linux-kernel - [patch 06/19] sched/mmcid: Prevent pointless work in mm_update_cpus

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Message-ID: <20251015172834.694547089@linutronix.de>
Date: Wed, 15 Oct 2025 19:29:34 +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>,
 TCMalloc Team <tcmalloc-eng@...gle.com>
Subject: [patch 06/19] sched/mmcid: Prevent pointless work in
 mm_update_cpus_allowed()

The @nr_cpus_allowed management does way too much useless work for the
common case where a process starts with unrestricted affinity,
i.e. @nr_cpus_allowed is equal to the number of possible CPUs right
away.

Add a check whether that limit is reached already and then avoid the whole
cpumask update and evaluation.

Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
---
 kernel/sched/core.c |   16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2719,6 +2719,7 @@ 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
@@ -2765,7 +2766,6 @@ 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);
@@ -10408,12 +10408,20 @@ 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)
+	if (!mm || READ_ONCE(mm->mm_cid.nr_cpus_allowed) == nr_cpu_ids)
 		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);
+	/* Check again under the lock */
+	if (mm->mm_cid.nr_cpus_allowed == nr_cpu_ids)
+		return;
+	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));
 }