[PATCH v2] sched/fair: Check a task has a fitting cpu when updating misfit

[Qais Yousef <qyousef@layalina.io>]

From: Qais Yousef <qais.yousef@arm.com>

If a misfit task is affined to a subset of the possible cpus, we need to
verify that one of these cpus can fit it. Otherwise the load balancer
code will continuously trigger needlessly leading the balance_interval
to increase in return and eventually end up with a situation where real
imbalances take a long time to address because of this impossible
imbalance situation.

This can happen in Android world where it's common for background tasks
to be restricted to little cores.

Similarly if we can't fit the biggest core, triggering misfit is
pointless as it is the best we can ever get on this system.

To be able to detect that; we use asym_cap_list to iterate through
capacities in the system to see if the task is able to run at a higher
capacity level based on its p->cpus_ptr.

To be able to iterate through capacity levels, export asym_cap_list to
allow for fast traversal of all available capacity levels in the system.

Test:
=====

Add

	trace_printk("balance_interval = %lu\n", interval)

in get_sd_balance_interval().

run
	if [ "$MASK" != "0" ]; then
		adb shell "taskset -a $MASK cat /dev/zero > /dev/null"
	fi
	sleep 10
	// parse ftrace buffer counting the occurrence of each valaue

Where MASK is either:

	* 0: no busy task running
	* 1: busy task is pinned to 1 cpu; handled today to not cause
	  misfit
	* f: busy task pinned to little cores, simulates busy background
	  task, demonstrates the problem to be fixed

Results:
========

Note how occurrence of balance_interval = 128 overshoots for MASK = f.

BEFORE
------

	MASK=0

		   1 balance_interval = 175
		 120 balance_interval = 128
		 846 balance_interval = 64
		  55 balance_interval = 63
		 215 balance_interval = 32
		   2 balance_interval = 31
		   2 balance_interval = 16
		   4 balance_interval = 8
		1870 balance_interval = 4
		  65 balance_interval = 2

	MASK=1

		  27 balance_interval = 175
		  37 balance_interval = 127
		 840 balance_interval = 64
		 167 balance_interval = 63
		 449 balance_interval = 32
		  84 balance_interval = 31
		 304 balance_interval = 16
		1156 balance_interval = 8
		2781 balance_interval = 4
		 428 balance_interval = 2

	MASK=f

		   1 balance_interval = 175
		1328 balance_interval = 128
		  44 balance_interval = 64
		 101 balance_interval = 63
		  25 balance_interval = 32
		   5 balance_interval = 31
		  23 balance_interval = 16
		  23 balance_interval = 8
		4306 balance_interval = 4
		 177 balance_interval = 2

AFTER
-----

Note how the high values almost disappear for all MASK values. The
system has background tasks that could trigger the problem without
simulate it even with MASK=0.

	MASK=0

		 103 balance_interval = 63
		  19 balance_interval = 31
		 194 balance_interval = 8
		4827 balance_interval = 4
		 179 balance_interval = 2

	MASK=1

		 131 balance_interval = 63
		   1 balance_interval = 31
		  87 balance_interval = 8
		3600 balance_interval = 4
		   7 balance_interval = 2

	MASK=f

		   8 balance_interval = 127
		 182 balance_interval = 63
		   3 balance_interval = 31
		   9 balance_interval = 16
		 415 balance_interval = 8
		3415 balance_interval = 4
		  21 balance_interval = 2

Signed-off-by: Qais Yousef <qais.yousef@arm.com>
Signed-off-by: Qais Yousef (Google) <qyousef@layalina.io>
---

Changes since v1:

	* Use asym_cap_list (thanks Dietmar) to iterate instead of iterating
	  through every cpu which Vincent was concerned about.
	* Use uclamped util to compare with capacity instead of util_fits_cpu()
	  when iterating through capcities (Dietmar).
	* Update commit log with test results to better demonstrate the problem

v1 discussion: https://lore.kernel.org/lkml/20230820203429.568884-1-qyousef@layalina.io/#t

Food for thoughts: should misfit cause balance_interval to double? This patch
will still be needed if the answer is yes to avoid unnecessary misfit-lb to
trigger repeatedly anyway.

