Merge branch 'sched-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull scheduler updates from Thomas Gleixner:
 "Third more careful attempt for this set of fixes:

   - Prevent a 32bit math overflow in the cpufreq code

   - Fix a buffer overflow when scanning the cgroup2 cpu.max property

   - A set of fixes for the NOHZ scheduler logic to prevent waking up
     CPUs even if the capacity of the busy CPUs is sufficient along with
     other tweaks optimizing the behaviour for asymmetric systems
     (big/little)"

* 'sched-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  sched/fair: Skip LLC NOHZ logic for asymmetric systems
  sched/fair: Tune down misfit NOHZ kicks
  sched/fair: Comment some nohz_balancer_kick() kick conditions
  sched/core: Fix buffer overflow in cgroup2 property cpu.max
  sched/cpufreq: Fix 32-bit math overflow

1 files changed, 63 insertions, 21 deletions

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index ea74d43924b2..fdab7eb6f351 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8060,6 +8060,18 @@ check_cpu_capacity(struct rq *rq, struct sched_domain *sd)
 }
 
 /*
+ * Check whether a rq has a misfit task and if it looks like we can actually
+ * help that task: we can migrate the task to a CPU of higher capacity, or
+ * the task's current CPU is heavily pressured.
+ */
+static inline int check_misfit_status(struct rq *rq, struct sched_domain *sd)
+{
+	return rq->misfit_task_load &&
+		(rq->cpu_capacity_orig < rq->rd->max_cpu_capacity ||
+		 check_cpu_capacity(rq, sd));
+}
+
+/*
  * Group imbalance indicates (and tries to solve) the problem where balancing
  * groups is inadequate due to ->cpus_allowed constraints.
  *
@@ -9586,35 +9598,21 @@ static void nohz_balancer_kick(struct rq *rq)
 	if (time_before(now, nohz.next_balance))
 		goto out;
 
-	if (rq->nr_running >= 2 || rq->misfit_task_load) {
+	if (rq->nr_running >= 2) {
 		flags = NOHZ_KICK_MASK;
 		goto out;
 	}
 
 	rcu_read_lock();
-	sds = rcu_dereference(per_cpu(sd_llc_shared, cpu));
-	if (sds) {
-		/*
-		 * If there is an imbalance between LLC domains (IOW we could
-		 * increase the overall cache use), we need some less-loaded LLC
-		 * domain to pull some load. Likewise, we may need to spread
-		 * load within the current LLC domain (e.g. packed SMT cores but
-		 * other CPUs are idle). We can't really know from here how busy
-		 * the others are - so just get a nohz balance going if it looks
-		 * like this LLC domain has tasks we could move.
-		 */
-		nr_busy = atomic_read(&sds->nr_busy_cpus);
-		if (nr_busy > 1) {
-			flags = NOHZ_KICK_MASK;
-			goto unlock;
-		}
-
-	}
 
 	sd = rcu_dereference(rq->sd);
 	if (sd) {
-		if ((rq->cfs.h_nr_running >= 1) &&
-		    check_cpu_capacity(rq, sd)) {
+		/*
+		 * If there's a CFS task and the current CPU has reduced
+		 * capacity; kick the ILB to see if there's a better CPU to run
+		 * on.
+		 */
+		if (rq->cfs.h_nr_running >= 1 && check_cpu_capacity(rq, sd)) {
 			flags = NOHZ_KICK_MASK;
 			goto unlock;
 		}
@@ -9622,6 +9620,11 @@ static void nohz_balancer_kick(struct rq *rq)
 
 	sd = rcu_dereference(per_cpu(sd_asym_packing, cpu));
 	if (sd) {
+		/*
+		 * When ASYM_PACKING; see if there's a more preferred CPU
+		 * currently idle; in which case, kick the ILB to move tasks
+		 * around.
+		 */
 		for_each_cpu_and(i, sched_domain_span(sd), nohz.idle_cpus_mask) {
 			if (sched_asym_prefer(i, cpu)) {
 				flags = NOHZ_KICK_MASK;
@@ -9629,6 +9632,45 @@ static void nohz_balancer_kick(struct rq *rq)
 			}
 		}
 	}
+
+	sd = rcu_dereference(per_cpu(sd_asym_cpucapacity, cpu));
+	if (sd) {
+		/*
+		 * When ASYM_CPUCAPACITY; see if there's a higher capacity CPU
+		 * to run the misfit task on.
+		 */
+		if (check_misfit_status(rq, sd)) {
+			flags = NOHZ_KICK_MASK;
+			goto unlock;
+		}
+
+		/*
+		 * For asymmetric systems, we do not want to nicely balance
+		 * cache use, instead we want to embrace asymmetry and only
+		 * ensure tasks have enough CPU capacity.
+		 *
+		 * Skip the LLC logic because it's not relevant in that case.
+		 */
+		goto unlock;
+	}
+
+	sds = rcu_dereference(per_cpu(sd_llc_shared, cpu));
+	if (sds) {
+		/*
+		 * If there is an imbalance between LLC domains (IOW we could
+		 * increase the overall cache use), we need some less-loaded LLC
+		 * domain to pull some load. Likewise, we may need to spread
+		 * load within the current LLC domain (e.g. packed SMT cores but
+		 * other CPUs are idle). We can't really know from here how busy
+		 * the others are - so just get a nohz balance going if it looks
+		 * like this LLC domain has tasks we could move.
+		 */
+		nr_busy = atomic_read(&sds->nr_busy_cpus);
+		if (nr_busy > 1) {
+			flags = NOHZ_KICK_MASK;
+			goto unlock;
+		}
+	}
 unlock:
 	rcu_read_unlock();
 out: