sched/deadline: Generalize unique visiting of root domains

Bandwidth checks and updates that work on root domains currently employ
a cookie mechanism for efficiency. This mechanism is very much tied to
when root domains are first created and initialized.

Generalize the cookie mechanism so that it can be used also later at
runtime while updating root domains. Also, additionally guard it with
sched_domains_mutex, since domains need to be stable while updating them
(and it will be required for further dynamic changes).

Fixes: 53916d5 ("sched/deadline: Check bandwidth overflow earlier for hotplug")
Reported-by: Jon Hunter <[email protected]>
Signed-off-by: Juri Lelli <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Reviewed-by: Valentin Schneider <[email protected]>
Reviewed-by: Dietmar Eggemann <[email protected]>
Tested-by: Waiman Long <[email protected]>
Tested-by: Jon Hunter <[email protected]>
Tested-by: Dietmar Eggemann <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: jlelli

include/linux/sched/deadline.h

@@ -37,4 +37,7 @@ extern void dl_clear_root_domain(struct root_domain *rd);
 
 #endif /* CONFIG_SMP */
 
+extern u64 dl_cookie;
+extern bool dl_bw_visited(int cpu, u64 cookie);
+
 #endif /* _LINUX_SCHED_DEADLINE_H */

kernel/sched/deadline.c

@@ -166,14 +166,14 @@ static inline unsigned long dl_bw_capacity(int i)
 	}
 }
 
-static inline bool dl_bw_visited(int cpu, u64 gen)
+static inline bool dl_bw_visited(int cpu, u64 cookie)
 {
 	struct root_domain *rd = cpu_rq(cpu)->rd;
 
-	if (rd->visit_gen == gen)
+	if (rd->visit_cookie == cookie)
 		return true;
 
-	rd->visit_gen = gen;
+	rd->visit_cookie = cookie;
 	return false;
 }
 
@@ -207,7 +207,7 @@ static inline unsigned long dl_bw_capacity(int i)
 	return SCHED_CAPACITY_SCALE;
 }
 
-static inline bool dl_bw_visited(int cpu, u64 gen)
+static inline bool dl_bw_visited(int cpu, u64 cookie)
 {
 	return false;
 }
@@ -3171,15 +3171,18 @@ DEFINE_SCHED_CLASS(dl) = {
 #endif
 };
 
-/* Used for dl_bw check and update, used under sched_rt_handler()::mutex */
-static u64 dl_generation;
+/*
+ * Used for dl_bw check and update, used under sched_rt_handler()::mutex and
+ * sched_domains_mutex.
+ */
+u64 dl_cookie;
 
 int sched_dl_global_validate(void)
 {
 	u64 runtime = global_rt_runtime();
 	u64 period = global_rt_period();
 	u64 new_bw = to_ratio(period, runtime);
-	u64 gen = ++dl_generation;
+	u64 cookie = ++dl_cookie;
 	struct dl_bw *dl_b;
 	int cpu, cpus, ret = 0;
 	unsigned long flags;
@@ -3192,7 +3195,7 @@ int sched_dl_global_validate(void)
 	for_each_online_cpu(cpu) {
 		rcu_read_lock_sched();
 
-		if (dl_bw_visited(cpu, gen))
+		if (dl_bw_visited(cpu, cookie))
 			goto next;
 
 		dl_b = dl_bw_of(cpu);
@@ -3229,7 +3232,7 @@ static void init_dl_rq_bw_ratio(struct dl_rq *dl_rq)
 void sched_dl_do_global(void)
 {
 	u64 new_bw = -1;
-	u64 gen = ++dl_generation;
+	u64 cookie = ++dl_cookie;
 	struct dl_bw *dl_b;
 	int cpu;
 	unsigned long flags;
@@ -3240,7 +3243,7 @@ void sched_dl_do_global(void)
 	for_each_possible_cpu(cpu) {
 		rcu_read_lock_sched();
 
-		if (dl_bw_visited(cpu, gen)) {
+		if (dl_bw_visited(cpu, cookie)) {
 			rcu_read_unlock_sched();
 			continue;
 		}

kernel/sched/rt.c

@@ -2910,6 +2910,7 @@ static int sched_rt_handler(const struct ctl_table *table, int write, void *buff
 	int ret;
 
 	mutex_lock(&mutex);
+	sched_domains_mutex_lock();
 	old_period = sysctl_sched_rt_period;
 	old_runtime = sysctl_sched_rt_runtime;
 
@@ -2936,6 +2937,7 @@ static int sched_rt_handler(const struct ctl_table *table, int write, void *buff
 		sysctl_sched_rt_period = old_period;
 		sysctl_sched_rt_runtime = old_runtime;
 	}
+	sched_domains_mutex_unlock();
 	mutex_unlock(&mutex);
 
 	return ret;

kernel/sched/sched.h

@@ -998,7 +998,7 @@ struct root_domain {
 	 * Also, some corner cases, like 'wrap around' is dangerous, but given
 	 * that u64 is 'big enough'. So that shouldn't be a concern.
 	 */
-	u64 visit_gen;
+	u64 visit_cookie;
 
 #ifdef HAVE_RT_PUSH_IPI
 	/*

kernel/sched/topology.c

@@ -568,7 +568,7 @@ static int init_rootdomain(struct root_domain *rd)
 	rd->rto_push_work = IRQ_WORK_INIT_HARD(rto_push_irq_work_func);
 #endif
 
-	rd->visit_gen = 0;
+	rd->visit_cookie = 0;
 	init_dl_bw(&rd->dl_bw);
 	if (cpudl_init(&rd->cpudl) != 0)
 		goto free_rto_mask;