linux-kernel - Re: [PATCH v11 08/16] sched: Allow task CPU affinity to be restricted on asymmetric systems

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Message-ID: <YRz0uI6V58eGNL1C@hirez.programming.kicks-ass.net>
Date:   Wed, 18 Aug 2021 13:53:28 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Will Deacon <will@...nel.org>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-arch@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Catalin Marinas <catalin.marinas@....com>,
        Marc Zyngier <maz@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Qais Yousef <qais.yousef@....com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Quentin Perret <qperret@...gle.com>, Tejun Heo <tj@...nel.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Mark Rutland <mark.rutland@....com>, kernel-team@...roid.com
Subject: Re: [PATCH v11 08/16] sched: Allow task CPU affinity to be
 restricted on asymmetric systems

On Wed, Aug 18, 2021 at 12:56:41PM +0200, Peter Zijlstra wrote:
> On Wed, Aug 18, 2021 at 11:42:28AM +0100, Will Deacon wrote:

> > I think the idea looks good, but perhaps we could wrap things up a bit:
> > 
> > /* Comment about why this is useful with RT */
> > static cpumask_t *clear_user_cpus_ptr(struct task_struct *p)
> > {
> > 	struct cpumask *user_mask = NULL;
> > 
> > 	swap(user_mask, p->user_cpus_ptr);
> > 	return user_mask;
> > }
> > 
> > void release_user_cpus_ptr(struct task_struct *p)
> > {
> > 	kfree(clear_user_cpus_ptr(p));
> > }
> > 
> > Then just use clear_user_cpus_ptr() in sched/core.c where we know what
> > we're doing (well, at least one of us does!).
> 
> OK, I'll go make it like that.

Something like so then?

--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2497,10 +2497,18 @@ int dup_user_cpus_ptr(struct task_struct
 	return 0;
 }
 
+static inline struct cpumask *clear_user_cpus_ptr(struct task_struct *p)
+{
+	struct cpumask *user_mask = NULL;
+
+	swap(p->user_cpus_ptr, user_mask);
+
+	return user_mask;
+}
+
 void release_user_cpus_ptr(struct task_struct *p)
 {
-	kfree(p->user_cpus_ptr);
-	p->user_cpus_ptr = NULL;
+	kfree(clear_user_cpus_ptr(p));
 }
 
 /*
@@ -2733,6 +2741,7 @@ static int __set_cpus_allowed_ptr_locked
 	const struct cpumask *cpu_allowed_mask = task_cpu_possible_mask(p);
 	const struct cpumask *cpu_valid_mask = cpu_active_mask;
 	bool kthread = p->flags & PF_KTHREAD;
+	struct cpumask *user_mask = NULL;
 	unsigned int dest_cpu;
 	int ret = 0;
 
@@ -2792,9 +2801,13 @@ static int __set_cpus_allowed_ptr_locked
 	__do_set_cpus_allowed(p, new_mask, flags);
 
 	if (flags & SCA_USER)
-		release_user_cpus_ptr(p);
+		user_mask = clear_user_cpus_ptr(p);
 
-	return affine_move_task(rq, p, rf, dest_cpu, flags);
+	ret = affine_move_task(rq, p, rf, dest_cpu, flags);
+
+	kfree(user_mask);
+
+	return ret;
 
 out:
 	task_rq_unlock(rq, p, rf);
@@ -2941,20 +2954,22 @@ __sched_setaffinity(struct task_struct *
  */
 void relax_compatible_cpus_allowed_ptr(struct task_struct *p)
 {
+	struct cpumask *user_mask = p->user_cpus_ptr;
 	unsigned long flags;
-	struct cpumask *mask = p->user_cpus_ptr;
 
 	/*
 	 * Try to restore the old affinity mask. If this fails, then
 	 * we free the mask explicitly to avoid it being inherited across
 	 * a subsequent fork().
 	 */
-	if (!mask || !__sched_setaffinity(p, mask))
+	if (!user_mask || !__sched_setaffinity(p, user_mask))
 		return;
 
 	raw_spin_lock_irqsave(&p->pi_lock, flags);
-	release_user_cpus_ptr(p);
+	user_mask = clear_user_cpus_ptr(p);
 	raw_spin_unlock_irqrestore(&p->pi_lock, flags);
+
+	kfree(user_mask);
 }
 
 void set_task_cpu(struct task_struct *p, unsigned int new_cpu)