Re: sched: Improve load balancing in the presence of idle CPUs

[Jason Low <jason.low2@hp.com>]

On Wed, 2015-04-01 at 18:04 +0100, Morten Rasmussen wrote:
> On Wed, Apr 01, 2015 at 07:49:56AM +0100, Preeti U Murthy wrote:

> > I am sorry I don't quite get this. Can you please elaborate?
> 
> I think the scenario is that we are in nohz_idle_balance() and decide to
> bail out because we have pulled some tasks, but before leaving
> nohz_idle_balance() we want to check if more balancing is necessary
> using nohz_kick_needed() and potentially kick somebody to continue.

Also, below is an example patch.

(Without the conversion to idle_cpu(), the check for rq->idle_balance
would not be accurate anymore)

---
 kernel/sched/fair.c |   17 ++++++++++-------
 1 files changed, 10 insertions(+), 7 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index fdae26e..7749a14 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7620,6 +7620,8 @@ out:
 }
 
 #ifdef CONFIG_NO_HZ_COMMON
+static inline bool nohz_kick_needed(struct rq *rq);
+
 /*
  * In CONFIG_NO_HZ_COMMON case, the idle balance kickee will do the
  * rebalancing for all the cpus for whom scheduler ticks are stopped.
@@ -7629,6 +7631,7 @@ static void nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
 	int this_cpu = this_rq->cpu;
 	struct rq *rq;
 	int balance_cpu;
+	bool done_balancing = false;
 
 	if (idle != CPU_IDLE ||
 	    !test_bit(NOHZ_BALANCE_KICK, nohz_flags(this_cpu)))
@@ -7644,7 +7647,7 @@ static void nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
 		 * balancing owner will pick it up.
 		 */
 		if (need_resched())
-			break;
+			goto end;
 
 		rq = cpu_rq(balance_cpu);
 
@@ -7663,9 +7666,12 @@ static void nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
 		if (time_after(this_rq->next_balance, rq->next_balance))
 			this_rq->next_balance = rq->next_balance;
 	}
+	done_balancing = true;
 	nohz.next_balance = this_rq->next_balance;
 end:
 	clear_bit(NOHZ_BALANCE_KICK, nohz_flags(this_cpu));
+	if (!done_balancing && nohz_kick_needed(this_rq))
+		nohz_balancer_kick();
 }
 
 /*
@@ -7687,7 +7693,7 @@ static inline bool nohz_kick_needed(struct rq *rq)
 	int nr_busy, cpu = rq->cpu;
 	bool kick = false;
 
-	if (unlikely(rq->idle_balance))
+	if (unlikely(idle_cpu(cpu)))
 		return false;
 
        /*
@@ -7757,16 +7763,13 @@ static void run_rebalance_domains(struct softirq_action *h)
 	enum cpu_idle_type idle = this_rq->idle_balance ?
 						CPU_IDLE : CPU_NOT_IDLE;
 
+	rebalance_domains(this_rq, idle);
 	/*
 	 * If this cpu has a pending nohz_balance_kick, then do the
 	 * balancing on behalf of the other idle cpus whose ticks are
-	 * stopped. Do nohz_idle_balance *before* rebalance_domains to
-	 * give the idle cpus a chance to load balance. Else we may
-	 * load balance only within the local sched_domain hierarchy
-	 * and abort nohz_idle_balance altogether if we pull some load.
+	 * stopped.
 	 */
 	nohz_idle_balance(this_rq, idle);
-	rebalance_domains(this_rq, idle);
 }
 
 /*
-- 
1.7.2.5