Re: [PATCH] sched: Optimize pick_next_task for idle_sched_class too

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Mar 01, 2017 at 10:53:03AM -0500, Steven Rostedt wrote:
> Peter, do we have a solution for this yet? Are you going to add the one
> with the linker magic?

I queued the below earlier today.

---
Subject: sched: Fix pick_next_task() for RT,DL
From: Peter Zijlstra <peterz@infradead.org>
Date: Wed Mar  1 10:51:47 CET 2017

Pavan noticed that commit 49ee576809d8 ("sched/core: Optimize
pick_next_task() for idle_sched_class") broke RT,DL balancing by
robbing them of the opportinty to do new-'idle' balancing when their
last runnable task (on that runqueue) goes away.

Cc: Steven Rostedt <rostedt@goodmis.org>
Reported-by: Pavan Kondeti <pkondeti@codeaurora.org>
Fixes: 49ee576809d8 ("sched/core: Optimize pick_next_task() for idle_sched_class")
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3273,10 +3273,15 @@ pick_next_task(struct rq *rq, struct tas
 	struct task_struct *p;
 
 	/*
-	 * Optimization: we know that if all tasks are in
-	 * the fair class we can call that function directly:
+	 * Optimization: we know that if all tasks are in the fair class we can
+	 * call that function directly, but only if the @pref task wasn't of a
+	 * higher scheduling class, because otherwise those loose the
+	 * opportunity to pull in more work from other CPUs.
 	 */
-	if (likely(rq->nr_running == rq->cfs.h_nr_running)) {
+	if (likely((prev->sched_class == &idle_sched_class ||
+		    prev->sched_class == &fair_sched_class) &&
+		   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;