[PATCH v3 2/2] sched/fair: Simplify balance_interval reset logic in sched_balance_rq()

[Xin Zhao]

In sched_balance_rq(), it is possible to call need_active_balance() twice
in quick succession, which is not appropriate. There are two conditions in
sched_balance_rq() that reset balance_interval to min_interval, one is
when the local variable active_balance is 0, and the other is when
need_active_balance() returns a non-zero value. The local variable
active_balance is initialized to 0. Therefore, the only situation in which
balance_interval can be reset to min_interval is if need_active_balance()
has been executed once, marking the local variable active_balance as 1,
and then the second call to need_active_balance() returns 0. In other
words, the case is that during the interval between two close calls to
need_active_balance(), busiest rq completes the recently dispatched active
balance stop work, which is quite rare.

There are mainly two scenarios that lead to reaching sched_balance_rq():
one is the newly idle balance triggered by __schedule(), and the other is
the periodic balance logic controlled by sd->balance_interval or
nohz.next_balance, which ultimately executes in the softirq context. The
vast majority of cases executing sched_balance_rq() in the system fall
under the first scenario. During the execution of __schedule(), preemption
is disabled, so the interval between two checks of need_active_balance()
in this case will not be long. Thus, before applying this patch,
balance_interval would only be unlikely to reset to min_interval during
the periodic balance logic, which is relatively much more less frequent
than newly balance. In the case which is in softirq context, the execution
of the two need_active_balance() checks can indeed be preempted by other
tasks, leading to a longer interval between the two checks. However, there
is no evidence to suggest that not resetting min_interval in these
low-probability cases caused by scheduling preemption offers any
significant benefits. It would be better to simplify this complex reset
logic for balance_interval to an unconditional reset.

Signed-off-by: Xin Zhao <jackzxcui1989@xxxxxxx>
---
 kernel/sched/fair.c | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 1c5043629..b562a610b 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -13457,10 +13457,8 @@ static int sched_balance_rq(int this_cpu, struct rq *this_rq,
 		sd->nr_balance_failed = 0;
 	}
 
-	if (likely(!active_balance) || need_active_balance(&env)) {
-		/* We were unbalanced, so reset the balancing interval */
-		sd->balance_interval = sd->min_interval;
-	}
+	/* We were unbalanced, so reset the balancing interval */
+	sd->balance_interval = sd->min_interval;
 
 	goto out;
 
-- 
2.34.1