Re: [RFC PATCH 3/6] sched: Add over-utilization/tipping point indicator

Dietmar Eggemann
2018-04-09T09:53:00Z

On 04/09/2018 11:40 AM, Peter Zijlstra wrote:
>
> (I know there is a new version out; but I was reading through this to
> catch up with the discussion)
>
> On Tue, Mar 20, 2018 at 09:43:09AM +0000, Dietmar Eggemann wrote:
>> +static inline int sd_overutilized(struct sched_domain *sd)
>> +{
>> + return READ_ONCE(sd->shared->overutilized);
>> +}
>> +
>> +static inline void update_overutilized_status(struct rq *rq)
>> +{
>> + struct sched_domain *sd;
>> +
>> + rcu_read_lock();
>> + sd = rcu_dereference(rq->sd);
>> + if (sd && !sd_overutilized(sd) && cpu_overutilized(rq->cpu))
>> + WRITE_ONCE(sd->shared->overutilized, 1);
>> + rcu_read_unlock();
>> +}
>> +#else
>
> I think you ought to go have a look at the end of
> kernel/sched/topology.c:sd_init(), where it says:
>
> /*
> * For all levels sharing cache; connect a sched_domain_shared
> * instance.
> */
> if (sd->flags & SD_SHARE_PKG_RESOURCES) {
> sd->shared = *per_cpu_ptr(sdd->sds, sd_id);
> atomic_inc(&sd->shared->ref);
> atomic_set(&sd->shared->nr_busy_cpus, sd_weight);
> }
>
> Because if I read all this correctly, your code assumes sd->shared
> exists unconditionally, while the quoted bit only ensures it does so <=
> LLC.
>

But the patch changes this part further down.

diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index 64cc564f5255..c8b7c7665ab2 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -1184,15 +1184,11 @@ sd_init(struct sched_domain_topology_level *tl,
sd->idle_idx = 1;
}

- /*
- * For all levels sharing cache; connect a sched_domain_shared
- * instance.
- */
- if (sd->flags & SD_SHARE_PKG_RESOURCES) {
- sd->shared = *per_cpu_ptr(sdd->sds, sd_id);
- atomic_inc(&sd->shared->ref);
+ sd->shared = *per_cpu_ptr(sdd->sds, sd_id);
+ atomic_inc(&sd->shared->ref);
+
+ if (sd->flags & SD_SHARE_PKG_RESOURCES)
atomic_set(&sd->shared->nr_busy_cpus, sd_weight);
- }