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Subject[PATCH v6 05/16] sched/core: uclamp: Update CPU's refcount on clamp changes
Date
Utilization clamp values enforced on a CPU by a task can be updated, for
example via a sched_setattr() syscall, while a task is RUNNABLE on that
CPU. A clamp value change always implies a clamp bucket refcount update
to ensure the new constraints are enforced.

Hook into uclamp_bucket_get() to trigger a CPU refcount syncup, via
uclamp_cpu_{inc,dec}_id(), whenever a task is RUNNABLE.

Signed-off-by: Patrick Bellasi <patrick.bellasi@arm.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>

---
Changes in v6:
Other:
- wholesale s/group/bucket/
- wholesale s/_{get,put}/_{inc,dec}/ to match refcount APIs
- small documentation updates
---
kernel/sched/core.c | 48 +++++++++++++++++++++++++++++++++++++++------
1 file changed, 42 insertions(+), 6 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 190137cd7b3b..67f059ee0a05 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -884,6 +884,38 @@ static inline void uclamp_cpu_dec(struct rq *rq, struct task_struct *p)
uclamp_cpu_dec_id(p, rq, clamp_id);
}

+static inline void
+uclamp_task_update_active(struct task_struct *p, unsigned int clamp_id)
+{
+ struct rq_flags rf;
+ struct rq *rq;
+
+ /*
+ * Lock the task and the CPU where the task is (or was) queued.
+ *
+ * We might lock the (previous) rq of a !RUNNABLE task, but that's the
+ * price to pay to safely serialize util_{min,max} updates with
+ * enqueues, dequeues and migration operations.
+ * This is the same locking schema used by __set_cpus_allowed_ptr().
+ */
+ rq = task_rq_lock(p, &rf);
+
+ /*
+ * Setting the clamp bucket is serialized by task_rq_lock().
+ * If the task is not yet RUNNABLE and its task_struct is not
+ * affecting a valid clamp bucket, the next time it's enqueued,
+ * it will already see the updated clamp bucket value.
+ */
+ if (!p->uclamp[clamp_id].active)
+ goto done;
+
+ uclamp_cpu_dec_id(p, rq, clamp_id);
+ uclamp_cpu_inc_id(p, rq, clamp_id);
+
+done:
+ task_rq_unlock(rq, p, &rf);
+}
+
static void uclamp_bucket_dec(unsigned int clamp_id, unsigned int bucket_id)
{
union uclamp_map *uc_maps = &uclamp_maps[clamp_id][0];
@@ -907,8 +939,8 @@ static void uclamp_bucket_dec(unsigned int clamp_id, unsigned int bucket_id)
&uc_map_old.data, uc_map_new.data));
}

-static void uclamp_bucket_inc(struct uclamp_se *uc_se, unsigned int clamp_id,
- unsigned int clamp_value)
+static void uclamp_bucket_inc(struct task_struct *p, 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;
@@ -979,6 +1011,9 @@ static void uclamp_bucket_inc(struct uclamp_se *uc_se, unsigned int clamp_id,
uc_se->value = clamp_value;
uc_se->bucket_id = bucket_id;

+ if (p)
+ uclamp_task_update_active(p, clamp_id);
+
if (uc_se->mapped)
uclamp_bucket_dec(clamp_id, prev_bucket_id);

@@ -1008,11 +1043,11 @@ static int __setscheduler_uclamp(struct task_struct *p,

mutex_lock(&uclamp_mutex);
if (attr->sched_flags & SCHED_FLAG_UTIL_CLAMP_MIN) {
- uclamp_bucket_inc(&p->uclamp[UCLAMP_MIN],
+ uclamp_bucket_inc(p, &p->uclamp[UCLAMP_MIN],
UCLAMP_MIN, lower_bound);
}
if (attr->sched_flags & SCHED_FLAG_UTIL_CLAMP_MAX) {
- uclamp_bucket_inc(&p->uclamp[UCLAMP_MAX],
+ uclamp_bucket_inc(p, &p->uclamp[UCLAMP_MAX],
UCLAMP_MAX, upper_bound);
}
mutex_unlock(&uclamp_mutex);
@@ -1049,7 +1084,8 @@ static void uclamp_fork(struct task_struct *p, bool reset)

p->uclamp[clamp_id].mapped = false;
p->uclamp[clamp_id].active = false;
- uclamp_bucket_inc(&p->uclamp[clamp_id], clamp_id, clamp_value);
+ uclamp_bucket_inc(NULL, &p->uclamp[clamp_id],
+ clamp_id, clamp_value);
}
}

@@ -1069,7 +1105,7 @@ static void __init init_uclamp(void)
memset(uclamp_maps, 0, sizeof(uclamp_maps));
for (clamp_id = 0; clamp_id < UCLAMP_CNT; ++clamp_id) {
uc_se = &init_task.uclamp[clamp_id];
- uclamp_bucket_inc(uc_se, clamp_id, uclamp_none(clamp_id));
+ uclamp_bucket_inc(NULL, uc_se, clamp_id, uclamp_none(clamp_id));
}
}

--
2.19.2
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