Re: [RFC PATCH 5/7] sched/schedutil: Add a new tunable to dictate response time

Peter Zijlstra
2023-09-07T15:41:02Z

On Mon, Aug 28, 2023 at 12:32:01AM +0100, Qais Yousef wrote:
> +static inline unsigned long
> +sugov_apply_response_time(struct sugov_policy *sg_policy, unsigned long util)
> +{
> + unsigned long mult;
> +
> + if (sg_policy->freq_response_time_ms == sg_policy->tunables->response_time_ms)
> + return util;
> +
> + mult = sg_policy->freq_response_time_ms * SCHED_CAPACITY_SCALE;
> + mult /= sg_policy->tunables->response_time_ms;
> + mult *= util;
> +
> + return mult >> SCHED_CAPACITY_SHIFT;
> +}
> +
> static bool sugov_should_update_freq(struct sugov_policy *sg_policy, u64 time)
> {
> s64 delta_ns;
> @@ -143,6 +184,7 @@ static unsigned int get_next_freq(struct sugov_policy *sg_policy,
> unsigned int freq = arch_scale_freq_invariant() ?
> policy->cpuinfo.max_freq : policy->cur;
>
> + util = sugov_apply_response_time(sg_policy, util);
> freq = map_util_freq(util, freq, max);
>
> if (freq == sg_policy->cached_raw_freq && !sg_policy->need_freq_update)

Urgh, so instead of caching the multiplier you keep computing what is
essentially a constant over and over and over and over again :/

That is, compute the whole 'freq_response_time_ms * SCHED_CAPACITY_SCALE
/ response_time_ms' thing *once*, when that file is written to, and then
reduce the whole thing to:

return (freq_response_mult * util) >> SCHED_CAPACITY_SHIFT;

No need for that special case, no need for divisions, just go.