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Subject[PATCH v2 1/2] sched/fair: Add NOHZ balancer flag for nohz.next_balance updates
Date
A following patch will trigger NOHZ idle balances as a means to update
nohz.next_balance. Vincent noted that blocked load updates can have
non-negligible overhead, which should be avoided if the intent is to only
update nohz.next_balance.

Add a new NOHZ balance kick flag, NOHZ_NEXT_KICK. Gate NOHZ blocked load
update by the presence of NOHZ_STATS_KICK - currently all NOHZ balance
kicks will have the NOHZ_STATS_KICK flag set, so no change in behaviour is
expected.

Suggested-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
---
kernel/sched/fair.c | 9 ++++++---
kernel/sched/sched.h | 8 +++++++-
2 files changed, 13 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 11d22943753f..5c88698c3664 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -10506,7 +10506,8 @@ static void _nohz_idle_balance(struct rq *this_rq, unsigned int flags,
* setting the flag, we are sure to not clear the state and not
* check the load of an idle cpu.
*/
- WRITE_ONCE(nohz.has_blocked, 0);
+ if (flags & NOHZ_STATS_KICK)
+ WRITE_ONCE(nohz.has_blocked, 0);

/*
* Ensures that if we miss the CPU, we must see the has_blocked
@@ -10528,13 +10529,15 @@ static void _nohz_idle_balance(struct rq *this_rq, unsigned int flags,
* balancing owner will pick it up.
*/
if (need_resched()) {
- has_blocked_load = true;
+ if (flags & NOHZ_STATS_KICK)
+ has_blocked_load = true;
goto abort;
}

rq = cpu_rq(balance_cpu);

- has_blocked_load |= update_nohz_stats(rq);
+ if (flags & NOHZ_STATS_KICK)
+ has_blocked_load |= update_nohz_stats(rq);

/*
* If time for next balance is due,
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 9a1c6aeb9165..b0f38b5d2550 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -2695,12 +2695,18 @@ extern void cfs_bandwidth_usage_dec(void);
#define NOHZ_BALANCE_KICK_BIT 0
#define NOHZ_STATS_KICK_BIT 1
#define NOHZ_NEWILB_KICK_BIT 2
+#define NOHZ_NEXT_KICK_BIT 3

+/* Run rebalance_domains() */
#define NOHZ_BALANCE_KICK BIT(NOHZ_BALANCE_KICK_BIT)
+/* Update blocked load */
#define NOHZ_STATS_KICK BIT(NOHZ_STATS_KICK_BIT)
+/* Update blocked load when entering idle */
#define NOHZ_NEWILB_KICK BIT(NOHZ_NEWILB_KICK_BIT)
+/* Update nohz.next_balance */
+#define NOHZ_NEXT_KICK BIT(NOHZ_NEXT_KICK_BIT)

-#define NOHZ_KICK_MASK (NOHZ_BALANCE_KICK | NOHZ_STATS_KICK)
+#define NOHZ_KICK_MASK (NOHZ_BALANCE_KICK | NOHZ_STATS_KICK | NOHZ_NEXT_KICK)

#define nohz_flags(cpu) (&cpu_rq(cpu)->nohz_flags)

--
2.25.1
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