[tip: core/rseq] sched/mmcid: Prevent pointless work in mm_update_cpus_allowed()

[tip-bot2 for Thomas Gleixner]

The following commit has been merged into the core/rseq branch of tip:

Commit-ID:     0d032a43ebeb9bf255cd7e3dad5f7a6371571648
Gitweb:        https://git.kernel.org/tip/0d032a43ebeb9bf255cd7e3dad5f7a6371571648
Author:        Thomas Gleixner <tglx@xxxxxxxxxxxxx>
AuthorDate:    Wed, 19 Nov 2025 18:26:55 +01:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Thu, 20 Nov 2025 12:14:54 +01:00

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@xxxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
Link: https://patch.msgid.link/20251119172549.385208276@xxxxxxxxxxxxx
---
 kernel/sched/core.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index f5e37c2..2ea77e7 100644
--- 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));
 }