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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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