[patch V3 06/20] sched/mmcid: Prevent pointless work in mm_update_cpus_allowed()

Thomas Gleixner
2025-10-29T13:09:07Z

mm_update_cpus_allowed() is not required to be invoked for affinity changes
due to migrate_disable() and migrate_enable().

migrate_disable() restricts the task temporarily to a CPU on which the task
was already allowed to run, so nothing changes. migrate_enable() restores
the actual task affinity mask.

If that mask changed between migrate_disable() and migrate_enable() then
that change was already accounted for.

Move the invocation to the proper place to avoid that.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
V2: Remove the nr_cpu_ids optimization which does not really work - PeterZ
---
kernel/sched/core.c | 11 ++++++++---
1 file changed, 8 insertions(+), 3 deletions(-)

--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2684,6 +2684,7 @@ void set_cpus_allowed_common(struct task

cpumask_copy(&p->cpus_mask, ctx->new_mask);
p->nr_cpus_allowed = cpumask_weight(ctx->new_mask);
+ mm_update_cpus_allowed(p->mm, ctx->new_mask);

/*
* Swap in a new user_cpus_ptr if SCA_USER flag set
@@ -2730,7 +2731,6 @@ static void
put_prev_task(rq, p);

p->sched_class->set_cpus_allowed(p, ctx);
- mm_update_cpus_allowed(p->mm, ctx->new_mask);

if (queued)
enqueue_task(rq, p, ENQUEUE_RESTORE | ENQUEUE_NOCLOCK);
@@ -10374,12 +10374,17 @@ void call_trace_sched_update_nr_running(
*/
static inline void mm_update_cpus_allowed(struct mm_struct *mm, const struct cpumask *affmsk)
{
- struct cpumask *mm_allowed = mm_cpus_allowed(mm);
+ struct cpumask *mm_allowed;

if (!mm)
return;
- /* The mm_cpus_allowed is the union of each thread allowed CPUs masks. */
+
+ /*
+ * mm::mm_cid::mm_cpus_allowed is the superset of each threads
+ * allowed CPUs mask which means it can only grow.
+ */
guard(raw_spinlock)(&mm->mm_cid.lock);
+ mm_allowed = mm_cpus_allowed(mm);
cpumask_or(mm_allowed, mm_allowed, affmsk);
WRITE_ONCE(mm->mm_cid.nr_cpus_allowed, cpumask_weight(mm_allowed));
}