sched: fair scheduler should not resched rt tasks

With use of ftrace Steven noticed that some RT tasks got rescheduled due
to sched_fair interaction.

What happens is that we reprogram the hrtick from enqueue/dequeue_fair_task()
because that can change nr_running, and thus a current tasks ideal runtime.
However, its possible the current task isn't a fair_sched_class task, and thus
doesn't have a hrtick set to change.

Fix this by wrapping those hrtick_start_fair() calls in a hrtick_update()
function, which will check for the right conditions.

Reported-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Acked-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 24 insertions, 4 deletions

diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
index 67084936b602..0c4bcac54761 100644
--- a/kernel/sched_fair.c
+++ b/kernel/sched_fair.c
@@ -73,6 +73,8 @@ unsigned int sysctl_sched_wakeup_granularity = 5000000UL;
 
 const_debug unsigned int sysctl_sched_migration_cost = 500000UL;
 
+static const struct sched_class fair_sched_class;
+
 /**************************************************************
  * CFS operations on generic schedulable entities:
  */
@@ -848,11 +850,31 @@ static void hrtick_start_fair(struct rq *rq, struct task_struct *p)
                 hrtick_start(rq, delta);
         }
 }
+
+/*
+ * called from enqueue/dequeue and updates the hrtick when the
+ * current task is from our class and nr_running is low enough
+ * to matter.
+ */
+static void hrtick_update(struct rq *rq)
+{
+        struct task_struct *curr = rq->curr;
+
+        if (curr->sched_class != &fair_sched_class)
+                return;
+
+        if (cfs_rq_of(&curr->se)->nr_running < sched_nr_latency)
+                hrtick_start_fair(rq, curr);
+}
 #else /* !CONFIG_SCHED_HRTICK */
 static inline void
 hrtick_start_fair(struct rq *rq, struct task_struct *p)
 {
 }
+
+static inline void hrtick_update(struct rq *rq)
+{
+}
 #endif
 
 /*
@@ -873,7 +895,7 @@ static void enqueue_task_fair(struct rq *rq, struct task_struct *p, int wakeup)
                 wakeup = 1;
         }
 
-        hrtick_start_fair(rq, rq->curr);
+        hrtick_update(rq);
 }
 
 /*
@@ -895,7 +917,7 @@ static void dequeue_task_fair(struct rq *rq, struct task_struct *p, int sleep)
                 sleep = 1;
         }
 
-        hrtick_start_fair(rq, rq->curr);
+        hrtick_update(rq);
 }
 
 /*
@@ -1001,8 +1023,6 @@ static inline int wake_idle(int cpu, struct task_struct *p)
 
 #ifdef CONFIG_SMP
 
-static const struct sched_class fair_sched_class;
-
 #ifdef CONFIG_FAIR_GROUP_SCHED
 /*
  * effective_load() calculates the load change as seen from the root_task_group