Re: [PATCH v6 04/16] sched/core: uclamp: Add CPU's clamp buckets refcounting
From: Peter Zijlstra <peterz@infradead.org>
Date: 2019-01-21 14:59:42
Also in:
linux-pm, lkml
On Tue, Jan 15, 2019 at 10:15:01AM +0000, Patrick Bellasi wrote:
quoted hunk ↗ jump to hunk
@@ -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;