[PATCH 18/24] sched: Update nohz rq clock before searching busiest group on load balancing

From: Frederic Weisbecker
Date: Thu Dec 20 2012 - 13:34:53 EST

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@xxxxxxxxx>
Cc: Alessio Igor Bogani <abogani@xxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Avi Kivity <avi@xxxxxxxxxx>
Cc: Chris Metcalf <cmetcalf@xxxxxxxxxx>
Cc: Christoph Lameter <cl@xxxxxxxxx>
Cc: Geoff Levand <geoff@xxxxxxxxxxxxx>
Cc: Gilad Ben Yossef <gilad@xxxxxxxxxxxxx>
Cc: Hakan Akkan <hakanakkan@xxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: Paul Gortmaker <paul.gortmaker@xxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
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 291e225..b1b791d 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4795,6 +4795,19 @@ 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);
+ }
group = find_busiest_group(&env, balance);


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