Re: [PATCH v6 04/16] sched/core: uclamp: Add CPU's clamp buckets refcounting

Peter Zijlstra
2019-01-21T14:59:29Z

On Tue, Jan 15, 2019 at 10:15:01AM +0000, Patrick Bellasi wrote:
> @@ -835,6 +954,28 @@ static void uclamp_bucket_inc(struct uclamp_se *uc_se, unsigned int clamp_id,
> } while (!atomic_long_try_cmpxchg(&uc_maps[bucket_id].adata,
> &uc_map_old.data, uc_map_new.data));
>
> + /*
> + * Ensure each CPU tracks the correct value for this clamp bucket.
> + * This initialization of per-CPU variables is required only when a
> + * clamp value is requested for the first time from a slow-path.
> + */

I'm confused; why is this needed?

> + if (unlikely(!uc_map_old.se_count)) {
> + for_each_possible_cpu(cpu) {
> + struct uclamp_cpu *uc_cpu =
> + &cpu_rq(cpu)->uclamp[clamp_id];
> +
> + /* CPU's tasks count must be 0 for free buckets */
> + SCHED_WARN_ON(uc_cpu->bucket[bucket_id].tasks);
> + if (unlikely(uc_cpu->bucket[bucket_id].tasks))
> + uc_cpu->bucket[bucket_id].tasks = 0;
> +
> + /* Minimize cache lines invalidation */
> + if (uc_cpu->bucket[bucket_id].value == bucket_value)
> + continue;
> + uc_cpu->bucket[bucket_id].value = bucket_value;
> + }
> + }
> +
> uc_se->value = clamp_value;
> uc_se->bucket_id = bucket_id;
>