 kernel/sched/fair.c     | 59 ++++++++++++++++++++++++++++++++++++-----
 kernel/sched/sched.h    | 13 +++++++++
 kernel/sched/topology.c | 13 +--------
 3 files changed, 66 insertions(+), 19 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index bcea3d55d95d..94e2f659fef9 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5065,17 +5065,59 @@ static inline int task_fits_cpu(struct task_struct *p, int cpu)
 
 static inline void update_misfit_status(struct task_struct *p, struct rq *rq)
 {
+	unsigned long uclamp_min, uclamp_max;
+	struct asym_cap_data *entry, *next;
+	bool has_fitting_cpu = false;
+	unsigned long util, cpu_cap;
+	int cpu = cpu_of(rq);
+
 	if (!sched_asym_cpucap_active())
 		return;
 
-	if (!p || p->nr_cpus_allowed == 1) {
-		rq->misfit_task_load = 0;
-		return;
-	}
+	if (!p || p->nr_cpus_allowed == 1)
+		goto out;
 
-	if (task_fits_cpu(p, cpu_of(rq))) {
-		rq->misfit_task_load = 0;
-		return;
+	cpu_cap = arch_scale_cpu_capacity(cpu);
+
+	/* If we can't fit the biggest CPU, that's the best we can ever get. */
+	if (cpu_cap == SCHED_CAPACITY_SCALE)
+		goto out;
+
+	uclamp_min = uclamp_eff_value(p, UCLAMP_MIN);
+	uclamp_max = uclamp_eff_value(p, UCLAMP_MAX);
+	util = task_util_est(p);
+
+	if (util_fits_cpu(util, uclamp_min, uclamp_max, cpu) > 0)
+		goto out;
+
+	/*
+	 * If the task affinity is not set to default, make sure it is not
+	 * restricted to a subset where no CPU can ever fit it. Triggering
+	 * misfit in this case is pointless as it has no where better to move
+	 * to. And it can lead to balance_interval to grow too high as we'll
+	 * continuously fail to move it anywhere.
+	 */
+	if (!cpumask_equal(p->cpus_ptr, cpu_possible_mask)) {
+		unsigned long clamped_util = clamp(util, uclamp_min, uclamp_max);
+
+		list_for_each_entry_safe(entry, next, &asym_cap_list, link) {
+			if (entry->capacity > cpu_cap) {
+				cpumask_t *cpumask;
+
+				if (clamped_util > entry->capacity)
+					continue;
+
+				cpumask = cpu_capacity_span(entry);
+				if (!cpumask_intersects(p->cpus_ptr, cpumask))
+					continue;
+
+				has_fitting_cpu = true;
+				break;
+			}
+		}
+
+		if (!has_fitting_cpu)
+			goto out;
 	}
 
 	/*
@@ -5083,6 +5125,9 @@ static inline void update_misfit_status(struct task_struct *p, struct rq *rq)
 	 * task_h_load() returns 0.
 	 */
 	rq->misfit_task_load = max_t(unsigned long, task_h_load(p), 1);
+	return;
+out:
+	rq->misfit_task_load = 0;
 }
 
 #else /* CONFIG_SMP */
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index e58a54bda77d..1b4c1ca3fb4c 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -109,6 +109,19 @@ extern int sysctl_sched_rt_period;
 extern int sysctl_sched_rt_runtime;
 extern int sched_rr_timeslice;
 
+/*
+ * Asymmetric CPU capacity bits
+ */
+struct asym_cap_data {
+	struct list_head link;
+	unsigned long capacity;
+	unsigned long cpus[];
+};
+
+extern struct list_head asym_cap_list;
+
+#define cpu_capacity_span(asym_data) to_cpumask((asym_data)->cpus)
+
 /*
  * Helpers for converting nanosecond timing to jiffy resolution
  */
diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index 10d1391e7416..e95a4c6e651f 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -1329,24 +1329,13 @@ static void init_sched_groups_capacity(int cpu, struct sched_domain *sd)
 	update_group_capacity(sd, cpu);
 }
 
-/*
- * Asymmetric CPU capacity bits
- */
-struct asym_cap_data {
-	struct list_head link;
-	unsigned long capacity;
-	unsigned long cpus[];
-};
-
 /*
  * Set of available CPUs grouped by their corresponding capacities
  * Each list entry contains a CPU mask reflecting CPUs that share the same
  * capacity.
  * The lifespan of data is unlimited.
  */
-static LIST_HEAD(asym_cap_list);
-
-#define cpu_capacity_span(asym_data) to_cpumask((asym_data)->cpus)
+LIST_HEAD(asym_cap_list);
 
 /*
  * Verify whether there is any CPU capacity asymmetry in a given sched domain.
-- 
2.34.1