[PATCH v5 01/12] cpuset: introduce partition_enable()

[Ridong Chen <ridong.chen@linux.dev>]

From: Ridong Chen <chenridong@xiaomi.com>

Add partition_enable() to consolidate updates to key cpuset structures
during partition enablement, including:
1. remote_partition
2. effective_xcpus
3. partition_root_state
4. prs_err

Key operations performed:
1. Invokes partition_xcpus_add() to assign exclusive CPUs
2. Maintains remote partition flag
3. Syncs the effective_xcpus mask
4. Updates partition_root_state and prs_err
5. Triggers scheduler domain rebuilds
6. Sends partition change notifications

This helper also handles the transition between root and isolated
states of a valid partition. In that case the cpumasks are unchanged
and only the isolation state of the CPUs in @new_excpus is updated. The
caller passes the CPUs used by the partition itself, as the CPUs of its
valid child partitions keep their own state.

A partition_state_update() helper is added to set partition_root_state
and prs_err, and to reset the partition data when the new state is not
a valid partition.

Signed-off-by: Ridong Chen <chenridong@xiaomi.com>
---
 kernel/cgroup/cpuset.c | 56 ++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 56 insertions(+)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 7e374210c123..860cb7ea3efb 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -1702,6 +1702,62 @@ static void compute_effective_nodemask(nodemask_t *new_mems,
 		nodes_copy(*new_mems, parent->effective_mems);
 }
 
+static void partition_state_update(struct cpuset *cs, int new_prs,
+				   enum prs_errcode prs_err)
+{
+	lockdep_assert_held(&callback_lock);
+
+	cs->partition_root_state = new_prs;
+	WRITE_ONCE(cs->prs_err, prs_err);
+	if (!is_partition_valid(cs))
+		reset_partition_data(cs);
+}
+
+/**
+ * partition_enable - Transitions a cpuset to a partition root
+ * @cs: The cpuset to enable partition for
+ * @parent: Parent cpuset of @cs, NULL for remote parent
+ * @new_prs: New partition state to set
+ * @new_excpus: New effective exclusive CPUs mask for the partition, or the
+ *              CPUs whose isolation state changes when switching between
+ *              root and isolated
+ *
+ * Transitions a cpuset to a partition root, only for v2.
+ * It also supports the transition between root and isolated partition.
+ */
+static void __maybe_unused partition_enable(struct cpuset *cs, struct cpuset *parent,
+					    int new_prs, struct cpumask *new_excpus)
+{
+	int old_prs = cs->partition_root_state;
+
+	lockdep_assert_cpuset_lock_held();
+	WARN_ON_ONCE(new_prs <= 0);
+	WARN_ON_ONCE(!cpuset_v2());
+
+	if (old_prs == new_prs)
+		return;
+
+	spin_lock_irq(&callback_lock);
+	if (old_prs > 0) {
+		/*
+		 * Switch between root and isolated, cpumasks are unchanged.
+		 * @new_excpus holds the CPUs whose isolation state changes.
+		 */
+		isolated_cpus_update(new_prs, new_excpus);
+	} else {
+		/* enable partition should only add exclusive cpus */
+		partition_xcpus_add(new_prs, parent, new_excpus);
+		/* enable remote partition */
+		if (!parent)
+			cs->remote_partition = true;
+		cpumask_copy(cs->effective_xcpus, new_excpus);
+	}
+	partition_state_update(cs, new_prs, PERR_NONE);
+	spin_unlock_irq(&callback_lock);
+	cpuset_force_rebuild();
+	notify_partition_change(cs, old_prs);
+}
+
 /*
  * remote_partition_enable - Enable current cpuset as a remote partition root
  * @cs: the cpuset to update
-- 
2.34.1