Re: [PATCH v6 03/16] sched/core: uclamp: Map TASK's clamp values into CPU's clamp buckets

Peter Zijlstra
2019-01-21T15:05:07Z

On Tue, Jan 15, 2019 at 10:15:00AM +0000, Patrick Bellasi wrote:
> +static inline unsigned int uclamp_bucket_value(unsigned int clamp_value)
> +{
> +#define UCLAMP_BUCKET_DELTA (SCHED_CAPACITY_SCALE / CONFIG_UCLAMP_BUCKETS_COUNT)
> +#define UCLAMP_BUCKET_UPPER (UCLAMP_BUCKET_DELTA * CONFIG_UCLAMP_BUCKETS_COUNT)
> +
> + if (clamp_value >= UCLAMP_BUCKET_UPPER)
> + return SCHED_CAPACITY_SCALE;
> +
> + return UCLAMP_BUCKET_DELTA * (clamp_value / UCLAMP_BUCKET_DELTA);
> +}

> +static void uclamp_bucket_inc(struct uclamp_se *uc_se, unsigned int clamp_id,
> + unsigned int clamp_value)
> +{
> + union uclamp_map *uc_maps = &uclamp_maps[clamp_id][0];
> + unsigned int prev_bucket_id = uc_se->bucket_id;
> + union uclamp_map uc_map_old, uc_map_new;
> + unsigned int free_bucket_id;
> + unsigned int bucket_value;
> + unsigned int bucket_id;
> +
> + bucket_value = uclamp_bucket_value(clamp_value);

Aahh!!

So why don't you do:

bucket_id = clamp_value / UCLAMP_BUCKET_DELTA;
bucket_value = bucket_id * UCLAMP_BUCKET_DELTA;

> + do {
> + /* Find the bucket_id of an already mapped clamp bucket... */
> + free_bucket_id = UCLAMP_BUCKETS;
> + for (bucket_id = 0; bucket_id < UCLAMP_BUCKETS; ++bucket_id) {
> + uc_map_old.data = atomic_long_read(&uc_maps[bucket_id].adata);
> + if (free_bucket_id == UCLAMP_BUCKETS && !uc_map_old.se_count)
> + free_bucket_id = bucket_id;
> + if (uc_map_old.value == bucket_value)
> + break;
> + }
> +
> + /* ... or allocate a new clamp bucket */
> + if (bucket_id >= UCLAMP_BUCKETS) {
> + /*
> + * A valid clamp bucket must always be available.
> + * If we cannot find one: refcounting is broken and we
> + * warn once. The sched_entity will be tracked in the
> + * fast-path using its previous clamp bucket, or not
> + * tracked at all if not yet mapped (i.e. it's new).
> + */
> + if (unlikely(free_bucket_id == UCLAMP_BUCKETS)) {
> + SCHED_WARN_ON(free_bucket_id == UCLAMP_BUCKETS);
> + return;
> + }
> + bucket_id = free_bucket_id;
> + uc_map_old.data = atomic_long_read(&uc_maps[bucket_id].adata);
> + }

And then skip all this?

> +
> + uc_map_new.se_count = uc_map_old.se_count + 1;
> + uc_map_new.value = bucket_value;
> +
> + } while (!atomic_long_try_cmpxchg(&uc_maps[bucket_id].adata,
> + &uc_map_old.data, uc_map_new.data));
> +
> + uc_se->value = clamp_value;
> + uc_se->bucket_id = bucket_id;
> +
> + if (uc_se->mapped)
> + uclamp_bucket_dec(clamp_id, prev_bucket_id);
> +
> + /*
> + * Task's sched_entity are refcounted in the fast-path only when they
> + * have got a valid clamp_bucket assigned.
> + */
> + uc_se->mapped = true;
> +}