[PATCH v11 5/9] cpuset: Make sure that partition flag work properly with CPU hotplug

[Waiman Long]

When there is a cpu hotplug event (CPU online or offline), the partitions
may need to be reconfigured and regenerated. So code is added to
the hotplug functions to make them work with new reserved_cpus mask to
compute the right effective_cpus for each of the affected cpusets.

Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
---
 Documentation/admin-guide/cgroup-v2.rst | 10 ++++++++++
 kernel/cgroup/cpuset.c                  | 26 ++++++++++++++++++++++++--
 2 files changed, 34 insertions(+), 2 deletions(-)

diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
index 5f3170f..f4efa40 100644
--- a/Documentation/admin-guide/cgroup-v2.rst
+++ b/Documentation/admin-guide/cgroup-v2.rst
@@ -1626,6 +1626,16 @@ Cpuset Interface Files
 	2) No CPU that has been distributed to child partition roots is
 	   is deleted.
 
+	When all the CPUs allocated to a partition are offlined, the
+	partition will be temporaily gone and all the tasks in it will
+	be migrated to another one that belongs to the parent of the
+	partition root.  This is a destructive operation and all the
+	existing CPU affinity that is narrower than the cpuset itself
+	will be lost.
+
+	When any of those offlined CPUs is onlined again, a new partition
+	will be re-created and the tasks will be migrated back.
+
 
 Device controller
 -----------------
diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 3a646a9..e118dd8 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -900,7 +900,8 @@ static void update_tasks_cpumask(struct cpuset *cs)
  * @parent: the parent cpuset
  *
  * If the parent has reserved CPUs, include them in the list of allowable
- * CPUs in computing the new effective_cpus mask.
+ * CPUs in computing the new effective_cpus mask. The cpu_active_mask is
+ * used to mask off cpus that are to be offlined.
  */
 static void compute_effective_cpumask(struct cpumask *new_cpus,
 				      struct cpuset *cs, struct cpuset *parent)
@@ -909,6 +910,7 @@ static void compute_effective_cpumask(struct cpumask *new_cpus,
 		cpumask_or(new_cpus, parent->effective_cpus,
 			   parent->reserved_cpus);
 		cpumask_and(new_cpus, new_cpus, cs->cpus_allowed);
+		cpumask_and(new_cpus, new_cpus, cpu_active_mask);
 	} else {
 		cpumask_and(new_cpus, cs->cpus_allowed, parent->effective_cpus);
 	}
@@ -2562,9 +2564,17 @@ static void cpuset_hotplug_update_tasks(struct cpuset *cs)
 		goto retry;
 	}
 
-	cpumask_and(&new_cpus, cs->cpus_allowed, parent_cs(cs)->effective_cpus);
+	compute_effective_cpumask(&new_cpus, cs, parent_cs(cs));
 	nodes_and(new_mems, cs->mems_allowed, parent_cs(cs)->effective_mems);
 
+	if (cs->nr_reserved) {
+		/*
+		 * Some of the CPUs may have been distributed to child
+		 * partition roots. So we need skip those when computing the
+		 * real effective cpus.
+		 */
+		cpumask_andnot(&new_cpus, &new_cpus, cs->reserved_cpus);
+	}
 	cpus_updated = !cpumask_equal(&new_cpus, cs->effective_cpus);
 	mems_updated = !nodes_equal(new_mems, cs->effective_mems);
 
@@ -2614,6 +2624,11 @@ static void cpuset_hotplug_workfn(struct work_struct *work)
 	cpumask_copy(&new_cpus, cpu_active_mask);
 	new_mems = node_states[N_MEMORY];
 
+	/*
+	 * If reserved_cpus is populated, it is likely that the check below
+	 * will produce a false positive on cpus_updated when the cpu list
+	 * isn't changed. It is extra work, but it is better to be safe.
+	 */
 	cpus_updated = !cpumask_equal(top_cpuset.effective_cpus, &new_cpus);
 	mems_updated = !nodes_equal(top_cpuset.effective_mems, new_mems);
 
@@ -2622,6 +2637,13 @@ static void cpuset_hotplug_workfn(struct work_struct *work)
 		spin_lock_irq(&callback_lock);
 		if (!on_dfl)
 			cpumask_copy(top_cpuset.cpus_allowed, &new_cpus);
+		/*
+		 * Make sure that the reserved cpus aren't in the
+		 * effective cpus.
+		 */
+		if (top_cpuset.nr_reserved)
+			cpumask_andnot(&new_cpus, &new_cpus,
+					top_cpuset.reserved_cpus);
 		cpumask_copy(top_cpuset.effective_cpus, &new_cpus);
 		spin_unlock_irq(&callback_lock);
 		/* we don't mess with cpumasks of tasks in top_cpuset */
-- 
1.8.3.1