Date:   Thu, 8 Dec 2022 11:31:36 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Lukasz Luba <lukasz.luba@....com>
Cc:     linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        rafael@...nel.org, dietmar.eggemann@....com, saravanak@...gle.com,
        wusamuel@...gle.com, isaacmanjarres@...gle.com,
        kernel-team@...roid.com, juri.lelli@...hat.com,
        peterz@...radead.org, mingo@...hat.com, rostedt@...dmis.org,
        bsegall@...gle.com, mgorman@...e.de, viresh.kumar@...aro.org
Subject: Re: [PATCH v2 2/2] cpufreq: schedutil: Optimize operations with
 single max CPU capacity

On Thu, 8 Dec 2022 at 11:06, Lukasz Luba <lukasz.luba@....com> wrote:
>
>
>
> On 12/8/22 08:37, Vincent Guittot wrote:
> > On Wed, 7 Dec 2022 at 11:17, Lukasz Luba <lukasz.luba@....com> wrote:
> >>
> >> The max CPU capacity is the same for all CPUs sharing frequency domain
> >> and thus 'policy' object. There is a way to avoid heavy operations
> >> in a loop for each CPU by leveraging this knowledge. Thus, simplify
> >> the looping code in the sugov_next_freq_shared() and drop heavy
> >> multiplications. Instead, use simple max() to get the highest utilization
> >> from these CPUs. This is useful for platforms with many (4 or 6) little
> >> CPUs.
> >>
> >> The max CPU capacity must be fetched every time we are called, due to
> >> difficulties during the policy setup, where we are not able to get the
> >> normalized CPU capacity at the right time.
> >>
> >> The stored value in sugov_policy::max is also than used in
> >> sugov_iowait_apply() to calculate the right boost. Thus, that field is
> >> useful to have in that sugov_policy struct.
> >>
> >> Signed-off-by: Lukasz Luba <lukasz.luba@....com>
> >> ---
> >>   kernel/sched/cpufreq_schedutil.c | 22 +++++++++++-----------
> >>   1 file changed, 11 insertions(+), 11 deletions(-)
> >>
> >> diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> >> index c19d6de67b7a..f9881f3d9488 100644
> >> --- a/kernel/sched/cpufreq_schedutil.c
> >> +++ b/kernel/sched/cpufreq_schedutil.c
> >> @@ -158,10 +158,8 @@ static unsigned int get_next_freq(struct sugov_policy *sg_policy,
> >>
> >>   static void sugov_get_util(struct sugov_cpu *sg_cpu)
> >>   {
> >> -       struct sugov_policy *sg_policy = sg_cpu->sg_policy;
> >>          struct rq *rq = cpu_rq(sg_cpu->cpu);
> >>
> >> -       sg_policy->max = arch_scale_cpu_capacity(sg_cpu->cpu);
> >>          sg_cpu->bw_dl = cpu_bw_dl(rq);
> >>          sg_cpu->util = effective_cpu_util(sg_cpu->cpu, cpu_util_cfs(sg_cpu->cpu),
> >>                                            FREQUENCY_UTIL, NULL);
> >> @@ -317,6 +315,8 @@ static inline void ignore_dl_rate_limit(struct sugov_cpu *sg_cpu)
> >>   static inline bool sugov_update_single_common(struct sugov_cpu *sg_cpu,
> >>                                                u64 time, unsigned int flags)
> >>   {
> >> +       struct sugov_policy *sg_policy = sg_cpu->sg_policy;
> >> +
> >>          sugov_iowait_boost(sg_cpu, time, flags);
> >>          sg_cpu->last_update = time;
> >>
> >> @@ -325,6 +325,9 @@ static inline bool sugov_update_single_common(struct sugov_cpu *sg_cpu,
> >>          if (!sugov_should_update_freq(sg_cpu->sg_policy, time))
> >>                  return false;
> >>
> >> +       /* Fetch the latest CPU capcity to avoid stale data */
> >> +       sg_policy->max = arch_scale_cpu_capacity(sg_cpu->cpu);
> >> +
> >>          sugov_get_util(sg_cpu);
> >>          sugov_iowait_apply(sg_cpu, time);
> >>
> >> @@ -414,25 +417,22 @@ static unsigned int sugov_next_freq_shared(struct sugov_cpu *sg_cpu, u64 time)
> >>   {
> >>          struct sugov_policy *sg_policy = sg_cpu->sg_policy;
> >>          struct cpufreq_policy *policy = sg_policy->policy;
> >> -       unsigned long util = 0, max = 1;
> >> +       unsigned long util = 0;
> >>          unsigned int j;
> >>
> >> +       /* Fetch the latest CPU capcity to avoid stale data */
> >> +       sg_policy->max = arch_scale_cpu_capacity(sg_cpu->cpu);
> >> +
> >>          for_each_cpu(j, policy->cpus) {
> >>                  struct sugov_cpu *j_sg_cpu = &per_cpu(sugov_cpu, j);
> >> -               unsigned long j_util, j_max;
> >>
> >>                  sugov_get_util(j_sg_cpu);
> >>                  sugov_iowait_apply(j_sg_cpu, time);
> >> -               j_util = j_sg_cpu->util;
> >> -               j_max = j_sg_cpu->max;
> >>
> >> -               if (j_util * max > j_max * util) {
> >> -                       util = j_util;
> >> -                       max = j_max;
> >> -               }
> >
> > With the code removed above, max is only used in 2 places:
> > - sugov_iowait_apply
> > - map_util_freq
> >
> > I wonder if it would be better to just call arch_scale_cpu_capacity()
> > in these 2 places instead of saving a copy in sg_policy and then
> > reading it twice.
>
> The sugov_iowait_apply() is called in that loop, so probably I will
> add a new argument to that call and just feed it with the capacity value
> from one CPU, which was read before the loop. So, similarly what is in
> this patch. Otherwise, all of those per-cpu capacity vars would be
> accessed inside the sugov_iowait_apply() with sg_cpu->cpu.

Yes make sense

>
> >
> > arch_scaleu_cpu_capacity is already a per_cpu variable so accessing it
> > should be pretty cheap.
>
> Yes and no, as you said this is per-cpu variable and would access them
> from one CPU, which is running that loop. They will have different pages
> and addresses so cache lines on that CPU. to avoiding trashing a cache
> lines on this running CPU let's read that capacity once, before the
> loop. Let's use the new arg to pass that value via one of the
> registers. In such, only one cache line would have to fetch that data
> into.
>
> So I thought this simple sg_policy->max would do the trick w/o a lot
> of hassle.

For the shared mode, everything is located in sugov_next_freq_shared
so you don't need to save the max value with your proposal above to
change sugov_iowait_apply interface.

This should be doable as well for single mode

> >
> > Thought ?
> >
>
> I can change that and drop the sg_policy->max and call differently
> those capacity values. I will have to unfortunately drop Viresh's ACKs,
> since this will be a way different code.
>
> Thanks Vincent for the suggestion. Do you want me to go further with
> such approach and send a v3?

Don't know what Rafael and Viresh think but it seems that we don't
need to save the return of arch_scale_cpu_capacity in ->max field but
directly use it

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