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

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Message-ID: <20250313203607.zod6lssjef37ynbf@offworld>
Date: Thu, 13 Mar 2025 13:36:07 -0700
From: Davidlohr Bueso <dave@...olabs.net>
To: Bharata B Rao <bharata@....com>
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Subject: Re: [RFC PATCH 2/4] mm: kpromoted: Hot page info collection and
 promotion daemon

On Thu, 06 Mar 2025, Bharata B Rao wrote:

>+/*
>+ * Go thro' page hotness information and migrate pages if required.
>+ *
>+ * Promoted pages are not longer tracked in the hot list.
>+ * Cold pages are pruned from the list as well.
>+ *
>+ * TODO: Batching could be done
>+ */
>+static void kpromoted_migrate(pg_data_t *pgdat)
>+{
>+	int nid = pgdat->node_id;
>+	struct page_hotness_info *phi;
>+	struct hlist_node *tmp;
>+	int nr_bkts = HASH_SIZE(page_hotness_hash);
>+	int bkt;
>+
>+	for (bkt = 0; bkt < nr_bkts; bkt++) {
>+		mutex_lock(&page_hotness_lock[bkt]);
>+		hlist_for_each_entry_safe(phi, tmp, &page_hotness_hash[bkt], hnode) {
>+			if (phi->hot_node != nid)
>+				continue;
>+
>+			if (page_should_be_promoted(phi)) {
>+				count_vm_event(KPROMOTED_MIG_CANDIDATE);
>+				if (!kpromote_page(phi)) {
>+					count_vm_event(KPROMOTED_MIG_PROMOTED);
>+					hlist_del_init(&phi->hnode);
>+					kfree(phi);
>+				}
>+			} else {
>+				/*
>+				 * Not a suitable page or cold page, stop tracking it.
>+				 * TODO: Identify cold pages and drive demotion?
>+				 */

I don't think kpromoted should drive demotion at all. No one is complaining about migrate
in lieu of discard, and there is also proactive reclaim which users can trigger. All the
in-kernel problems are wrt promotion. The simpler any of these kthreads are the better.

>+				count_vm_event(KPROMOTED_MIG_DROPPED);
>+				hlist_del_init(&phi->hnode);
>+				kfree(phi);
>+			}
>+		}
>+		mutex_unlock(&page_hotness_lock[bkt]);
>+	}
>+}