sched/fair: Replace CFS internal cpu_util() with cpu_util_cfs()

cpu_util_cfs() was created by commit d4edd662ac16 ("sched/cpufreq: Use
the DEADLINE utilization signal") to enable the access to CPU
utilization from the Schedutil CPUfreq governor.

Commit a07630b8b2c1 ("sched/cpufreq/schedutil: Use util_est for OPP
selection") added util_est support later.

The only thing cpu_util() is doing on top of what cpu_util_cfs() already
does is to clamp the return value to the [0..capacity_orig] capacity
range of the CPU. Integrating this into cpu_util_cfs() is not harming
the existing users (Schedutil and CPUfreq cooling (latter via
sched_cpu_util() wrapper)).

For straightforwardness, prefer to keep using `int cpu` as the function
parameter over using `struct rq *rq` which might avoid some calls to
cpu_rq(cpu) -> per_cpu(runqueues, cpu) -> RELOC_HIDE().
Update cfs_util()'s documentation and reuse it for cpu_util_cfs().
Remove cpu_util().

Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Link: https://lore.kernel.org/r/20211118164240.623551-1-dietmar.eggemann@arm.com

1 files changed, 40 insertions, 4 deletions

diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index eb971151e7e4..de53be905739 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -2966,16 +2966,52 @@ static inline unsigned long cpu_util_dl(struct rq *rq)
 	return READ_ONCE(rq->avg_dl.util_avg);
 }
 
-static inline unsigned long cpu_util_cfs(struct rq *rq)
+/**
+ * cpu_util_cfs() - Estimates the amount of CPU capacity used by CFS tasks.
+ * @cpu: the CPU to get the utilization for.
+ *
+ * The unit of the return value must be the same as the one of CPU capacity
+ * so that CPU utilization can be compared with CPU capacity.
+ *
+ * CPU utilization is the sum of running time of runnable tasks plus the
+ * recent utilization of currently non-runnable tasks on that CPU.
+ * It represents the amount of CPU capacity currently used by CFS tasks in
+ * the range [0..max CPU capacity] with max CPU capacity being the CPU
+ * capacity at f_max.
+ *
+ * The estimated CPU utilization is defined as the maximum between CPU
+ * utilization and sum of the estimated utilization of the currently
+ * runnable tasks on that CPU. It preserves a utilization "snapshot" of
+ * previously-executed tasks, which helps better deduce how busy a CPU will
+ * be when a long-sleeping task wakes up. The contribution to CPU utilization
+ * of such a task would be significantly decayed at this point of time.
+ *
+ * CPU utilization can be higher than the current CPU capacity
+ * (f_curr/f_max * max CPU capacity) or even the max CPU capacity because
+ * of rounding errors as well as task migrations or wakeups of new tasks.
+ * CPU utilization has to be capped to fit into the [0..max CPU capacity]
+ * range. Otherwise a group of CPUs (CPU0 util = 121% + CPU1 util = 80%)
+ * could be seen as over-utilized even though CPU1 has 20% of spare CPU
+ * capacity. CPU utilization is allowed to overshoot current CPU capacity
+ * though since this is useful for predicting the CPU capacity required
+ * after task migrations (scheduler-driven DVFS).
+ *
+ * Return: (Estimated) utilization for the specified CPU.
+ */
+static inline unsigned long cpu_util_cfs(int cpu)
 {
-	unsigned long util = READ_ONCE(rq->cfs.avg.util_avg);
+	struct cfs_rq *cfs_rq;
+	unsigned long util;
+
+	cfs_rq = &cpu_rq(cpu)->cfs;
+	util = READ_ONCE(cfs_rq->avg.util_avg);
 
 	if (sched_feat(UTIL_EST)) {
 		util = max_t(unsigned long, util,
-			     READ_ONCE(rq->cfs.avg.util_est.enqueued));
+			     READ_ONCE(cfs_rq->avg.util_est.enqueued));
 	}
 
-	return util;
+	return min(util, capacity_orig_of(cpu));
 }
 
 static inline unsigned long cpu_util_rt(struct rq *rq)