[PATCH] cgroup/cpuset: Support multiple source/destination cpusets using pids pattern

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

The current cpuset_can_attach() and cpuset_attach() functions assume task
migration is from one source cpuset to one destination cpuset. This can be
wrong in several scenarios:
 - Moving a multi-threaded process with threads in different cpusets
 - Disabling the cpuset controller (many children to one parent)
 - Enabling the cpuset controller (one parent to many children)

Fix this by adopting the pids subsystem's per-task accounting pattern.
In cpuset_can_attach(), use task_cs(task) to get the correct source cpuset
for each task (like pids_can_attach uses task_css), adjust nr_deadline_tasks
and reserve DL bandwidth per-task, and increment attach_in_progress per-task
on the destination cpuset. In cpuset_attach(), handle destination cpuset
changes within the task iteration loop.

A shared helper cpuset_undo_attach() reverses the per-task operations for
both partial rollback in cpuset_can_attach() and full reversal in
cpuset_cancel_attach().

When multiple source cpusets are detected in can_attach(), set
attach_many_sources so that cpuset_attach() forces cpus_updated and
mems_updated to true, ensuring all tasks get properly updated regardless
of which source cpuset cpuset_attach_old_cs points to.

This eliminates the need for nr_migrate_dl_tasks, sum_migrate_dl_bw, and
dl_bw_cpu fields in struct cpuset.

Fixes: 4ec22e9c5a90 ("cpuset: Enable cpuset controller in default hierarchy")
Signed-off-by: Ridong Chen <ridong.chen@linux.dev>
---
 kernel/cgroup/cpuset-internal.h |   8 --
 kernel/cgroup/cpuset.c          | 177 ++++++++++++++++----------------
 2 files changed, 89 insertions(+), 96 deletions(-)

diff --git a/kernel/cgroup/cpuset-internal.h b/kernel/cgroup/cpuset-internal.h
index f7aaf01f7cd5..601e38b3c75b 100644
--- a/kernel/cgroup/cpuset-internal.h
+++ b/kernel/cgroup/cpuset-internal.h
@@ -166,14 +166,6 @@ struct cpuset {
 	 * know when to rebuild associated root domain bandwidth information.
 	 */
 	int nr_deadline_tasks;
-	int nr_migrate_dl_tasks;
-	/* DL bandwidth that needs destination reservation for this attach. */
-	u64 sum_migrate_dl_bw;
-	/*
-	 * CPU used for temporary DL bandwidth allocation during attach;
-	 * -1 if no DL bandwidth was allocated in the current attach.
-	 */
-	int dl_bw_cpu;
 
 	/* Invalid partition error code, not lock protected */
 	enum prs_errcode prs_err;
diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index e52a5a40d607..be222eb6078c 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -288,7 +288,6 @@ struct cpuset top_cpuset = {
 	.flags = BIT(CS_CPU_EXCLUSIVE) |
 		 BIT(CS_MEM_EXCLUSIVE) | BIT(CS_SCHED_LOAD_BALANCE),
 	.partition_root_state = PRS_ROOT,
-	.dl_bw_cpu = -1,
 };
 
 /**
@@ -580,8 +579,6 @@ static struct cpuset *dup_or_alloc_cpuset(struct cpuset *cs)
 	if (!trial)
 		return NULL;
 
-	trial->dl_bw_cpu = -1;
-
 	/* Setup cpumask pointer array */
 	cpumask_var_t *pmask[4] = {
 		&trial->cpus_allowed,
@@ -2984,6 +2981,7 @@ static int update_prstate(struct cpuset *cs, int new_prs)
 static struct cpuset *cpuset_attach_old_cs;
 static bool attach_cpus_updated;
 static bool attach_mems_updated;
+static bool attach_many_sources;
 
 /*
  * Check to see if a cpuset can accept a new task
@@ -3026,30 +3024,36 @@ static int cpuset_can_attach_check(struct cpuset *cs, struct cpuset *oldcs,
 	return 0;
 }
 
-static int cpuset_reserve_dl_bw(struct cpuset *cs)
+/*
+ * Reverse per-task operations done in cpuset_can_attach().
+ * If @stop_at is non-NULL, only undo tasks before it (partial rollback).
+ * If @stop_at is NULL, undo all tasks (full reversal for cancel_attach).
+ * Must be called with cpuset_mutex held.
+ */
+static void cpuset_undo_attach(struct cgroup_taskset *tset,
+			       struct task_struct *stop_at)
 {
-	int cpu, ret;
-
-	if (!cs->sum_migrate_dl_bw)
-		return 0;
-
-	cpu = cpumask_any_and(cpu_active_mask, cs->effective_cpus);
-	if (unlikely(cpu >= nr_cpu_ids))
-		return -EINVAL;
+	struct cgroup_subsys_state *css;
+	struct task_struct *task;
 
