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

Qais Yousef
2023-09-10T19:25:08Z

On 09/07/23 13:44, Peter Zijlstra wrote:
> 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.

Yes! I was too focused am I doing the right thing, I didn't stop to think this
is actually a constant and can be done once too. I will fix it if this knobs
ends up hanging around.

Thanks!

--
Qais Yousef