[PATCH 18/27] sched: Update nohz rq clock before searching busiest group on load balancing

[Frederic Weisbecker <fweisbec@gmail.com>]

While load balancing an rq target, we look for the busiest group.
This operation may require an uptodate rq clock if we end up calling
scale_rt_power(). To this end, update it manually if the target is
running tickless.

DOUBT: don't we actually also need this in vanilla kernel, in case
this_cpu is in dyntick-idle mode?

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Alessio Igor Bogani <abogani@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Chris Metcalf <cmetcalf@tilera.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Geoff Levand <geoff@infradead.org>
Cc: Gilad Ben Yossef <gilad@benyossef.com>
Cc: Hakan Akkan <hakanakkan@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
---
 kernel/sched/fair.c |   13 +++++++++++++
 1 files changed, 13 insertions(+), 0 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 698137d..473f50f 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5023,6 +5023,19 @@ static int load_balance(int this_cpu, struct rq *this_rq,
 
 	schedstat_inc(sd, lb_count[idle]);
 
+	/*
+	 * find_busiest_group() may need an uptodate cpu clock
+	 * for find_busiest_group() (see scale_rt_power()). If
+	 * the CPU is nohz, it's clock may be stale.
+	 */
+	if (tick_nohz_full_cpu(this_cpu)) {
+		local_irq_save(flags);
+		raw_spin_lock(&this_rq->lock);
+		update_rq_clock(this_rq);
+		raw_spin_unlock(&this_rq->lock);
+		local_irq_restore(flags);
+	}
+
 redo:
 	group = find_busiest_group(&env, balance);
 
-- 
1.7.5.4