-	ret = dl_bw_alloc(cpu, cs->sum_migrate_dl_bw);
-	if (ret)
-		return ret;
+	cgroup_taskset_for_each(task, css, tset) {
+		struct cpuset *cs = css_cs(css);
+		struct cpuset *oldcs = task_cs(task);
 
-	cs->dl_bw_cpu = cpu;
-	return 0;
-}
+		if (task == stop_at)
+			break;
 
-static void reset_migrate_dl_data(struct cpuset *cs)
-{
-	cs->nr_migrate_dl_tasks = 0;
-	cs->sum_migrate_dl_bw = 0;
-	cs->dl_bw_cpu = -1;
+		if (dl_task(task)) {
+			cs->nr_deadline_tasks--;
+			oldcs->nr_deadline_tasks++;
+			if (dl_task_needs_bw_move(task, cs->effective_cpus)) {
+				int cpu = cpumask_any_and(cpu_active_mask,
+							 cs->effective_cpus);
+				dl_bw_free(cpu, task->dl.dl_bw);
+			}
+		}
+		dec_attach_in_progress_locked(cs);
+	}
 }
 
 /* Called by cgroups to determine if a cpuset is usable; cpuset_mutex held */
@@ -3061,96 +3065,79 @@ static int cpuset_can_attach(struct cgroup_taskset *tset)
 	bool setsched_check;
 	int ret;
 
-	/* used later by cpuset_attach() */
 	cpuset_attach_old_cs = task_cs(cgroup_taskset_first(tset, &css));
 	oldcs = cpuset_attach_old_cs;
 	cs = css_cs(css);
 
 	mutex_lock(&cpuset_mutex);
+	attach_many_sources = false;
 
-	/* Check to see if task is allowed in the cpuset */
 	ret = cpuset_can_attach_check(cs, oldcs, &setsched_check);
 	if (ret)
 		goto out_unlock;
 
-	/*
-	 * The cpuset_attach_old_cs is used mainly by cpuset_migrate_mm() to get
-	 * the old_mems_allowed value. There are two ways that many-to-one
-	 * cpuset migration can happen:
-	 * 1) A multithread application with threads in different cpusets is
-	 *    wholely migrated to a new cpuset.
-	 * 2) Disabling v2 cpuset controller will move all the tasks in child
-	 *    cpusets to the parent cpuset.
-	 *
-	 * In the former case, it is the mm setting of the group leader that
-	 * really matters. So cpuset_attach_old_cs should track the oldcs of the
-	 * group leader. It falls back to the oldcs of the first task if there
-	 * is no group leader in the taskset. In the latter case, effective_mems
-	 * of child cpusets must always be a subset of the parent. So no real
-	 * page migration will be necessary no matter which child cpuset is
-	 * selected as cpuset_attach_old_cs.
-	 */
 	cgroup_taskset_for_each(task, css, tset) {
+		struct cpuset *newcs = css_cs(css);
+		struct cpuset *new_oldcs = task_cs(task);
+
+		if (newcs != cs || new_oldcs != oldcs) {
+			if (new_oldcs != oldcs)
+				attach_many_sources = true;
+			cs = newcs;
+			oldcs = new_oldcs;
+			ret = cpuset_can_attach_check(cs, oldcs,
+						      &setsched_check);
+			if (ret)
+				goto out_rollback;
+		}
+
 		ret = task_can_attach(task);
 		if (ret)
-			goto out_unlock;
+			goto out_rollback;
 
