sched/core: Optimize pick_next_task() for idle_sched_class

Steve noticed that when we switch from IDLE to SCHED_OTHER we fail to
take the shortcut, even though all runnable tasks are of the fair
class, because prev->sched_class != &fair_sched_class.

Since I reworked the put_prev_task() stuff, we don't really care about
prev->class here, so removing that condition will allow this case.

This increases the likely case from 78% to 98% correct for Steve's
workload.

Reported-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Tested-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20170119174408.GN6485@twins.programming.kicks-ass.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 49ce1cb3d32074591670616e030abb334aa80558..51ca21ef7ae52c7ade8538dcbeec1cb6ee3cb4f1 100644 (file)
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3321,15 +3321,14 @@ static inline void schedule_debug(struct task_struct *prev)
 static inline struct task_struct *
 pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
 {
-       const struct sched_class *class = &fair_sched_class;
+       const struct sched_class *class;
        struct task_struct *p;
 
        /*
         * Optimization: we know that if all tasks are in
         * the fair class we can call that function directly:
         */
-       if (likely(prev->sched_class == class &&
-                  rq->nr_running == rq->cfs.h_nr_running)) {
+       if (likely(rq->nr_running == rq->cfs.h_nr_running)) {
                p = fair_sched_class.pick_next_task(rq, prev, rf);
                if (unlikely(p == RETRY_TASK))
                        goto again;