Re: [PATCH -V3 0/3] memory tiering: hot page selection

[Johannes Weiner]

Hi Huang,

Have you had a chance to look at our hot page detection patch that
Hasan has sent out some time ago? [1]

It hooks into page reclaim to determine what is and isn't hot. Reclaim
is an existing, well-tested mechanism to do just that. It's just 13
lines of code: set active bit on the first hint fault; promote on the
second one if the active bit is still set. This promotes only pages
hot enough that they can compete with toptier access frequencies.

It's not just convenient, it's also essential to link tier promotion
rate to page aging. Tiered NUMA balancing is about establishing a
global LRU order across two (or more) nodes. LRU promotions *within* a
node require multiple LRU cycles with references. LRU promotions
*between* nodes must follow the same rules, and be subject to the same
aging pressure, or you can get much colder pages promoted into a very
hot workingset and wreak havoc.

We've hammered this patch quite extensively with several Meta
production workloads and it's been working reliably at keeping
reasonable promotion rates.

@@ -4202,6 +4202,19 @@ static vm_fault_t do_numa_page(struct vm_fault *vmf)
 
 	last_cpupid = page_cpupid_last(page);
 	page_nid = page_to_nid(page);
+
+	/* Only migrate pages that are active on non-toptier node */
+	if (numa_promotion_tiered_enabled &&
+		!node_is_toptier(page_nid) &&
+		!PageActive(page)) {
+		count_vm_numa_event(NUMA_HINT_FAULTS);
+		if (page_nid == numa_node_id())
+			count_vm_numa_event(NUMA_HINT_FAULTS_LOCAL);
+		mark_page_accessed(page);
+		pte_unmap_unlock(vmf->pte, vmf->ptl);
+		goto out;
+	}
+
 	target_nid = numa_migrate_prep(page, vma, vmf->address, page_nid,
 			&flags);
 	pte_unmap_unlock(vmf->pte, vmf->ptl);

[1] https://lore.kernel.org/all/20211130003634.35468-1-hasanalmaruf@xxxxxx/t/#m85b95624622f175ca17a00cc8cc0fc9cc4eeb6d2