Re: [PATCH] sched/fair: Fix nohz.next_balance update

From: Valentin Schneider
Date: Wed May 06 2020 - 06:29:16 EST



On 05/05/20 15:27, Vincent Guittot wrote:
> So I would be in favor of something as simple as :
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 04098d678f3b..e028bc1c4744 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -10457,6 +10457,14 @@ 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);
> @@ -10477,14 +10485,6 @@ static bool _nohz_idle_balance(struct rq *this_rq, unsigned int flags,
> 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;
> }
>

But then we may skip an update if we goto abort, no? Imagine we have just
NOHZ_STATS_KICK, so we don't call any rebalance_domains(), and then as we
go through the last NOHZ CPU in the loop we hit need_resched(). We would
end in the abort part without any update to nohz.next_balance, despite
having accumulated relevant data in the local next_balance variable.

Also note that in this case, nohz_idle_balance() will still return true.

If we rip out just the one update we need from rebalance_domains(), then
perhaps we could go with what Peng was initially suggesting? i.e. something
like the below.

---
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 46b7bd41573f..0a292e0a0731 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -9934,22 +9934,8 @@ 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)
@@ -10315,6 +10301,11 @@ static bool _nohz_idle_balance(struct rq *this_rq, unsigned int flags,
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));

@@ -10551,6 +10542,17 @@ static __latent_entropy void run_rebalance_domains(struct softirq_action *h)
/* normal load balance */
update_blocked_averages(this_rq->cpu);
rebalance_domains(this_rq, idle);
+
+#ifdef CONFIG_NO_HZ_COMMON
+ /*
+ * NOHZ idle CPUs will be rebalanced with nohz_idle_balance() and thus
+ * nohz.next_balance will be updated accordingly. If there was no NOHZ
+ * kick, then we just need to update nohz.next_balance wrt *this* CPU.
+ */
+ if ((idle == CPU_IDLE) &&
+ time_after(nohz.next_balance, this_rq->next_balance))
+ nohz.next_balance = this_rq->next_balance;
+#endif
}

/*
---