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SubjectRe: [PATCH v8 08/16] sched/core: uclamp: Set default clamps for RT tasks
On Tue, Apr 2, 2019 at 3:42 AM Patrick Bellasi <patrick.bellasi@arm.com> wrote:
>
> By default FAIR tasks start without clamps, i.e. neither boosted nor
> capped, and they run at the best frequency matching their utilization
> demand. This default behavior does not fit RT tasks which instead are
> expected to run at the maximum available frequency, if not otherwise
> required by explicitly capping them.
>
> Enforce the correct behavior for RT tasks by setting util_min to max
> whenever:
>
> 1. the task is switched to the RT class and it does not already have a
> user-defined clamp value assigned.
>
> 2. an RT task is forked from a parent with RESET_ON_FORK set.
>
> NOTE: utilization clamp values are cross scheduling class attributes and
> thus they are never changed/reset once a value has been explicitly
> defined from user-space.
>
> Signed-off-by: Patrick Bellasi <patrick.bellasi@arm.com>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> ---
> kernel/sched/core.c | 26 ++++++++++++++++++++++++++
> 1 file changed, 26 insertions(+)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index bdebdabe9bc4..71c9dd6487b1 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1042,6 +1042,28 @@ static int uclamp_validate(struct task_struct *p,
> static void __setscheduler_uclamp(struct task_struct *p,
> const struct sched_attr *attr)
> {
> + unsigned int clamp_id;
> +
> + /*
> + * On scheduling class change, reset to default clamps for tasks
> + * without a task-specific value.
> + */
> + for (clamp_id = 0; clamp_id < UCLAMP_CNT; ++clamp_id) {
> + struct uclamp_se *uc_se = &p->uclamp_req[clamp_id];
> + unsigned int clamp_value = uclamp_none(clamp_id);
> +
> + /* Keep using defined clamps across class changes */
> + if (uc_se->user_defined)
> + continue;
> +
> + /* By default, RT tasks always get 100% boost */
> + if (unlikely(rt_task(p) && clamp_id == UCLAMP_MIN))
> + clamp_value = uclamp_none(UCLAMP_MAX);
> +
> + uc_se->bucket_id = uclamp_bucket_id(clamp_value);
> + uc_se->value = clamp_value;

Is it possible for p->uclamp_req[UCLAMP_MAX].value to be less than
uclamp_none(UCLAMP_MAX) for this RT task? If that's a possibility then
I think we will end up with a case of p->uclamp_req[UCLAMP_MIN].value
> p->uclamp_req[UCLAMP_MAX].value after these assignments are done.

> + }
> +
> if (likely(!(attr->sched_flags & SCHED_FLAG_UTIL_CLAMP)))
> return;
>
> @@ -1077,6 +1099,10 @@ static void uclamp_fork(struct task_struct *p)
> for (clamp_id = 0; clamp_id < UCLAMP_CNT; ++clamp_id) {
> unsigned int clamp_value = uclamp_none(clamp_id);
>
> + /* By default, RT tasks always get 100% boost */
> + if (unlikely(rt_task(p) && clamp_id == UCLAMP_MIN))
> + clamp_value = uclamp_none(UCLAMP_MAX);
> +
> p->uclamp_req[clamp_id].user_defined = false;
> p->uclamp_req[clamp_id].value = clamp_value;
> p->uclamp_req[clamp_id].bucket_id = uclamp_bucket_id(clamp_value);
> --
> 2.20.1
>

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