Re: [cgroup/for-6.20 PATCH 3/4] cgroup/cpuset: Don't fail cpuset.cpus change in v2

[Waiman Long]

On 12/25/25 4:30 AM, Chen Ridong wrote:

On 2025/12/25 15:30, Waiman Long wrote:

Commit fe8cd2736e75 ("cgroup/cpuset: Delay setting of CS_CPU_EXCLUSIVE
until valid partition") introduced a new check to disallow the setting
of a new cpuset.cpus.exclusive value that is a superset of a sibling's
cpuset.cpus value so that there will at least be one CPU left in the
sibling in case the cpuset becomes a valid partition root. This new
check does have the side effect of failing a cpuset.cpus change that
make it a subset of a sibling's cpuset.cpus.exclusive value.

With v2, users are supposed to be allowed to set whatever value they
want in cpuset.cpus without failure. To maintain this rule, the check
is now restricted to only when cpuset.cpus.exclusive is being changed
not when cpuset.cpus is changed.

Signed-off-by: Waiman Long <long...@redhat.com>
---
  kernel/cgroup/cpuset.c | 30 +++++++++++++++---------------
  1 file changed, 15 insertions(+), 15 deletions(-)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 850334dbc36a..83bf6b588e5f 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -609,33 +609,31 @@ static inline bool cpusets_are_exclusive(struct cpuset 
*cs1, struct cpuset *cs2)

/**

   * cpus_excl_conflict - Check if two cpusets have exclusive CPU conflicts
- * @cs1: first cpuset to check
- * @cs2: second cpuset to check
+ * @trial:     the trial cpuset to be checked
+ * @sibling:   a sibling cpuset to be checked against
+ * @new_xcpus: new exclusive_cpus in trial cpuset
   *

Can we rename it to xcpus_changed?

The current name new_xcpus gives me the impression that CPUs are being added.
For example: if exclusive_cpus is 1, and it changes to 1-7, then new_xcpus 
would be 2-7.

Sure. I will make the change in the next version.

Cheers,
Longman

   * Returns: true if CPU exclusivity conflict exists, false otherwise
   *
   * Conflict detection rules:
   * 1. If either cpuset is CPU exclusive, they must be mutually exclusive
   * 2. exclusive_cpus masks cannot intersect between cpusets
- * 3. The allowed CPUs of one cpuset cannot be a subset of another's exclusive 
CPUs
+ * 3. The allowed CPUs of a sibling cpuset cannot be a subset of the new 
exclusive CPUs
   */
-static inline bool cpus_excl_conflict(struct cpuset *cs1, struct cpuset *cs2)
+static inline bool cpus_excl_conflict(struct cpuset *trial, struct cpuset 
*sibling,
+                                     bool new_xcpus)
  {
        /* If either cpuset is exclusive, check if they are mutually exclusive 
*/
-       if (is_cpu_exclusive(cs1) || is_cpu_exclusive(cs2))
-               return !cpusets_are_exclusive(cs1, cs2);
+       if (is_cpu_exclusive(trial) || is_cpu_exclusive(sibling))
+               return !cpusets_are_exclusive(trial, sibling);

/* Exclusive_cpus cannot intersect */

-       if (cpumask_intersects(cs1->exclusive_cpus, cs2->exclusive_cpus))
+       if (cpumask_intersects(trial->exclusive_cpus, sibling->exclusive_cpus))
                return true;

- /* The cpus_allowed of one cpuset cannot be a subset of another cpuset's exclusive_cpus */

-       if (!cpumask_empty(cs1->cpus_allowed) &&
-           cpumask_subset(cs1->cpus_allowed, cs2->exclusive_cpus))
-               return true;
-
-       if (!cpumask_empty(cs2->cpus_allowed) &&
-           cpumask_subset(cs2->cpus_allowed, cs1->exclusive_cpus))
+       /* The cpus_allowed of a sibling cpuset cannot be a subset of the new 
exclusive_cpus */
+       if (new_xcpus && !cpumask_empty(sibling->cpus_allowed) &&
+           cpumask_subset(sibling->cpus_allowed, trial->exclusive_cpus))
                return true;

return false;

@@ -672,6 +670,7 @@ static int validate_change(struct cpuset *cur, struct 
cpuset *trial)
  {
        struct cgroup_subsys_state *css;
        struct cpuset *c, *par;
+       bool new_xcpus;
        int ret = 0;

rcu_read_lock();

@@ -728,10 +727,11 @@ static int validate_change(struct cpuset *cur, struct 
cpuset *trial)
         * overlap. exclusive_cpus cannot overlap with each other if set.
         */
        ret = -EINVAL;
+       new_xcpus = !cpumask_equal(cur->exclusive_cpus, trial->exclusive_cpus);
        cpuset_for_each_child(c, css, par) {
                if (c == cur)
                        continue;
-               if (cpus_excl_conflict(trial, c))
+               if (cpus_excl_conflict(trial, c, new_xcpus))
                        goto out;
                if (mems_excl_conflict(trial, c))
                        goto out;