

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.

Does the Reviewed-by stand with that?

-- 
  Kiryl Shutsemau / Kirill A. Shutemov