sched/mmcid: Prevent pointless work in mm_update_cpus_allowed()

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>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Link: https://patch.msgid.link/20251119172549.385208276@linutronix.de

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index f5e37c233b0162e0c7fa37fbafdf70f11c3bf7ae..2ea77e72f7c679a5899a1aac780346d795b3f499 100644 (file)
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2684,6 +2684,7 @@ void set_cpus_allowed_common(struct task_struct *p, struct affinity_context *ctx
 
        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 @@ __do_set_cpus_allowed(struct task_struct *p, struct affinity_context *ctx)
                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);
@@ -10376,12 +10376,17 @@ void call_trace_sched_update_nr_running(struct rq *rq, int count)
  */
 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));
 }