Hi Morten,
On 04/02/15 18:31, Morten Rasmussen wrote:
> With energy-aware scheduling enabled nohz_kick_needed() generates many
> nohz idle-balance kicks which lead to nothing when multiple tasks get
> packed on a single cpu to save energy. This causes unnecessary wake-ups
> and hence wastes energy. Make these conditions depend on !energy_aware()
> for now until the energy-aware nohz story gets sorted out.
>
> cc: Ingo Molnar <mi...@redhat.com>
> cc: Peter Zijlstra <pet...@infradead.org>
>
> Signed-off-by: Morten Rasmussen <morten.rasmus...@arm.com>
> ---
> kernel/sched/fair.c | 7 +++++--
> 1 file changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 1c248f8..cfe65ae 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -8195,6 +8195,8 @@ static void nohz_idle_balance(struct rq *this_rq, enum
> cpu_idle_type idle)
> clear_bit(NOHZ_BALANCE_KICK, nohz_flags(this_cpu));
> }
>
> +static int cpu_overutilized(int cpu, struct sched_domain *sd);
> +
> /*
> * Current heuristic for kicking the idle load balancer in the presence
> * of an idle cpu in the system.
> @@ -8234,12 +8236,13 @@ static inline bool nohz_kick_needed(struct rq *rq)
> if (time_before(now, nohz.next_balance))
> return false;
>
> - if (rq->nr_running >= 2)
> + sd = rcu_dereference(rq->sd);
> + if (rq->nr_running >= 2 && (!energy_aware() || cpu_overutilized(cpu,
> sd)))
> return true;
CONFIG_PROVE_RCU checking revealed this one:
[ 3.814454] ===============================
[ 3.826989] [ INFO: suspicious RCU usage. ]
[ 3.839526] 3.19.0-rc7+ #10 Not tainted
[ 3.851018] -------------------------------
[ 3.863554] kernel/sched/fair.c:8239 suspicious
rcu_dereference_check() usage!
[ 3.885216]
[ 3.885216] other info that might help us debug this:
[ 3.885216]
[ 3.909236]
[ 3.909236] rcu_scheduler_active = 1, debug_locks = 1
[ 3.928817] no locks held by kthreadd/437.
The RCU read-side critical section has to be extended to incorporate
this sd = rcu_dereference(rq->sd):
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index cfe65aec3237..145360ee6e4a 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8236,11 +8236,13 @@ static inline bool nohz_kick_needed(struct rq *rq)
if (time_before(now, nohz.next_balance))
return false;
+ rcu_read_lock();
sd = rcu_dereference(rq->sd);
- if (rq->nr_running >= 2 && (!energy_aware() ||
cpu_overutilized(cpu, sd)))
- return true;
+ if (rq->nr_running >= 2 && (!energy_aware() ||
cpu_overutilized(cpu, sd))) {
+ kick = true;
+ goto unlock;
+ }
- rcu_read_lock();
sd = rcu_dereference(per_cpu(sd_busy, cpu));
if (sd && !energy_aware()) {
sgc = sd->groups->sgc;
-- Dietmar
>
> rcu_read_lock();
> sd = rcu_dereference(per_cpu(sd_busy, cpu));
> - if (sd) {
> + if (sd && !energy_aware()) {
> sgc = sd->groups->sgc;
> nr_busy = atomic_read(&sgc->nr_busy_cpus);
>
>
--
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