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sched/fair: Trigger nohz.next_balance updates when a CPU goes NOHZ-idle 

Consider a system with some NOHZ-idle CPUs, such that

  nohz.idle_cpus_mask = S
  nohz.next_balance = T

When a new CPU k goes NOHZ idle (nohz_balance_enter_idle()), we end up
with:

  nohz.idle_cpus_mask = S \U {k}
  nohz.next_balance = T

Note that the nohz.next_balance hasn't changed - it won't be updated until
a NOHZ balance is triggered. This is problematic if the newly NOHZ idle CPU
has an earlier rq.next_balance than the other NOHZ idle CPUs, IOW if:

  cpu_rq(k).next_balance < nohz.next_balance

In such scenarios, the existing nohz.next_balance will prevent any NOHZ
balance from happening, which itself will prevent nohz.next_balance from
being updated to this new cpu_rq(k).next_balance. Unnecessary load balance
delays of over 12ms caused by this were observed on an arm64 RB5 board.

Use the new nohz.needs_update flag to mark the presence of newly-idle CPUs
that need their rq->next_balance to be collated into
nohz.next_balance. Trigger a NOHZ_NEXT_KICK when the flag is set.

Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Link: https://lkml.kernel.org/r/20210823111700.2842997-3-valentin.schneider@arm.com
This commit is contained in:
Valentin Schneider 2021-08-23 12:17:00 +01:00 committed by Peter Zijlstra
parent efd984c481
commit 7fd7a9e0ca
1 changed files with 13 additions and 2 deletions
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15
kernel/sched/fair.c
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@ -5787,6 +5787,7 @@ static struct {
cpumask_var_t idle_cpus_mask;
atomic_t nr_cpus;
int has_blocked; /* Idle CPUS has blocked load */
int needs_update; /* Newly idle CPUs need their next_balance collated */
unsigned long next_balance; /* in jiffy units */
unsigned long next_blocked; /* Next update of blocked load in jiffies */
} nohz ____cacheline_aligned;
@ -10450,6 +10451,9 @@ static void nohz_balancer_kick(struct rq *rq)
unlock:
rcu_read_unlock();
out:
if (READ_ONCE(nohz.needs_update))
flags |= NOHZ_NEXT_KICK;
if (flags)
kick_ilb(flags);
}
@ -10546,12 +10550,13 @@ void nohz_balance_enter_idle(int cpu)
/*
* Ensures that if nohz_idle_balance() fails to observe our
* @idle_cpus_mask store, it must observe the @has_blocked
* store.
* and @needs_update stores.
*/
smp_mb__after_atomic();
set_cpu_sd_state_idle(cpu);
WRITE_ONCE(nohz.needs_update, 1);
out:
/*
* Each time a cpu enter idle, we assume that it has blocked load and
@ -10600,13 +10605,17 @@ static void _nohz_idle_balance(struct rq *this_rq, unsigned int flags,
/*
* We assume there will be no idle load after this update and clear
* the has_blocked flag. If a cpu enters idle in the mean time, it will
* set the has_blocked flag and trig another update of idle load.
* set the has_blocked flag and trigger another update of idle load.
* Because a cpu that becomes idle, is added to idle_cpus_mask before
* setting the flag, we are sure to not clear the state and not
* check the load of an idle cpu.
*
* Same applies to idle_cpus_mask vs needs_update.
*/
if (flags & NOHZ_STATS_KICK)
WRITE_ONCE(nohz.has_blocked, 0);
if (flags & NOHZ_NEXT_KICK)
WRITE_ONCE(nohz.needs_update, 0);
/*
* Ensures that if we miss the CPU, we must see the has_blocked
@ -10630,6 +10639,8 @@ static void _nohz_idle_balance(struct rq *this_rq, unsigned int flags,
if (need_resched()) {
if (flags & NOHZ_STATS_KICK)
has_blocked_load = true;
if (flags & NOHZ_NEXT_KICK)
WRITE_ONCE(nohz.needs_update, 1);
goto abort;
}