sched: Trivial core scheduling cookie management

In order to not have to use pid_struct, create a new, smaller,
structure to manage task cookies for core scheduling.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Don Hiatt <dhiatt@digitalocean.com>
Tested-by: Hongyu Ning <hongyu.ning@linux.intel.com>
Tested-by: Vincent Guittot <vincent.guittot@linaro.org>
Link: https://lkml.kernel.org/r/20210422123308.919768100@infradead.org

include/linux/sched.h

@@ -2179,4 +2179,10 @@ int sched_trace_rq_nr_running(struct rq *rq);
 
 const struct cpumask *sched_trace_rd_span(struct root_domain *rd);
 
+#ifdef CONFIG_SCHED_CORE
+extern void sched_core_free(struct task_struct *tsk);
+#else
+static inline void sched_core_free(struct task_struct *tsk) { }
+#endif
+
 #endif

kernel/fork.c

@@ -742,6 +742,7 @@ void __put_task_struct(struct task_struct *tsk)
 	exit_creds(tsk);
 	delayacct_tsk_free(tsk);
 	put_signal_struct(tsk->signal);
+	sched_core_free(tsk);
 
 	if (!profile_handoff_task(tsk))
 		free_task(tsk);

kernel/sched/Makefile

@@ -36,3 +36,4 @@ obj-$(CONFIG_CPU_FREQ_GOV_SCHEDUTIL) += cpufreq_schedutil.o
 obj-$(CONFIG_MEMBARRIER) += membarrier.o
 obj-$(CONFIG_CPU_ISOLATION) += isolation.o
 obj-$(CONFIG_PSI) += psi.o
+obj-$(CONFIG_SCHED_CORE) += core_sched.o

kernel/sched/core.c

@@ -167,7 +167,7 @@ static inline int rb_sched_core_cmp(const void *key, const struct rb_node *node)
 	return 0;
 }
 
-static void sched_core_enqueue(struct rq *rq, struct task_struct *p)
+void sched_core_enqueue(struct rq *rq, struct task_struct *p)
 {
 	rq->core->core_task_seq++;
 
@@ -177,14 +177,15 @@ static void sched_core_enqueue(struct rq *rq, struct task_struct *p)
 	rb_add(&p->core_node, &rq->core_tree, rb_sched_core_less);
 }
 
-static void sched_core_dequeue(struct rq *rq, struct task_struct *p)
+void sched_core_dequeue(struct rq *rq, struct task_struct *p)
 {
 	rq->core->core_task_seq++;
 
-	if (!p->core_cookie)
+	if (!sched_core_enqueued(p))
 		return;
 
 	rb_erase(&p->core_node, &rq->core_tree);
+	RB_CLEAR_NODE(&p->core_node);
 }
 
 /*

kernel/sched/core_sched.c

@@ -0,0 +1,109 @@
+// SPDX-License-Identifier: GPL-2.0-only
+
+#include "sched.h"
+
+/*
+ * A simple wrapper around refcount. An allocated sched_core_cookie's
+ * address is used to compute the cookie of the task.
+ */
+struct sched_core_cookie {
+	refcount_t refcnt;
+};
+
+unsigned long sched_core_alloc_cookie(void)
+{
+	struct sched_core_cookie *ck = kmalloc(sizeof(*ck), GFP_KERNEL);
+	if (!ck)
+		return 0;
+
+	refcount_set(&ck->refcnt, 1);
+	sched_core_get();
+
+	return (unsigned long)ck;
+}
+
+void sched_core_put_cookie(unsigned long cookie)
+{
+	struct sched_core_cookie *ptr = (void *)cookie;
+
+	if (ptr && refcount_dec_and_test(&ptr->refcnt)) {
+		kfree(ptr);
+		sched_core_put();
+	}
+}
+
+unsigned long sched_core_get_cookie(unsigned long cookie)
+{
+	struct sched_core_cookie *ptr = (void *)cookie;
+
+	if (ptr)
+		refcount_inc(&ptr->refcnt);
+
+	return cookie;
+}
+
+/*
+ * sched_core_update_cookie - replace the cookie on a task
+ * @p: the task to update
+ * @cookie: the new cookie
+ *
+ * Effectively exchange the task cookie; caller is responsible for lifetimes on
+ * both ends.
+ *
+ * Returns: the old cookie
+ */
+unsigned long sched_core_update_cookie(struct task_struct *p, unsigned long cookie)
+{
+	unsigned long old_cookie;
+	struct rq_flags rf;
+	struct rq *rq;
+	bool enqueued;
+
+	rq = task_rq_lock(p, &rf);
+
+	/*
+	 * Since creating a cookie implies sched_core_get(), and we cannot set
+	 * a cookie until after we've created it, similarly, we cannot destroy
+	 * a cookie until after we've removed it, we must have core scheduling
+	 * enabled here.
+	 */
+	SCHED_WARN_ON((p->core_cookie || cookie) && !sched_core_enabled(rq));
+
+	enqueued = sched_core_enqueued(p);
+	if (enqueued)
+		sched_core_dequeue(rq, p);
+
+	old_cookie = p->core_cookie;
+	p->core_cookie = cookie;
+
+	if (enqueued)
+		sched_core_enqueue(rq, p);
+
+	/*
+	 * If task is currently running, it may not be compatible anymore after
+	 * the cookie change, so enter the scheduler on its CPU to schedule it
+	 * away.
+	 */
+	if (task_running(rq, p))
+		resched_curr(rq);
+
+	task_rq_unlock(rq, p, &rf);
+
+	return old_cookie;
+}
+
+static unsigned long sched_core_clone_cookie(struct task_struct *p)
+{
+	unsigned long cookie, flags;
+
+	raw_spin_lock_irqsave(&p->pi_lock, flags);
+	cookie = sched_core_get_cookie(p->core_cookie);
+	raw_spin_unlock_irqrestore(&p->pi_lock, flags);
+
+	return cookie;
+}
+
+void sched_core_free(struct task_struct *p)
+{
+	sched_core_put_cookie(p->core_cookie);
+}

kernel/sched/sched.h

@@ -1229,6 +1229,22 @@ static inline bool sched_group_cookie_match(struct rq *rq,
 
 extern void queue_core_balance(struct rq *rq);
 
+static inline bool sched_core_enqueued(struct task_struct *p)
+{
+	return !RB_EMPTY_NODE(&p->core_node);
+}
+
+extern void sched_core_enqueue(struct rq *rq, struct task_struct *p);
+extern void sched_core_dequeue(struct rq *rq, struct task_struct *p);
+
+extern void sched_core_get(void);
+extern void sched_core_put(void);
+
+extern unsigned long sched_core_alloc_cookie(void);
+extern void sched_core_put_cookie(unsigned long cookie);
+extern unsigned long sched_core_get_cookie(unsigned long cookie);
+extern unsigned long sched_core_update_cookie(struct task_struct *p, unsigned long cookie);
+
 #else /* !CONFIG_SCHED_CORE */
 
 static inline bool sched_core_enabled(struct rq *rq)