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

[Dietmar Eggemann]

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);
-	}