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

Bharata B Rao
2025-03-17T03:39:33Z

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.

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 */
kpromoted_mig_cold_old 0
kpromoted_mig_cold_not_accessed 1373
kpromoted_mig_candidate 319635
kpromoted_mig_promoted 319635
kpromoted_mig_dropped 1599

Need to check why is this the case.

>
>> +
>> +    /* If the page was hot a while ago, don't promote */
>> +    if ((now - phi->last_update) > 2 *
>> msecs_to_jiffies(KPROMOTED_FREQ_WINDOW)) {
>> +        count_vm_event(KPROMOTED_MIG_COLD_OLD);
>> +        return false;
>> +    }
>> +
>> +    /* If the page hasn't been accessed enough number of times, don't
>> promote */
>> +    if (phi->frequency < KPRMOTED_FREQ_THRESHOLD) {
>> +        count_vm_event(KPROMOTED_MIG_COLD_NOT_ACCESSED);
>> +        return false;
>> +    }
>> +    return true;
>> +}
>
> ...
>
>> +static int kpromoted(void *p)
>> +{
>> +    pg_data_t *pgdat = (pg_data_t *)p;
>> +    struct task_struct *tsk = current;
>> +    long timeout = msecs_to_jiffies(KPROMOTE_DELAY);
>> +
>> +    const struct cpumask *cpumask = cpumask_of_node(pgdat->node_id);
>> +
>> +    if (!cpumask_empty(cpumask))
>> +        set_cpus_allowed_ptr(tsk, cpumask);
>
> Explicit cpumasks are not needed if you use kthread_create_on_node().

Thanks, will incorporate.

Regards,
Bharata.