On 08/16/2018 06:47 PM, Patrick Bellasi wrote:
On 16-Aug 17:43, Dietmar Eggemann wrote:
On 08/06/2018 06:39 PM, Patrick Bellasi wrote:
When a util_max clamped task sleeps, its clamp constraints are removed
>from the CPU. However, the blocked utilization on that CPU can still be
higher than the max clamp value enforced while that task was running.
This max clamp removal when a CPU is going to be idle could thus allow
unwanted CPU frequency increases, right while the task is not running.
So 'rq->uclamp.flags == UCLAMP_FLAG_IDLE' means CPU is IDLE because
non-clamped tasks are tracked as well ((group_id = 0)).
Right, but... with (group_id = 0) you mean that "non-clamped tasks are
tracked" in the first clamp group?
Yes. I was asking myself what will happen if there are only non-clamped
tasks runnable ...
Maybe this is worth mentioning here?
Maybe I can explicitely say that we detect that there are not RUNNABLE
tasks because all the clamp groups are in UCLAMP_NOT_VALID status.
Yes, would have helped me the grasp this earlier ...
[...]
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index bc2beedec7bf..ff76b000bbe8 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -906,7 +906,8 @@ uclamp_group_find(int clamp_id, unsigned int clamp_value)
* For the specified clamp index, this method computes the new CPU utilization
* clamp to use until the next change on the set of RUNNABLE tasks on that
CPU.
*/
-static inline void uclamp_cpu_update(struct rq *rq, int clamp_id)
+static inline void uclamp_cpu_update(struct rq *rq, int clamp_id,
+ unsigned int last_clamp_value)
{
struct uclamp_group *uc_grp = &rq->uclamp.group[clamp_id][0];
int max_value = UCLAMP_NOT_VALID;
@@ -924,6 +925,19 @@ static inline void uclamp_cpu_update(struct rq *rq, int
clamp_id)
The condition:
if (!uclamp_group_active(uc_grp, group_id))
continue;
in 'for (group_id = 0; group_id <= CONFIG_UCLAMP_GROUPS_COUNT; ++group_id)
{}' makes sure that 'max_value == UCLAMP_NOT_VALID' is true for the if
condition (*):
if (max_value >= SCHED_CAPACITY_SCALE)
break;
}
+
+ /*
+ * Just for the UCLAMP_MAX value, in case there are no RUNNABLE
+ * task, we keep the CPU clamped to the last task's clamp value.
+ * This avoids frequency spikes to MAX when one CPU, with an high
+ * blocked utilization, sleeps and another CPU, in the same frequency
+ * domain, do not see anymore the clamp on the first CPU.
+ */
+ if (clamp_id == UCLAMP_MAX && max_value == UCLAMP_NOT_VALID) {
+ rq->uclamp.flags |= UCLAMP_FLAG_IDLE;
+ max_value = last_clamp_value;
+ }
+
(*): So the uc_grp[group_id].value stays last_clamp_value?
A bit confusing... but I think you've got the point.
OK.
What do you do when the blocked utilization decays below this enforced
last_clamp_value on that CPU?
This is done _just_ for max_util:
- it clamps a blocked utilization bigger then last_clamp_value
thus avoiding the selection of an OPP bigger then the one enforced
while the task was runnable
- it has not effect on a blocked utilization smaller then last_clamp_value
thus allowing to reduce gracefully the OPP as long as the blocked
utilization is decayed
Ah correct, max_util is about capping, not boosting.
I assume there are plenty of this kind of corner cases because we have
blocked signals (including all tasks) and clamping (including runnable
tasks).
This is a pretty compelling one I've noticed in my tests and thus
worth a fix... I don't have on hand other similar corner cases, do
you?
No not right now, will continue to watch out for them ...
