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SubjectRe: [PATCH] sched/uclamp: Fix getting unreasonable ucalmp_max when rq is idle
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+Cc Patrick's current address

On 18/06/21 15:23, 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 task's UCLAMP_MAX;
> (2)the p's uc_eff_max < uc_rq_max.
>
> The result is 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.
>
> Inserts whether the rq is idle in the uclamp_rq_util_with().
>
> Signed-off-by: Xuewen Yan <xuewen.yan@unisoc.com>
> ---
> kernel/sched/sched.h | 5 ++++-
> 1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index a189bec13729..0feef6af89f2 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -2550,7 +2550,10 @@ unsigned long uclamp_rq_util_with(struct rq *rq, unsigned long util,
>
> if (p) {
> min_util = max(min_util, uclamp_eff_value(p, UCLAMP_MIN));
> - max_util = max(max_util, uclamp_eff_value(p, UCLAMP_MAX));
> + if (rq->uclamp_flags & UCLAMP_FLAG_IDLE)
> + max_util = uclamp_eff_value(p, UCLAMP_MAX);
> + else
> + max_util = max(max_util, uclamp_eff_value(p, UCLAMP_MAX));

That makes sense to me - enqueuing the task will lift UCLAMP_FLAG_IDLE and
set the rq clamp as the task's via uclamp_idle_reset().

Does this want a

Fixes: 9d20ad7dfc9a ("sched/uclamp: Add uclamp_util_with()")

?

Also, when we have UCLAMP_FLAG_IDLE, we don't even need to read the rq max
- and I'm pretty sure the same applies to the rq min. What about something like:

---
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 6510f0a46789..a2c6f6ae6392 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -2833,23 +2833,27 @@ 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));
+ 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)
- max_util = uclamp_eff_value(p, UCLAMP_MAX);
- else
- max_util = max(max_util, uclamp_eff_value(p, UCLAMP_MAX));
+ 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
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