Re: [PATCH] sched/numa, mm: Skip page promotion if cpu pid is valid

[Huang, Ying]

Donet Tom <donettom@xxxxxxxxxxxxx> writes:

> Hi
>
> On 3/31/26 2:03 PM, Huang, Ying wrote:
>> Hi, Donet,
>>
>> Donet Tom <donettom@xxxxxxxxxxxxx> writes:
>>
>>> On 3/26/26 3:59 PM, David Hildenbrand (Arm) wrote:
>>>> On 3/26/26 08:12, Donet Tom wrote:
>>>>> If memory tiering is disabled, cpupid of slow memory pages may
>>>>> contain a valid CPU and PID. If tiering is enabled at runtime,
>>>>> there is a chance that in should_numa_migrate_memory(), this
>>>>> valid CPU/PID is treated as a last access timestamp, leading
>>>>> to unnecessary promotion.
>>>> Is that measurable? Should we at least have a Fixes: ?
>>>>
>>>>> Prevent this by skipping promotion when cpupid is valid.
>>>>>
>>>>> Signed-off-by: Donet Tom <donettom@xxxxxxxxxxxxx>
>>>>> ---
>>>>>    kernel/sched/fair.c | 7 +++++++
>>>>>    1 file changed, 7 insertions(+)
>>>>>
>>>>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>>>>> index 4b43809a3fb1..f5830a5a94d5 100644
>>>>> --- a/kernel/sched/fair.c
>>>>> +++ b/kernel/sched/fair.c
>>>>> @@ -2001,6 +2001,13 @@ bool should_numa_migrate_memory(struct task_struct *p, struct folio *folio,
>>>>>    		unsigned int latency, th, def_th;
>>>>>    		long nr = folio_nr_pages(folio);
>>>>>    
>>>> /*
>>>>    * When ...
>>>>
>>>>> +		/* When tiering is enabled at runtime, last_cpupid may
>>>>> +		 * hold a valid cpupid instead of an access timestamp.
>>>>> +		 * If so, skip page promotion.
>>>>> +		 */
>>>>> +		if (cpupid_valid(folio_last_cpupid(folio)))
>>>>> +			return false;
>>>>> +
>>>> IIUC, as timestamp we use jiffies_to_msecs(). So, soon after bootup,
>>>> we would no longer get false positives for cpupid_valid().
>>>> I suppose overflows are not a problem, correct?
>>> Thank you, David, for guiding me in the right direction.
>>>
>>> I initially thought that overflows would not occur, and therefore
>>> cpupid_valid() would not produce false positives. However,
>>> after looking into it further, it appears that overflow can
>>> happen when storing the access time.
>>>
>>> The last_cpupid field is used to store the last access time.
>>>  From the code, it appears that 21 bits are used for this
>>> (#define LAST_CPUPID_SHIFT (LAST__PID_SHIFT + LAST__CPU_SHIFT)).
>>>
>>> With 21 bits, the maximum value that can be stored is
>> It can be less than 21 bits, if CONFIG_NR_CPUS is small.
>>
>> 	DEFINE(NR_CPUS_BITS, order_base_2(CONFIG_NR_CPUS));
>>
>>> 2097151ms (35Hrs) . If the access time exceeds this
>>> range, it can overflow, which may lead to cpupid_valid()
>>> returning false positives.
>>>
>>> I think we need a reliable way to determine cpupid_valid() that
>>> does not produce false positives.
>> Yes.  IMHO, false positives is unavoidable.  So, the patch fixes a
>> temporal performance issue at the cost of a longstanding performance
>> issue.  Right?
>
>
> I was trying to fix a functional issue. When memory tiering is
>
> enabled at runtime, treating last_cpupid as access time is incorrect, right?

I don't think that it's a functional issue.  It has only performance
impact.  Did you find any functionality bug?

---
Best Regards,
Huang, Ying