[PATCH 1/2] sched/fair: scale nohz.next_balance according to number of idle CPUs.

[Imran Khan <imran.f.khan@oracle.com>]

On large scale systems, for example with 768 CPUs and cpusets consisting
of 380+ CPUs, there may always be some idle CPU with it's rq->next_balance
close to or same as now.
This causes nohz.next_balance to be perpetually same as current jiffies and
thus causing time based check in nohz_balancer_kick() to awlays fail.

For example putting dtrace probe at nohz_balancer_kick, on such a system,
we can see that nohz.next_balance is at current jiffy at almost each tick:

447 9536  nohz_balancer_kick:entry jiffies=9764770863 nohz.next_balance=9764770863
447 9536  nohz_balancer_kick:entry jiffies=9764770864 nohz.next_balance=9764770864
447 9536  nohz_balancer_kick:entry jiffies=9764770865 nohz.next_balance=9764770865
447 9536  nohz_balancer_kick:entry jiffies=9764770866 nohz.next_balance=9764770866
447 9536  nohz_balancer_kick:entry jiffies=9764770867 nohz.next_balance=9764770867
447 9536  nohz_balancer_kick:entry jiffies=9764770868 nohz.next_balance=9764770868
447 9536  nohz_balancer_kick:entry jiffies=9764770869 nohz.next_balance=9764770870
447 9536  nohz_balancer_kick:entry jiffies=9764770870 nohz.next_balance=9764770870
447 9536  nohz_balancer_kick:entry jiffies=9764770871 nohz.next_balance=9764770871
447 9536  nohz_balancer_kick:entry jiffies=9764770872 nohz.next_balance=9764770872
447 9536  nohz_balancer_kick:entry jiffies=9764770873 nohz.next_balance=9764770873
447 9536  nohz_balancer_kick:entry jiffies=9764770874 nohz.next_balance=9764770874
447 9536  nohz_balancer_kick:entry jiffies=9764770875 nohz.next_balance=9764770876
447 9536  nohz_balancer_kick:entry jiffies=9764770876 nohz.next_balance=9764770876
447 9536  nohz_balancer_kick:entry jiffies=9764770877 nohz.next_balance=9764770877
447 9536  nohz_balancer_kick:entry jiffies=9764770878 nohz.next_balance=9764770878

On such system setting nohz.next_balance to next jiffy can cause kick_ilb()
to run almost every tick and this in turn can consume a lot of CPU cycles in
subsequenet nohz idle balancing.
So set nohz.next_balance based on number of currently idle CPUs, such that
for 32 idle CPUs nohz.next_balance is advanced further by 1 jiffy.
This will nohz_balancer_kick to bail out early.

Signed-off-by: Imran Khan <imran.f.khan@oracle.com>
---
 kernel/sched/fair.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index ab4114712be74..bd35275a05b38 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -12447,8 +12447,17 @@ static void kick_ilb(unsigned int flags)
 	 * Increase nohz.next_balance only when if full ilb is triggered but
 	 * not if we only update stats.
 	 */
-	if (flags & NOHZ_BALANCE_KICK)
-		nohz.next_balance = jiffies+1;
+	if (flags & NOHZ_BALANCE_KICK) {
+		unsigned int nr_idle = cpumask_weight(nohz.idle_cpus_mask);
+
+		/*
+		 * On large systems, there may always be some idle CPU(s) with
+		 * rq->next_balance close to or at current time, thus causing
+		 * frequent invocation of kick_ilb() from nohz_balancer_kick().
+		 * Adjust next_balance based on the number of idle CPUs.
+		 */
+		nohz.next_balance = jiffies + 1 + ((nr_idle > 32) ? ilog2(nr_idle) - 4 : 0);
+	}
 
 	ilb_cpu = find_new_ilb();
 	if (ilb_cpu < 0)
-- 
2.34.1