[RFC PATCH v3 1/3] sched/isolation: Add infrastructure for dynamic CPU isolation

[Costa Shulyupin <costa.shul@redhat.com>]

Introduce the infrastructure function housekeeping_update()
to modify housekeeping_cpumask at runtime and update the
configurations of dependent subsystems accordingly in
following patches.

Parent patch:
sched/isolation: Exclude dynamically isolated CPUs from housekeeping masks
https://lore.kernel.org/lkml/20240821142312.236970-1-longman@redhat.com/

Signed-off-by: Costa Shulyupin <costa.shul@redhat.com>

---

Note: In theory, updating housekeeping.flags only once is possible.
However, this approach would make this and following code less clear
and harder to follow and review.

Changes in v3:
- Remove redundant WRITE_ONCE. The first WRITE_ONCE is located
in housekeeping_update() because this function will
update dependent subsystems with changes of the housekeeping masks.

Changes in v2:
- remove unnecessary `err` variable
- add for_each_clear_bit... to clear isolated CPUs
- Address Gleixner's comments:
- use WRITE_ONCE to change housekeeping.flags
- use `struct cpumask *update` in signature of housekeeping_update

v1:
- https://lore.kernel.org/lkml/20240516190437.3545310-2-costa.shul@redhat.com/
---
 kernel/sched/isolation.c | 43 ++++++++++++++++++++++++++++++++++++----
 1 file changed, 39 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c
index 2ee47bc25aea..ebbb215505e8 100644
--- a/kernel/sched/isolation.c
+++ b/kernel/sched/isolation.c
@@ -127,6 +127,40 @@ static void __init housekeeping_setup_type(enum hk_type type,
 		     housekeeping_staging);
 }
 
+/*
+ * housekeeping_update - change housekeeping.cpumasks[type] and propagate the
+ * change.
+ */
+static int housekeeping_update(enum hk_type type, const struct cpumask *update)
+{
+	struct {
+		struct cpumask changed;
+		struct cpumask enable;
+		struct cpumask disable;
+	} *masks;
+
+	masks = kmalloc(sizeof(*masks), GFP_KERNEL);
+	if (!masks)
+		return -ENOMEM;
+
+	lockdep_assert_cpus_held();
+	cpumask_xor(&masks->changed, housekeeping_cpumask(type), update);
+	cpumask_and(&masks->enable, &masks->changed, update);
+	cpumask_andnot(&masks->disable, &masks->changed, update);
+	cpumask_copy(housekeeping.cpumasks[type], update);
+	WRITE_ONCE(housekeeping.flags, housekeeping.flags | BIT(type));
+	if (!static_branch_unlikely(&housekeeping_overridden))
+		static_key_enable_cpuslocked(&housekeeping_overridden.key);
+
+	/* Add here code to update dependent subsystems with
+	 * changes of the housekeeping masks.
+	 */
+
+	kfree(masks);
+
+	return 0;
+}
+
 static int __init housekeeping_setup(char *str, unsigned long flags)
 {
 	cpumask_var_t non_housekeeping_mask, housekeeping_staging;
@@ -330,10 +364,12 @@ int housekeeping_exlude_isolcpus(const struct cpumask *isolcpus, unsigned long f
 		/*
 		 * Reset housekeeping to bootup default
 		 */
+
+		for_each_clear_bit(type, &boot_hk_flags, HK_TYPE_MAX)
+			housekeeping_update(type, cpu_possible_mask);
 		for_each_set_bit(type, &boot_hk_flags, HK_TYPE_MAX)
-			cpumask_copy(housekeeping.cpumasks[type], boot_hk_cpumask);
+			housekeeping_update(type, boot_hk_cpumask);
 
-		WRITE_ONCE(housekeeping.flags, boot_hk_flags);
 		if (!boot_hk_flags && static_key_enabled(&housekeeping_overridden))
 			static_key_disable_cpuslocked(&housekeeping_overridden.key);
 		return 0;
@@ -358,9 +394,8 @@ int housekeeping_exlude_isolcpus(const struct cpumask *isolcpus, unsigned long f
 		cpumask_andnot(tmp_mask, src_mask, isolcpus);
 		if (!cpumask_intersects(tmp_mask, cpu_online_mask))
 			return -EINVAL;	/* Invalid isolated CPUs */
-		cpumask_copy(housekeeping.cpumasks[type], tmp_mask);
+		housekeeping_update(type, tmp_mask);
 	}
-	WRITE_ONCE(housekeeping.flags, boot_hk_flags | flags);
 	excluded = true;
 	if (!static_key_enabled(&housekeeping_overridden))
 		static_key_enable_cpuslocked(&housekeeping_overridden.key);
-- 
2.47.0