| Date | Fri, 20 Feb 2015 19:26:38 +0000 | From | Dietmar Eggemann <> | Subject | Re: [RFCv3 PATCH 48/48] sched: Disable energy-unfriendly nohz kicks |
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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 <mingo@redhat.com> > cc: Peter Zijlstra <peterz@infradead.org> > > Signed-off-by: Morten Rasmussen <morten.rasmussen@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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