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 19:10:58 +0200 |
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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 ...
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