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SubjectRe: [PATCH 1/3] sched: Fix nohz_kick_needed to consider the nr_busy of the parent domain's group
On Thu, Oct 24, 2013 at 01:37:38PM +0530, Preeti U Murthy wrote:
> kernel/sched/core.c | 5 +++++
> kernel/sched/fair.c | 38 ++++++++++++++++++++------------------
> kernel/sched/sched.h | 1 +
> 3 files changed, 26 insertions(+), 18 deletions(-)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index c06b8d3..c540392 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -5271,6 +5271,7 @@ DEFINE_PER_CPU(struct sched_domain *, sd_llc);
> DEFINE_PER_CPU(int, sd_llc_size);
> DEFINE_PER_CPU(int, sd_llc_id);
> DEFINE_PER_CPU(struct sched_domain *, sd_numa);
> +DEFINE_PER_CPU(struct sched_domain *, sd_busy);
>
> static void update_top_cache_domain(int cpu)
> {
> @@ -5290,6 +5291,10 @@ static void update_top_cache_domain(int cpu)
>
> sd = lowest_flag_domain(cpu, SD_NUMA);
> rcu_assign_pointer(per_cpu(sd_numa, cpu), sd);
> +
> + sd = highest_flag_domain(cpu, SD_SHARE_PKG_RESOURCES);
> + if (sd)
> + rcu_assign_pointer(per_cpu(sd_busy, cpu), sd->parent);
> }
>
> /*
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index e9c9549..f66cfd9 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6515,16 +6515,16 @@ static inline void nohz_balance_exit_idle(int cpu)
> static inline void set_cpu_sd_state_busy(void)
> {
> struct sched_domain *sd;
> + int cpu = smp_processor_id();
>
> rcu_read_lock();
> + sd = rcu_dereference(per_cpu(sd_busy, cpu));
>
> if (!sd || !sd->nohz_idle)
> goto unlock;
> sd->nohz_idle = 0;
>
> + atomic_inc(&sd->groups->sgp->nr_busy_cpus);
> unlock:
> rcu_read_unlock();
> }
> @@ -6532,16 +6532,16 @@ unlock:
> void set_cpu_sd_state_idle(void)
> {
> struct sched_domain *sd;
> + int cpu = smp_processor_id();
>
> rcu_read_lock();
> + sd = rcu_dereference(per_cpu(sd_busy, cpu));
>
> if (!sd || sd->nohz_idle)
> goto unlock;
> sd->nohz_idle = 1;
>
> + atomic_dec(&sd->groups->sgp->nr_busy_cpus);
> unlock:
> rcu_read_unlock();
> }

Oh nice, that gets rid of the multiple atomics, and it nicely splits
this nohz logic into per topology groups -- now if only we could split
the rest too :-)

> @@ -6748,6 +6748,8 @@ static inline int nohz_kick_needed(struct rq *rq, int cpu)
> {
> unsigned long now = jiffies;
> struct sched_domain *sd;
> + struct sched_group_power *sgp;
> + int nr_busy;
>
> if (unlikely(idle_cpu(cpu)))
> return 0;
> @@ -6773,22 +6775,22 @@ static inline int nohz_kick_needed(struct rq *rq, int cpu)
> goto need_kick;
>
> rcu_read_lock();
> + sd = rcu_dereference(per_cpu(sd_busy, cpu));
>
> + if (sd) {
> + sgp = sd->groups->sgp;
> + nr_busy = atomic_read(&sgp->nr_busy_cpus);
>
> + if (nr_busy > 1)
> goto need_kick_unlock;
> }

OK, so far so good.

> +
> + sd = highest_flag_domain(cpu, SD_ASYM_PACKING);
> +
> + if (sd && (cpumask_first_and(nohz.idle_cpus_mask,
> + sched_domain_span(sd)) < cpu))
> + goto need_kick_unlock;
> +
> rcu_read_unlock();
> return 0;

This again is a bit sad; most archs will not have SD_ASYM_PACKING set at
all; this means that they all will do a complete (and pointless) sched
domain tree walk here.

It would be much better to also introduce sd_asym and do the analogous
thing to the new sd_busy.



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