sched: Introduce task_struct::user_cpus_ptr to track requested affinity

In preparation for saving and restoring the user-requested CPU affinity
mask of a task, add a new cpumask_t pointer to 'struct task_struct'.

If the pointer is non-NULL, then the mask is copied across fork() and
freed on task exit.

Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Valentin Schneider <Valentin.Schneider@arm.com>
Link: https://lore.kernel.org/r/20210730112443.23245-7-will@kernel.org

include/linux/sched.h

@@ -748,6 +748,7 @@ struct task_struct {
 	unsigned int			policy;
 	int				nr_cpus_allowed;
 	const cpumask_t			*cpus_ptr;
+	cpumask_t			*user_cpus_ptr;
 	cpumask_t			cpus_mask;
 	void				*migration_pending;
 #ifdef CONFIG_SMP
@@ -1706,6 +1707,8 @@ extern int task_can_attach(struct task_struct *p, const struct cpumask *cs_cpus_
 #ifdef CONFIG_SMP
 extern void do_set_cpus_allowed(struct task_struct *p, const struct cpumask *new_mask);
 extern int set_cpus_allowed_ptr(struct task_struct *p, const struct cpumask *new_mask);
+extern int dup_user_cpus_ptr(struct task_struct *dst, struct task_struct *src, int node);
+extern void release_user_cpus_ptr(struct task_struct *p);
 #else
 static inline void do_set_cpus_allowed(struct task_struct *p, const struct cpumask *new_mask)
 {
@@ -1716,6 +1719,16 @@ static inline int set_cpus_allowed_ptr(struct task_struct *p, const struct cpuma
 		return -EINVAL;
 	return 0;
 }
+static inline int dup_user_cpus_ptr(struct task_struct *dst, struct task_struct *src, int node)
+{
+	if (src->user_cpus_ptr)
+		return -EINVAL;
+	return 0;
+}
+static inline void release_user_cpus_ptr(struct task_struct *p)
+{
+	WARN_ON(p->user_cpus_ptr);
+}
 #endif
 
 extern int yield_to(struct task_struct *p, bool preempt);

init/init_task.c

@@ -80,6 +80,7 @@ struct task_struct init_task
 	.normal_prio	= MAX_PRIO - 20,
 	.policy		= SCHED_NORMAL,
 	.cpus_ptr	= &init_task.cpus_mask,
+	.user_cpus_ptr	= NULL,
 	.cpus_mask	= CPU_MASK_ALL,
 	.nr_cpus_allowed= NR_CPUS,
 	.mm		= NULL,

kernel/fork.c

@@ -446,6 +446,7 @@ void put_task_stack(struct task_struct *tsk)
 
 void free_task(struct task_struct *tsk)
 {
+	release_user_cpus_ptr(tsk);
 	scs_release(tsk);
 
 #ifndef CONFIG_THREAD_INFO_IN_TASK
@@ -919,6 +920,7 @@ static struct task_struct *dup_task_struct(struct task_struct *orig, int node)
 #endif
 	if (orig->cpus_ptr == &orig->cpus_mask)
 		tsk->cpus_ptr = &tsk->cpus_mask;
+	dup_user_cpus_ptr(tsk, orig, node);
 
 	/*
 	 * One for the user space visible state that goes away when reaped.

kernel/sched/core.c

@@ -2480,6 +2480,26 @@ void do_set_cpus_allowed(struct task_struct *p, const struct cpumask *new_mask)
 	__do_set_cpus_allowed(p, new_mask, 0);
 }
 
+int dup_user_cpus_ptr(struct task_struct *dst, struct task_struct *src,
+		      int node)
+{
+	if (!src->user_cpus_ptr)
+		return 0;
+
+	dst->user_cpus_ptr = kmalloc_node(cpumask_size(), GFP_KERNEL, node);
+	if (!dst->user_cpus_ptr)
+		return -ENOMEM;
+
+	cpumask_copy(dst->user_cpus_ptr, src->user_cpus_ptr);
+	return 0;
+}
+
+void release_user_cpus_ptr(struct task_struct *p)
+{
+	kfree(p->user_cpus_ptr);
+	p->user_cpus_ptr = NULL;
+}
+
 /*
  * This function is wildly self concurrent; here be dragons.
  *