sched: fair: avoid little cpus due to sync, prev bias

Important threads can get forced to little cpu's
when the sync or prev_bias hints are followed
blindly. This patch adds a check to see whether
those paths are forcing the task to a cpu that
has less capacity than other cpu's available for
the task. If so, we ignore the sync and prev_bias
and allow the scheduler to make a free decision.

Bug: 117438867
Change-Id: Ie5a99f9a8b65ba9382a8d0de2ae0aad843e558d1
Signed-off-by: Miguel de Dios <migueldedios@google.com>
Signed-off-by: Alexander Winkowski <dereference23@outlook.com>

kernel/sched/fair.c

@@ -76,6 +76,8 @@ walt_dec_cfs_rq_stats(struct cfs_rq *cfs_rq, struct task_struct *p) {}
 
 #endif
 
+static inline bool cpu_is_in_target_set(struct task_struct *p, int cpu);
+
 /*
  * Targeted preemption latency for CPU-bound tasks:
  *
@@ -5806,7 +5808,8 @@ static inline bool
 bias_to_this_cpu(struct task_struct *p, int cpu, struct cpumask *rtg_target)
 {
 	bool base_test = cpumask_test_cpu(cpu, &p->cpus_allowed) &&
-			cpu_active(cpu) && task_fits_max(p, cpu);
+			cpu_active(cpu) && task_fits_max(p, cpu) &&
+			cpu_is_in_target_set(p, cpu);
 	bool rtg_test = rtg_target && cpumask_test_cpu(cpu, rtg_target);
 
 	return base_test && (!rtg_target || rtg_test);
@@ -7442,6 +7445,21 @@ enum fastpaths {
 	MANY_WAKEUP,
 };
 
+/*
+ * Check whether cpu is in the fastest set of cpu's that p should run on.
+ * If p is boosted, prefer that p runs on a faster cpu; otherwise, allow p
+ * to run on any cpu.
+ */
+static inline bool
+cpu_is_in_target_set(struct task_struct *p, int cpu)
+{
+	struct root_domain *rd = cpu_rq(cpu)->rd;
+	int first_cpu = (schedtune_task_boost(p)) ?
+		rd->mid_cap_orig_cpu : rd->min_cap_orig_cpu;
+	int next_usable_cpu = cpumask_next(first_cpu - 1, &p->cpus_allowed);
+	return cpu >= next_usable_cpu || next_usable_cpu >= nr_cpu_ids;
+}
+
 static inline int find_best_target(struct task_struct *p, int *backup_cpu,
 				   bool boosted, bool prefer_idle,
 				   struct find_best_target_env *fbt_env)