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

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

On 3/31/26 10:33, Huang, Ying wrote:
> Hi, Donet,
> 
> Donet Tom <donettom@linux.ibm.com> writes:
> 
>> On 3/26/26 3:59 PM, David Hildenbrand (Arm) wrote:
>>> Is that measurable? Should we at least have a Fixes: ?
>>>
>>> /*
>>>   * When ...
>>>
>>> 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?


Could we set aside a bit to indicate "cpuid vs. time" ? We'd lose one
bit for time, to we care?

Would make it all easier to get ...

-- 
Cheers,

David