-		/* Update cpuset_attach_old_cs to the latest group leader */
 		if (task == task->group_leader)
 			cpuset_attach_old_cs = task_cs(task);
 
 		if (setsched_check) {
 			ret = security_task_setscheduler(task);
 			if (ret)
-				goto out_unlock;
+				goto out_rollback;
 		}
 
 		if (dl_task(task)) {
-			/*
-			 * Count all migrating DL tasks for cpuset task accounting.
-			 * Only tasks that need a root-domain bandwidth move
-			 * contribute to sum_migrate_dl_bw.
-			 */
-			cs->nr_migrate_dl_tasks++;
-			if (dl_task_needs_bw_move(task, cs->effective_cpus))
-				cs->sum_migrate_dl_bw += task->dl.dl_bw;
+			cs->nr_deadline_tasks++;
+			oldcs->nr_deadline_tasks--;
+
+			if (dl_task_needs_bw_move(task, cs->effective_cpus)) {
+				int cpu = cpumask_any_and(cpu_active_mask,
+							 cs->effective_cpus);
+				if (unlikely(cpu >= nr_cpu_ids)) {
+					ret = -EINVAL;
+					goto out_rollback;
+				}
+				ret = dl_bw_alloc(cpu, task->dl.dl_bw);
+				if (ret)
+					goto out_rollback;
+			}
 		}
-	}
-
-	ret = cpuset_reserve_dl_bw(cs);
 
-out_unlock:
-	if (ret) {
-		reset_migrate_dl_data(cs);
-	} else {
-		/*
-		 * Mark attach is in progress.  This makes validate_change() fail
-		 * changes which zero cpus/mems_allowed.
-		 */
 		cs->attach_in_progress++;
 	}
 
+	goto out_unlock;
+
+out_rollback:
+	cpuset_undo_attach(tset, task);
+
+out_unlock:
 	mutex_unlock(&cpuset_mutex);
 	return ret;
 }
 
 static void cpuset_cancel_attach(struct cgroup_taskset *tset)
 {
-	struct cgroup_subsys_state *css;
-	struct cpuset *cs;
-
-	cgroup_taskset_first(tset, &css);
-	cs = css_cs(css);
-
 	mutex_lock(&cpuset_mutex);
-	dec_attach_in_progress_locked(cs);
-
-	if (cs->dl_bw_cpu >= 0)
-		dl_bw_free(cs->dl_bw_cpu, cs->sum_migrate_dl_bw);
-
-	if (cs->nr_migrate_dl_tasks)
-		reset_migrate_dl_data(cs);
-
+	cpuset_undo_attach(tset, NULL);
 	mutex_unlock(&cpuset_mutex);
 }
 
@@ -3232,8 +3219,15 @@ static void cpuset_attach(struct cgroup_taskset *tset)
 	mutex_lock(&cpuset_mutex);
 	queue_task_work = false;
 
-	attach_cpus_updated = !cpumask_equal(cs->effective_cpus, oldcs->effective_cpus);
-	attach_mems_updated = !nodes_equal(cs->effective_mems, oldcs->effective_mems);
+	if (attach_many_sources) {
+		attach_cpus_updated = true;
+		attach_mems_updated = true;
+	} else {
+		attach_cpus_updated = !cpumask_equal(cs->effective_cpus,
+						     oldcs->effective_cpus);
+		attach_mems_updated = !nodes_equal(cs->effective_mems,
+						    oldcs->effective_mems);
+	}
 
 	/*
 	 * In the default hierarchy, enabling cpuset in the child cgroups
@@ -3249,21 +3243,28 @@ static void cpuset_attach(struct cgroup_taskset *tset)
 		guarantee_online_mems(cs, &cpuset_attach_nodemask_to);
 	}
 
-	cgroup_taskset_for_each(task, css, tset)
+	cgroup_taskset_for_each(task, css, tset) {
+		struct cpuset *newcs = css_cs(css);
+
+		if (newcs != cs) {
+			cs->old_mems_allowed = cpuset_attach_nodemask_to;
+			cs = newcs;
+			if (cpuset_v2())
+				cpuset_attach_nodemask_to = cs->effective_mems;
+			else
+				guarantee_online_mems(cs, &cpuset_attach_nodemask_to);
+		}
 		cpuset_attach_task(cs, task);
+	}
 
 out:
 	if (queue_task_work)
 		schedule_flush_migrate_mm();
 	cs->old_mems_allowed = cpuset_attach_nodemask_to;
 
-	if (cs->nr_migrate_dl_tasks) {
-		cs->nr_deadline_tasks += cs->nr_migrate_dl_tasks;
-		oldcs->nr_deadline_tasks -= cs->nr_migrate_dl_tasks;
-		reset_migrate_dl_data(cs);
-	}
-
-	dec_attach_in_progress_locked(cs);
+	/* Decrement per-task attach_in_progress */
+	cgroup_taskset_for_each(task, css, tset)
+		dec_attach_in_progress_locked(css_cs(css));
 
 	mutex_unlock(&cpuset_mutex);
 }
-- 
2.43.0