Re: [PATCH] selftests: mm: fix and speedup "droppable" test

["David Hildenbrand (Arm)" <david@kernel.org>]

On 6/11/26 13:07, Sarthak Sharma wrote:
> 
> 
> On 6/11/26 3:31 PM, David Hildenbrand (Arm) wrote:
>> The droppable test currently relies on creating memory pressure in a
>> child process to trigger dropping the droppable pages.
>>
>> That not only takes a long time on some machines (allocating and filling
>> all that memory), on large machines this will not work as we hardcode the
>> area size to 134217728 bytes.
>>
>> ... further, we rely on timeouts to detect that memory was not dropped,
>> which is really suboptimal.
>>
>> Instead, let's just use MADV_PAGEOUT on a 2 MiB region. MADV_PAGEOUT works
>> with droppable memory even without swap.
>>
>> There is the low chance of MADV_PAGEOUT failing to drop a page because
>> of speculative references. We'll wait 1s and retry 10 times to
>> rule that unlikely case out as best as we can.
>>
>> On a machine without swap:
>>
>> 	$ ./droppable
>> 	TAP version 13
>> 	1..1
>> 	ok 1 madvise(MADV_PAGEOUT) behavior
>> 	# Totals: pass:1 fail:0 xfail:0 xpass:0 skip:0 error:0
>>
>> Reported-by: Aishwarya TCV <Aishwarya.TCV@arm.com>
>> Fixes: 9651fcedf7b9 ("mm: add MAP_DROPPABLE for designating always lazily freeable mappings")
>> Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
>> ---
> 
> I ran this test before and after applying the patch and recorded the
> execution time over 5 runs on an Orion O6 board.
> 

Thanks!

> Before the patch, average execution time = 3.87 s

Out of interest, how much memory did your machine have?

I ran into this myself on a 200gig machine, and there was essentially no
progress ...

-- 
Cheers,

David