Re: [PATCH v4 06/16] sched/cpufreq: uclamp: add utilization clamping for FAIR tasks

Peter Zijlstra
2018-09-14T09:32:52Z

On Tue, Aug 28, 2018 at 02:53:14PM +0100, Patrick Bellasi wrote:
> diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> index 3fffad3bc8a8..949082555ee8 100644
> --- a/kernel/sched/cpufreq_schedutil.c
> +++ b/kernel/sched/cpufreq_schedutil.c
> @@ -222,8 +222,13 @@ static unsigned long sugov_get_util(struct sugov_cpu *sg_cpu)
> * CFS tasks and we use the same metric to track the effective
> * utilization (PELT windows are synchronized) we can directly add them
> * to obtain the CPU's actual utilization.
> + *
> + * CFS utilization can be boosted or capped, depending on utilization
> + * clamp constraints configured for currently RUNNABLE tasks.
> */
> util = cpu_util_cfs(rq);
> + if (util)
> + util = uclamp_util(rq, util);

Should that not be:

util = clamp_util(rq, cpu_util_cfs(rq));

Because if !util might we not still want to enforce the min clamp?

> util += cpu_util_rt(rq);
>
> /*

> @@ -322,11 +328,24 @@ static void sugov_iowait_boost(struct sugov_cpu *sg_cpu, u64 time,
> return;
> sg_cpu->iowait_boost_pending = true;
>
> + /*
> + * Boost FAIR tasks only up to the CPU clamped utilization.
> + *
> + * Since DL tasks have a much more advanced bandwidth control, it's
> + * safe to assume that IO boost does not apply to those tasks.
> + * Instead, since RT tasks are not utiliation clamped, we don't want
> + * to apply clamping on IO boost while there is blocked RT
> + * utilization.
> + */
> + max_boost = sg_cpu->iowait_boost_max;
> + if (!cpu_util_rt(cpu_rq(sg_cpu->cpu)))
> + max_boost = uclamp_util(cpu_rq(sg_cpu->cpu), max_boost);

OK I suppose.

> +
> /* Double the boost at each request */
> if (sg_cpu->iowait_boost) {
> sg_cpu->iowait_boost <<= 1;
> - if (sg_cpu->iowait_boost > sg_cpu->iowait_boost_max)
> - sg_cpu->iowait_boost = sg_cpu->iowait_boost_max;
> + if (sg_cpu->iowait_boost > max_boost)
> + sg_cpu->iowait_boost = max_boost;
> return;
> }
>

> +static inline unsigned int uclamp_value(struct rq *rq, int clamp_id)
> +{
> + struct uclamp_cpu *uc_cpu = &rq->uclamp;
> +
> + if (uc_cpu->value[clamp_id] == UCLAMP_NOT_VALID)
> + return uclamp_none(clamp_id);
> +
> + return uc_cpu->value[clamp_id];
> +}

Would that not be more readable as:

static inline unsigned int uclamp_value(struct rq *rq, int clamp_id)
{
unsigned int val = rq->uclamp.value[clamp_id];

if (unlikely(val == UCLAMP_NOT_VALID))
val = uclamp_none(clamp_id);

return val;
}

And how come NOT_VALID is possible? I thought the idea was to always
have all things a valid value.