linux-kernel - [patch V2 06/20] sched/mmcid: Prevent pointless work in mm_update_cpus

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Message-ID: <20251022110555.775050699@linutronix.de>
Date: Wed, 22 Oct 2025 14:55:27 +0200 (CEST)
From: Thomas Gleixner <tglx@...utronix.de>
To: LKML <linux-kernel@...r.kernel.org>
Cc: Peter Zijlstra <peterz@...radead.org>,
 Gabriele Monaco <gmonaco@...hat.com>,
 Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
 Michael Jeanson <mjeanson@...icios.com>,
 Jens Axboe <axboe@...nel.dk>,
 "Paul E. McKenney" <paulmck@...nel.org>,
 "Gautham R. Shenoy" <gautham.shenoy@....com>,
 Florian Weimer <fweimer@...hat.com>,
 Tim Chen <tim.c.chen@...el.com>,
 Yury Norov <yury.norov@...il.com>
Subject: [patch V2 06/20] 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@...utronix.de>
---
 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));
 }