[RFC][PATCH 5/7] cpufreq / sched: UUF_IO flag to indicate iowait condition

["Rafael J. Wysocki" <rjw@rjwysocki.net>]

From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Testing indicates that it is possible to improve performace
significantly without increasing energy consumption too much by
teaching cpufreq governors to bump up the CPU performance level if
the in_iowait flag is set for the task in enqueue_task_fair().

For this purpose, define a new cpufreq_update_util() flag
UUF_IO and modify enqueue_task_fair() to pass that flag to
cpufreq_update_util() in the in_iowait case.  That generally
requires cpufreq_update_util() to be called directly from there,
because update_load_avg() is not likely to be invoked in that
case.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
---
 include/linux/sched.h |    1 +
 kernel/sched/fair.c   |    8 ++++++++
 2 files changed, 9 insertions(+)

Index: linux-pm/kernel/sched/fair.c
===================================================================
--- linux-pm.orig/kernel/sched/fair.c
+++ linux-pm/kernel/sched/fair.c
@@ -4459,6 +4459,14 @@ enqueue_task_fair(struct rq *rq, struct
 	struct cfs_rq *cfs_rq;
 	struct sched_entity *se = &p->se;
 
+	/*
+	 * If in_iowait is set, it is likely that the loops below will not
+	 * trigger any cpufreq utilization updates, so do it here explicitly
+	 * with the IO flag passed.
+	 */
+	if (p->in_iowait)
+		cpufreq_update_util(rq, UUF_IO);
+
 	for_each_sched_entity(se) {
 		if (se->on_rq)
 			break;
Index: linux-pm/include/linux/sched.h
===================================================================
--- linux-pm.orig/include/linux/sched.h
+++ linux-pm/include/linux/sched.h
@@ -3376,6 +3376,7 @@ static inline unsigned long rlimit_max(u
 }
 
 #define UUF_RT	0x01
+#define UUF_IO	0x02
 
 #ifdef CONFIG_CPU_FREQ
 struct update_util_data {