commit c5afb6a87f23 ("sched/fair: Fix nohz.next_balance update")
During idle load balance, this_cpu(ilb) do load balance for the other
idle CPUs, also gather the earliest (nohz.)next_balance.
Since commit:
'b7031a02ec75 ("sched/fair: Add NOHZ_STATS_KICK")'
We update nohz.next_balance like this:
_nohz_idle_balance() {
for_each_cpu(nohz.idle_cpus_mask) {
rebalance_domains() {
update nohz.next_balance <-- compare and update
}
}
rebalance_domains(this_cpu) {
update nohz.next_balance <-- compare and update
}
update nohz.next_balance <-- unconditionally update
}
For instance, nohz.idle_cpus_mask spans {cpu2,3,5,8}, and this_cpu is
cpu5. After the above loop we could gather the earliest *next_balance*
among {cpu2,3,8}, then rebalance_domains(this_cpu) update
nohz.next_balance with this_rq->next_balance, but finally overwrite
nohz.next_balance with the earliest *next_balance* among {cpu2,3,8},
we may end up with not getting the earliest next_balance.
Since we can gather all the updated rq->next_balance, including this_cpu,
in _nohz_idle_balance(), it's safe to remove the extra lines in
rebalance_domains() which are originally intended for this_cpu. And
finally the updating only happen in _nohz_idle_balance().
Signed-off-by: Peng Liu <iwtba...@gmail.com>
Cc: Ingo Molnar <mi...@redhat.com>
Cc: Peter Zijlstra <pet...@infradead.org>
Cc: Juri Lelli <juri.le...@redhat.com>
Cc: Vincent Guittot <vincent.guit...@linaro.org>
Cc: Dietmar Eggemann <dietmar.eggem...@arm.com>
Cc: Steven Rostedt <rost...@goodmis.org>
Cc: Ben Segall <bseg...@google.com>
Cc: Mel Gorman <mgor...@suse.de>
Cc: Valentin Schneider <valentin.schnei...@arm.com>
---
kernel/sched/fair.c | 24 ++++++++----------------
1 file changed, 8 insertions(+), 16 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 02f323b85b6d..1d0cf33fefad 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -9943,22 +9943,8 @@ static void rebalance_domains(struct rq *rq, enum
cpu_idle_type idle)
* When the cpu is attached to null domain for ex, it will not be
* updated.
*/
- if (likely(update_next_balance)) {
+ if (likely(update_next_balance))
rq->next_balance = next_balance;
-
-#ifdef CONFIG_NO_HZ_COMMON
- /*
- * If this CPU has been elected to perform the nohz idle
- * balance. Other idle CPUs have already rebalanced with
- * nohz_idle_balance() and nohz.next_balance has been
- * updated accordingly. This CPU is now running the idle load
- * balance for itself and we need to update the
- * nohz.next_balance accordingly.
- */
- if ((idle == CPU_IDLE) && time_after(nohz.next_balance,
rq->next_balance))
- nohz.next_balance = rq->next_balance;
-#endif
- }
}
static inline int on_null_domain(struct rq *rq)
@@ -10321,9 +10307,15 @@ static bool _nohz_idle_balance(struct rq *this_rq,
unsigned int flags,
has_blocked_load |= this_rq->has_blocked_load;
}
- if (flags & NOHZ_BALANCE_KICK)
+ if (flags & NOHZ_BALANCE_KICK) {
rebalance_domains(this_rq, CPU_IDLE);
+ if (time_after(next_balance, this_rq->next_balance)) {
+ next_balance = this_rq->next_balance;
+ update_next_balance = 1;
+ }
+ }
+
WRITE_ONCE(nohz.next_blocked,
now + msecs_to_jiffies(LOAD_AVG_PERIOD));
--
2.17.1
