Messages in this thread | | | Subject | Re: [PATCH v3 07/14] sched/core: uclamp: enforce last task UCLAMP_MAX | From | Dietmar Eggemann <> | Date | Thu, 16 Aug 2018 17:43:18 +0200 |
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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)).
Maybe this is worth mentioning here?
> This can happen, for example, where there is another (smaller) task > running on a different CPU of the same frequency domain. > In this case, when we aggregate the utilization of all the CPUs in a > shared frequency domain, schedutil can still see the full non clamped > blocked utilization of all the CPUs and thus eventually increase the > frequency. > > Let's fix this by using: > > uclamp_cpu_put_id(UCLAMP_MAX) > uclamp_cpu_update(last_clamp_value) > > to detect when a CPU has no more RUNNABLE clamped tasks and to flag this > condition. Thus, while a CPU is idle, we can still enforce the last used > clamp value for it. > > To the contrary, we do not track any UCLAMP_MIN since, while a CPU is > idle, we don't want to enforce any minimum frequency > Indeed, we rely just on blocked load decay to smoothly reduce the > frequency.
[...]
> 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?
What do you do when the blocked utilization decays below this enforced last_clamp_value on that CPU?
I assume there are plenty of this kind of corner cases because we have blocked signals (including all tasks) and clamping (including runnable tasks).
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