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SubjectRe: [PATCH v2] sched/uclamp: Avoid getting unreasonable ucalmp value when rq is idle
On Wed, Jun 30, 2021 at 10:12:04PM +0800, Xuewen Yan wrote:
> From: Xuewen Yan <xuewen.yan@unisoc.com>
>
> Now in uclamp_rq_util_with(), when the task != NULL, the uclamp_max as following:
> uc_rq_max = rq->uclamp[UCLAMP_MAX].value;
> uc_eff_max = uclamp_eff_value(p, UCLAMP_MAX);
> uclamp_max = max{uc_rq_max, uc_eff_max};
>
> Consider the following scenario:
> (1)the rq is idle, the uc_rq_max is last runnable task's UCLAMP_MAX;
> (2)the p's uc_eff_max < uc_rq_max.
>
> As a result, the uclamp_max = uc_rq_max instead of uc_eff_max, it is unreasonable.
>
> The scenario often happens in find_energy_efficient_cpu(), when the task has smaller UCLAMP_MAX.
>
> When rq has UCLAMP_FLAG_IDLE flag, enqueuing the task will lift UCLAMP_FLAG_IDLE
> and set the rq clamp as the task's via uclamp_idle_reset(). It doesn't need
> to read the rq clamp. And it can also avoid the problems described above.
>
> Fixes: 9d20ad7dfc9a ("sched/uclamp: Add uclamp_util_with()")
>
> Signed-off-by: Xuewen Yan <xuewen.yan@unisoc.com>

Valentin, Qais, can either of you write a Changelog/comment for this, I
can't seem to make any sense of it.

Is this about wake-from-idle, where the first task's uclamp goes amis
because the rq->uclamp values haven't been updated yet?


> ---
> kernel/sched/sched.h | 21 ++++++++++++++-------
> 1 file changed, 14 insertions(+), 7 deletions(-)
>
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index c80d42e9589b..14a41a243f7b 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -2818,20 +2818,27 @@ static __always_inline
> unsigned long uclamp_rq_util_with(struct rq *rq, unsigned long util,
> struct task_struct *p)
> {
> - unsigned long min_util;
> - unsigned long max_util;
> + unsigned long min_util = 0;
> + unsigned long max_util = 0;
>
> if (!static_branch_likely(&sched_uclamp_used))
> return util;
>
> - min_util = READ_ONCE(rq->uclamp[UCLAMP_MIN].value);
> - max_util = READ_ONCE(rq->uclamp[UCLAMP_MAX].value);
> -
> if (p) {
> - min_util = max(min_util, uclamp_eff_value(p, UCLAMP_MIN));
> - max_util = max(max_util, uclamp_eff_value(p, UCLAMP_MAX));
> + min_util = uclamp_eff_value(p, UCLAMP_MIN);
> + max_util = uclamp_eff_value(p, UCLAMP_MAX);
> +
> + /*
> + * Ignore last runnable task's max clamp, as this task will
> + * reset it. Similarly, no need to read the rq's min clamp.
> + */
> + if (rq->uclamp_flags & UCLAMP_FLAG_IDLE)
> + goto out;
> }
>
> + min_util = max_t(unsigned long, min_util, READ_ONCE(rq->uclamp[UCLAMP_MIN].value));
> + max_util = max_t(unsigned long, max_util, READ_ONCE(rq->uclamp[UCLAMP_MAX].value));
> +out:
> /*
> * Since CPU's {min,max}_util clamps are MAX aggregated considering
> * RUNNABLE tasks with _different_ clamps, we can end up with an
> --
> 2.25.1
>

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