sched/fair: Fix NOHZ next idle balance

With commit:
  'b7031a02ec75 ("sched/fair: Add NOHZ_STATS_KICK")'
rebalance_domains of the local cfs_rq happens before others idle cpus have
updated nohz.next_balance and its value is overwritten.

Move the update of nohz.next_balance for other idles cpus before balancing
and updating the next_balance of local cfs_rq.

Also, the nohz.next_balance is now updated only if all idle cpus got a
chance to rebalance their domains and the idle balance has not been aborted
because of new activities on the CPU. In case of need_resched, the idle
load balance will be kick the next jiffie in order to address remaining
ilb.

Fixes: b7031a02ec75 ("sched/fair: Add NOHZ_STATS_KICK")
Reported-by: Peng Liu <iwtbavbm@gmail.com>
Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>
Acked-by: Mel Gorman <mgorman@suse.de>
Link: https://lkml.kernel.org/r/20200609123748.18636-1-vincent.guittot@linaro.org

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index a785a9b..295c9ff 100644 (file)
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -10022,7 +10022,12 @@ static void kick_ilb(unsigned int flags)
 {
        int ilb_cpu;
 
-       nohz.next_balance++;
+       /*
+        * 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;
 
        ilb_cpu = find_new_ilb();
 
@@ -10343,6 +10348,14 @@ static bool _nohz_idle_balance(struct rq *this_rq, unsigned int flags,
                }
        }
 
+       /*
+        * next_balance will be updated only when there is a need.
+        * When the CPU is attached to null domain for ex, it will not be
+        * updated.
+        */
+       if (likely(update_next_balance))
+               nohz.next_balance = next_balance;
+
        /* Newly idle CPU doesn't need an update */
        if (idle != CPU_NEWLY_IDLE) {
                update_blocked_averages(this_cpu);
@@ -10363,14 +10376,6 @@ abort:
        if (has_blocked_load)
                WRITE_ONCE(nohz.has_blocked, 1);
 
-       /*
-        * next_balance will be updated only when there is a need.
-        * When the CPU is attached to null domain for ex, it will not be
-        * updated.
-        */
-       if (likely(update_next_balance))
-               nohz.next_balance = next_balance;
-
        return ret;
 }