[RFC PATCH -v4 16/22] cpuset: simplify partition update logic for hotplug tasks

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

From: Chen Ridong <chenridong@huawei.com>

Simplify the partition update logic in cpuset_hotplug_update_tasks() by
calling the unified local_partition_update() interface.

For local partitions, the previous patch introduced local_partition_update
which handles both validation state transitions:
- Invalidates local partitions that fail validation checks
- Transitions invalid partitions to valid state when no errors are detected

This eliminates the need for separate transition logic in
cpuset_hotplug_update_tasks(). The only exception is when
subpartitions_cpus becomes empty because no CPU is left for the
top_cpuset, in which case the local partition is disabled directly
with PERR_HOTPLUG. The effective cpumask is then always recomputed.

For remote partitions, only the hotplug invalidation check is needed,
so jump to update_tasks directly after it instead of going through the
local partition checks.

The partition_cmd enum type can now be safely removed as it is no longer
referenced by any code paths after the partition update logic
simplification.

Signed-off-by: Chen Ridong <chenridong@huawei.com>
Signed-off-by: Ridong Chen <chenridong@xiaomi.com>
---
 kernel/cgroup/cpuset.c | 78 ++++++++++++++----------------------------
 1 file changed, 25 insertions(+), 53 deletions(-)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 78a783cb6308..00afa4987381 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -1135,17 +1135,6 @@ void cpuset_update_tasks_cpumask(struct cpuset *cs, struct cpumask *new_cpus)
 	css_task_iter_end(&it);
 }
 
-/*
- * Commands for update_parent_effective_cpumask
- */
-enum partition_cmd {
-	partcmd_enable,		/* Enable partition root	  */
-	partcmd_enablei,	/* Enable isolated partition root */
-	partcmd_disable,	/* Disable partition root	  */
-	partcmd_update,		/* Update parent's effective_cpus */
-	partcmd_invalidate,	/* Make partition invalid	  */
-};
-
 static void update_sibling_cpumasks(struct cpuset *parent, struct cpuset *cs,
 				    struct tmpmasks *tmp);
 
@@ -3944,7 +3933,6 @@ static void cpuset_hotplug_update_tasks(struct cpuset *cs, struct tmpmasks *tmp)
 	static nodemask_t new_mems;
 	bool cpus_updated;
 	bool mems_updated;
-	int partcmd = -1;
 	struct cpuset *parent;
 
 	wait_attach_done_lock();
@@ -3960,54 +3948,38 @@ static void cpuset_hotplug_update_tasks(struct cpuset *cs, struct tmpmasks *tmp)
 	if (!tmp || !cs->partition_root_state)
 		goto update_tasks;
 
-	if (is_remote_partition(cs) && (cpumask_empty(subpartitions_cpus) ||
-				       (cpumask_empty(&new_cpus) &&
-					partition_is_populated(cs, NULL)))) {
-		WRITE_ONCE(cs->prs_err, PERR_HOTPLUG);
-		remote_partition_disable(cs, tmp);
-		compute_effective_cpumask(&new_cpus, cs, parent);
-	}
-
 	/*
-	 * Force the partition to become invalid if either one of
-	 * the following conditions hold:
-	 * 1) empty effective cpus but not valid empty partition.
-	 * 2) parent is invalid or doesn't grant any cpus to child
-	 *    partitions.
-	 * 3) subpartitions_cpus is empty.
+	 * Compute effective_cpus for valid partition root, may invalidate
+	 * child partition roots if necessary.
 	 */
-	if (is_local_partition(cs) &&
-	    (!is_partition_valid(parent) ||
-	     tasks_nocpu_error(parent, cs, &new_cpus) ||
-	     cpumask_empty(subpartitions_cpus))) {
-		enum prs_errcode prs_err;
-
-		if (!is_partition_valid(parent))
-			prs_err = is_partition_invalid(parent)
-				  ? PERR_INVPARENT : PERR_NOTPART;
-		else if (cpumask_empty(subpartitions_cpus))
-			prs_err = PERR_HOTPLUG;
-		else
-			prs_err = PERR_NOCPUS;
-		partcmd = partcmd_invalidate;
-		local_partition_disable(cs, prs_err, tmp);
+	if (is_remote_partition(cs)) {
+		if (cpumask_empty(subpartitions_cpus) ||
+		    (cpumask_empty(&new_cpus) &&
+		     partition_is_populated(cs, NULL))) {
+			WRITE_ONCE(cs->prs_err, PERR_HOTPLUG);
+			remote_partition_disable(cs, tmp);
+			compute_effective_cpumask(&new_cpus, cs, parent);
+		}
+		goto update_tasks;
 	}
+
 	/*
-	 * On the other hand, an invalid partition root may be transitioned
-	 * back to a regular one with a non-empty user xcpus.
+	 * The subpartitions_cpus may be cleared if no CPUs remain available for
+	 * the top_cpuset. In this case, disable the local partition directly.
+	 * Otherwise, local_partition_update() will handle the automatic
+	 * transition between valid and invalid partition states.
 	 */
-	else if (is_partition_valid(parent) && is_partition_invalid(cs) &&
-		 !cpumask_empty(user_xcpus(cs))) {
-		partcmd = partcmd_update;
+	if (is_local_partition(cs) && cpumask_empty(subpartitions_cpus))
+		local_partition_disable(cs, PERR_HOTPLUG, tmp);
+	else
 		local_partition_update(cs, tmp);
-	}
 
-	if (partcmd >= 0) {
-		if (is_partition_valid(cs) || (partcmd == partcmd_invalidate)) {
-			compute_effective_cpumask(&new_cpus, cs, parent);
-			cpuset_force_rebuild();
-		}
-	}
+	/*
+	 * Recompute effective CPU mask after partition state update:
+	 * - For valid partitions: calculate partition-specific effective CPUs
+	 * - For invalid partitions: compute member effective CPU mask
+	 */
+	compute_effective_cpumask(&new_cpus, cs, parent);
 
 update_tasks:
 	cpus_updated = !cpumask_equal(&new_cpus, &old_cpus);
-- 
2.34.1