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SubjectRe: [PATCH v3 07/14] sched/core: uclamp: enforce last task UCLAMP_MAX
On 06-Aug 17:39, 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.
>
> 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.

[...]

> @@ -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)
> 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;
> + }
> +
> rq->uclamp.value[clamp_id] = max_value;
> }
>
> @@ -953,13 +967,26 @@ static inline void uclamp_cpu_get_id(struct task_struct *p,
> uc_grp = &rq->uclamp.group[clamp_id][0];
> uc_grp[group_id].tasks += 1;
>
> + /* Force clamp update on idle exit */
> + uc_cpu = &rq->uclamp;
> + clamp_value = p->uclamp[clamp_id].value;
> + if (unlikely(uc_cpu->flags & UCLAMP_FLAG_IDLE)) {
> + /*
> + * This function is called for both UCLAMP_MIN (before) and
> + * UCLAMP_MAX (after). Let's reset the flag only the second
> + * once we know that UCLAMP_MIN has been already updated.
> + */
> + if (clamp_id == UCLAMP_MAX)
> + uc_cpu->flags &= ~UCLAMP_FLAG_IDLE;
> + uc_cpu->value[clamp_id] = clamp_value;
> + return;
> + }

Just notice that the code block above is not reached when we enqueue a task
without a valid clamp group, i.e. an un-clamped task, which is the
default for all tasks.

The fix should be as simple as moving this block at the beginning of
uclamp_cpu_update() so that we always un-conditionally release the
clamp holding as soon as we enqueue the first task after a CPU wakeup,
i.e. when the UCLAMP_FLAG_IDLE flag is set.

Will fix this on v4.

--
#include <best/regards.h>

Patrick Bellasi

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