[PATCH v2] mm/demotion: print demotion targets

[Li Zhijian <lizhijian@fujitsu.com>]

Currently, when a demotion occurs, it will prioritize selecting a node
from the preferred targets as the destination node for the demotion. If
the preferred node does not meet the requirements, it will try from all
the lower memory tier nodes until it finds a suitable demotion destination
node or ultimately fails.

However, the demotion target information isn't exposed to the users,
especially the preferred target information, which relies on more factors.
This makes users hard to understand the exact demotion behavior.

Rather than having a new sys interface to expose this information,
printing directly to kernel messages, just like the current page
allocation fallback order does.

A dmesg example with this patch is as follows:
[    0.704860] Demotion targets for Node 0: null
[    0.705456] Demotion targets for Node 1: null
// node 2 is onlined
[   32.259775] Demotion targets for Node 0: perferred: 2, fallback: 2
[   32.261290] Demotion targets for Node 1: perferred: 2, fallback: 2
[   32.262726] Demotion targets for Node 2: null
// node 3 is onlined
[   42.448809] Demotion targets for Node 0: perferred: 2, fallback: 2-3
[   42.450704] Demotion targets for Node 1: perferred: 2, fallback: 2-3
[   42.452556] Demotion targets for Node 2: perferred: 3, fallback: 3
[   42.454136] Demotion targets for Node 3: null
// node 4 is onlined
[   52.676833] Demotion targets for Node 0: perferred: 2, fallback: 2-4
[   52.678735] Demotion targets for Node 1: perferred: 2, fallback: 2-4
[   52.680493] Demotion targets for Node 2: perferred: 4, fallback: 3-4
[   52.682154] Demotion targets for Node 3: null
[   52.683405] Demotion targets for Node 4: null
// node 5 is onlined
[   62.931902] Demotion targets for Node 0: perferred: 2, fallback: 2-5
[   62.938266] Demotion targets for Node 1: perferred: 5, fallback: 2-5
[   62.943515] Demotion targets for Node 2: perferred: 4, fallback: 3-4
[   62.947471] Demotion targets for Node 3: null
[   62.949908] Demotion targets for Node 4: null
[   62.952137] Demotion targets for Node 5: perferred: 3, fallback: 3-4

CC: "Huang, Ying" <ying.huang@intel.com>
CC: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Signed-off-by: Li Zhijian <lizhijian@fujitsu.com>
---
V2:
Regarding this requirement, we have previously discussed [1].
The initial proposal involved introducing a new sys interface.
However, due to concerns about potential changes and compatibility
issues with the interface in the future, a consensus was not
reached with the community. Therefore, this time, we are directly
printing out the information.

[1] https://lore.kernel.org/all/d1d5add8-8f4a-4578-8bf0-2cbe79b09989@fujitsu.com/
---
 mm/memory-tiers.c | 24 +++++++++++++++++++++++-
 1 file changed, 23 insertions(+), 1 deletion(-)

diff --git a/mm/memory-tiers.c b/mm/memory-tiers.c
index 5462d9e3c84c..4d3506a290b7 100644
--- a/mm/memory-tiers.c
+++ b/mm/memory-tiers.c
@@ -359,6 +359,26 @@ static void disable_all_demotion_targets(void)
 	synchronize_rcu();
 }
 
+static void dump_demotion_targets(void)
+{
+	int node;
+
+	for_each_node_state(node, N_MEMORY) {
+		struct memory_tier *memtier = __node_get_memory_tier(node);
+		nodemask_t preferred = node_demotion[node].preferred;
+
+		if (!memtier)
+			continue;
+
+		if (nodes_empty(preferred))
+			pr_info("Demotion targets for Node %d: null\n", node);
+		else
+			pr_info("Demotion targets for Node %d: preferred: %*pbl, fallback: %*pbl\n",
+				node, nodemask_pr_args(&preferred),
+				nodemask_pr_args(&memtier->lower_tier_mask));
+	}
+}
+
 /*
  * Find an automatic demotion target for all memory
  * nodes. Failing here is OK.  It might just indicate
@@ -443,7 +463,7 @@ static void establish_demotion_targets(void)
 	 * Now build the lower_tier mask for each node collecting node mask from
 	 * all memory tier below it. This allows us to fallback demotion page
 	 * allocation to a set of nodes that is closer the above selected
-	 * perferred node.
+	 * preferred node.
 	 */
 	lower_tier = node_states[N_MEMORY];
 	list_for_each_entry(memtier, &memory_tiers, list) {
@@ -456,6 +476,8 @@ static void establish_demotion_targets(void)
 		nodes_andnot(lower_tier, lower_tier, tier_nodes);
 		memtier->lower_tier_mask = lower_tier;
 	}
+
+	dump_demotion_targets();
 }
 
 #else
-- 
2.29.2