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

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Message-ID: <20250313164430.bzkfyqmx6a5dj7d2@offworld>
Date: Thu, 13 Mar 2025 09:44:30 -0700
From: Davidlohr Bueso <dave@...olabs.net>
To: Bharata B Rao <bharata@....com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
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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:

>+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:

if (folio_test_active())
    return true;

>+
>+	/* 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().

See https://web.git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/commit/?id=c6a566f6c1b4d5dff659acd221f95a72923f4085

>+
>+	while (!kthread_should_stop()) {
>+		wait_event_timeout(pgdat->kpromoted_wait,
>+				   kpromoted_work_requested(pgdat), timeout);
>+		kpromoted_do_work(pgdat);
>+	}
>+	return 0;
>+}
>+
>+static void kpromoted_run(int nid)
>+{
>+	pg_data_t *pgdat = NODE_DATA(nid);
>+
>+	if (pgdat->kpromoted)
>+		return;
>+
>+	pgdat->kpromoted = kthread_run(kpromoted, pgdat, "kpromoted%d", nid);
>+	if (IS_ERR(pgdat->kpromoted)) {
>+		pr_err("Failed to start kpromoted on node %d\n", nid);
>+		pgdat->kpromoted = NULL;
>+	}
>+}
>+