Re: [RFC PATCH 2/4] mm: kpromoted: Hot page info collection and promotion daemon

Bharata B Rao
2025-03-17T16:22:45Z

On 17-Mar-25 8:35 PM, Gregory Price wrote:
> On Mon, Mar 17, 2025 at 09:09:18AM +0530, Bharata B Rao wrote:
>> On 13-Mar-25 10:14 PM, Davidlohr Bueso wrote:
>>> On Thu, 06 Mar 2025, Bharata B Rao wrote:
>>>
>>>> +static int page_should_be_promoted(struct page_hotness_info *phi)
>>>> +{
>>>> +    struct page *page = pfn_to_online_page(phi->pfn);
>>>> +    unsigned long now = jiffies;
>>>> +    struct folio *folio;
>>>> +
>>>> +    if (!page || is_zone_device_page(page))
>>>> +        return false;
>>>> +
>>>> +    folio = page_folio(page);
>>>> +    if (!folio_test_lru(folio)) {
>>>> +        count_vm_event(KPROMOTED_MIG_NON_LRU);
>>>> +        return false;
>>>> +    }
>>>> +    if (folio_nid(folio) == phi->hot_node) {
>>>> +        count_vm_event(KPROMOTED_MIG_RIGHT_NODE);
>>>> +        return false;
>>>> +    }
>>>
>>> How about using the LRU age itself:
>>
>> Sounds like a good check for page hotness.
>>
>>>
>>> if (folio_test_active())
>>>    return true;
>>
>> But the numbers I obtained with this check added, didn't really hit this
>> condition all that much. I was running a multi-threaded application that
>> allocates enough memory such that the allocation spills over from DRAM node
>> to the CXL node. Threads keep touching the memory pages in random order.
>>
>
> Is demotion enabled by any chance?

Yes, I thought enabling demotion is required to create enough room in
the toptier to handle promotion.

>
> i.e. are you sure it's actually allocating from CXL and not demoting
> cold stuff to CXL?

But then I realized that spill over was caused by demotion rather than
initial allocation even when I used MPOL_BIND | MPOL_F_NUMA_BALANCING
policy with both toptier and CXL node in the nodemask.

>
>> kpromoted_recorded_accesses 960620 /* Number of recorded accesses */
>> kpromoted_recorded_hwhints 960620 /* Nr accesses via HW hints, IBS in this
>> case */
>> kpromoted_recorded_pgtscans 0
>> kpromoted_record_toptier 638006 /* Nr toptier accesses */
>> kpromoted_record_added 321234 /* Nr (CXL) accesses that are tracked */
>> kpromoted_record_exists 1380
>> kpromoted_mig_right_node 0
>> kpromoted_mig_non_lru 226
>> kpromoted_mig_lru_active 47 /* Number of accesses considered for promotion
>> as determined by folio_test_active() check */

However disabling demotion has no impact on this number (and hence the
folio_test_active() check)

Regards,
Bharata.