[PATCH] mm/mempolicy: recompute automatic interleave weights on memory hotplug

["Gregory Price (Meta)" <gourry@gourry.net>]

In auto mode, weighted interleave derives each node's weight from its
bandwidth, reduced over the nodes in N_MEMORY. HMAT and CXL report a
node's bandwidth once, when the node first gains memory, and nothing
else on the hotplug path recomputes the weights.

When a node loses its memory, the other nodes keep weights computed
with its bandwidth until another node reports. If one does while the
node is away, the reduction skips the node and resets its weight to 1,
which it keeps once its memory returns.

With HMAT bandwidths of 100G for nodes 0-3, and 50G and 25G for kmem
nodes 4 and 5, weighted_interleave/nodeN reads:

  event             node 0-3   node 4   node 5
  node 4 online         7         3        -
  node 4 removed        7         -        -
  node 5 online         7         -        1
  node 4 back           7         1        1

Toggling weighted_interleave/auto to 0 and back to 1 gives the correct
6, 3 and 1.

Recompute the weights from the recorded bandwidth whenever a node gains
its first memory or loses its last.

mempolicy_set_node_perf() now only records the bandwidth and calls the
same helper, so the bandwidth is kept even if allocating the new weights
fails. wi_state_lock is dropped between the two steps, which should be
safe because the helper reads node_bw_table and the mode under the lock,
so a racing writer only makes it work from newer state.

With this patch, weighted_interleave/nodeN reads 6, 3 and 1 as soon as
node 4 is back.

Fixes: e341f9c3c841 ("mm/mempolicy: Weighted Interleave Auto-tuning")
Cc: stable@vger.kernel.org
Signed-off-by: Gregory Price (Meta) <gourry@gourry.net>
---
 mm/mempolicy.c | 66 +++++++++++++++++++++++++++++---------------------
 1 file changed, 38 insertions(+), 28 deletions(-)

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index 0884c629670d..ffb293a4cc56 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -238,27 +238,45 @@ static void reduce_interleave_weights(unsigned int *bw, u8 *new_iw)
 		new_iw[nid] /= iw_gcd;
 }
 
+/* Recompute the weights from node_bw_table, unless they were set manually */
+static int wi_state_refresh(void)
+{
+	struct weighted_interleave_state *new_wi_state, *old_wi_state;
+	int i;
+
+	new_wi_state = kmalloc_flex(*new_wi_state, iw_table, nr_node_ids);
+	if (!new_wi_state)
+		return -ENOMEM;
+	new_wi_state->mode_auto = true;
+	for (i = 0; i < nr_node_ids; i++)
+		new_wi_state->iw_table[i] = 1;
+
+	mutex_lock(&wi_state_lock);
+	old_wi_state = rcu_dereference_protected(wi_state,
+					lockdep_is_held(&wi_state_lock));
+	/* NULL wi_state assumes auto=true */
+	if (!node_bw_table || (old_wi_state && !old_wi_state->mode_auto)) {
+		mutex_unlock(&wi_state_lock);
+		kfree(new_wi_state);
+		return 0;
+	}
+	reduce_interleave_weights(node_bw_table, new_wi_state->iw_table);
+	rcu_assign_pointer(wi_state, new_wi_state);
+	mutex_unlock(&wi_state_lock);
+	wi_state_retire(old_wi_state);
+	return 0;
+}
+
 int mempolicy_set_node_perf(unsigned int node, struct access_coordinate *coords)
 {
-	struct weighted_interleave_state *new_wi_state, *old_wi_state = NULL;
 	unsigned int *old_bw, *new_bw;
 	unsigned int bw_val;
-	int i;
 
 	bw_val = min(coords->read_bandwidth, coords->write_bandwidth);
 	new_bw = kcalloc(nr_node_ids, sizeof(unsigned int), GFP_KERNEL);
 	if (!new_bw)
 		return -ENOMEM;
 
-	new_wi_state = kmalloc_flex(*new_wi_state, iw_table, nr_node_ids);
-	if (!new_wi_state) {
-		kfree(new_bw);
-		return -ENOMEM;
-	}
-	new_wi_state->mode_auto = true;
-	for (i = 0; i < nr_node_ids; i++)
-		new_wi_state->iw_table[i] = 1;
-
 	/*
 	 * Update bandwidth info, even in manual mode. That way, when switching
 	 * to auto mode in the future, iw_table can be overwritten using
@@ -271,25 +289,10 @@ int mempolicy_set_node_perf(unsigned int node, struct access_coordinate *coords)
 		memcpy(new_bw, old_bw, nr_node_ids * sizeof(*old_bw));
 	new_bw[node] = bw_val;
 	node_bw_table = new_bw;
-
-	old_wi_state = rcu_dereference_protected(wi_state,
-					lockdep_is_held(&wi_state_lock));
-	if (old_wi_state && !old_wi_state->mode_auto) {
-		/* Manual mode; skip reducing weights and updating wi_state */
-		mutex_unlock(&wi_state_lock);
-		kfree(new_wi_state);
-		goto out;
-	}
-
-	/* NULL wi_state assumes auto=true; reduce weights and update wi_state*/
-	reduce_interleave_weights(new_bw, new_wi_state->iw_table);
-	rcu_assign_pointer(wi_state, new_wi_state);
-
 	mutex_unlock(&wi_state_lock);
-	wi_state_retire(old_wi_state);
-out:
+
 	kfree(old_bw);
-	return 0;
+	return wi_state_refresh();
 }
 
 /**
@@ -3900,8 +3903,15 @@ static int wi_node_notifier(struct notifier_block *nb,
 	case NODE_REMOVED_LAST_MEMORY:
 		sysfs_wi_node_delete(nid);
 		break;
+	default:
+		return NOTIFY_OK;
 	}
 
+	/*
+	 * HMAT and CXL report a node's bandwidth only when it first gains
+	 * memory, so recompute the weights whenever N_MEMORY changes.
+	 */
+	wi_state_refresh();
 	return NOTIFY_OK;
 }
 
-- 
2.56.0