Re: [PATCH v3 03/14] sched/core: uclamp: add CPU's clamp groups accounting

Dietmar Eggemann
2018-08-14T15:44:15Z

On 08/06/2018 06:39 PM, Patrick Bellasi wrote:

[...]

> +/**
> + * uclamp_cpu_put_id(): decrease reference count for a clamp group on a CPU
> + * @p: the task being dequeued from a CPU
> + * @cpu: the CPU from where the clamp group has to be released
> + * @clamp_id: the utilization clamp (e.g. min or max utilization) to release
> + *
> + * When a task is dequeued from a CPU's RQ, the CPU's clamp group reference
> + * counted by the task is decreased.
> + * If this was the last task defining the current max clamp group, then the
> + * CPU clamping is updated to find the new max for the specified clamp
> + * index.
> + */
> +static inline void uclamp_cpu_put_id(struct task_struct *p,
> + struct rq *rq, int clamp_id)
> +{
> + struct uclamp_group *uc_grp;
> + struct uclamp_cpu *uc_cpu;
> + unsigned int clamp_value;
> + int group_id;
> +
> + /* No task specific clamp values: nothing to do */
> + group_id = p->uclamp[clamp_id].group_id;
> + if (group_id == UCLAMP_NOT_VALID)
> + return;
> +
> + /* Decrement the task's reference counted group index */
> + uc_grp = &rq->uclamp.group[clamp_id][0];
> +#ifdef SCHED_DEBUG
> + if (unlikely(uc_grp[group_id].tasks == 0)) {
> + WARN(1, "invalid CPU[%d] clamp group [%d:%d] refcount\n",
> + cpu_of(rq), clamp_id, group_id);
> + uc_grp[group_id].tasks = 1;
> + }
> +#endif

This one indicates that there are some holes in your ref-counting. It's
probably easier to debug that there is still a task but the
uc_grp[group_id].tasks value == 0 (A). I assume the other problem exists
as well, i.e. last task and uc_grp[group_id].tasks > 1 (B)?

You have uclamp_cpu_[get/put](_id)() in [enqueue/dequeue]_task.

Patch 04/14 introduces its use in uclamp_task_update_active().

Do you know why (A) (and (B)) are happening?

> + uc_grp[group_id].tasks -= 1;
> +
> + /* If this is not the last task, no updates are required */
> + if (uc_grp[group_id].tasks > 0)
> + return;
> +
> + /*
> + * Update the CPU only if this was the last task of the group
> + * defining the current clamp value.
> + */
> + uc_cpu = &rq->uclamp;
> + clamp_value = uc_grp[group_id].value;
> + if (clamp_value >= uc_cpu->value[clamp_id])

'clamp_value > uc_cpu->value[clamp_id]' should indicate another
inconsistency in the uclamp machinery, right? …