linux-kernel - Re: [PATCH] cpufreq: schedutil: next_freq need update when cpufreq

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Message-ID: <CAB8ipk8b8ZfwXN7KK-zFVPQ-8i37h64v-wz2ErB3AANaZ9w7aA@mail.gmail.com>
Date:   Mon, 24 Jul 2023 11:36:59 +0800
From:   Xuewen Yan <xuewen.yan94@...il.com>
To:     Qais Yousef <qyousef@...alina.io>
Cc:     Xuewen Yan <xuewen.yan@...soc.com>, rafael@...nel.org,
        viresh.kumar@...aro.org, mingo@...hat.com, peterz@...radead.org,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        bristot@...hat.com, vschneid@...hat.com, guohua.yan@...soc.com,
        linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] cpufreq: schedutil: next_freq need update when
 cpufreq_limits changed

On Sat, Jul 22, 2023 at 7:02 AM Qais Yousef <qyousef@...alina.io> wrote:
>
> On 07/19/23 21:05, Xuewen Yan wrote:
> > When cpufreq's policy is single, there is a scenario that will
> > cause sg_policy's next_freq to be unable to update.
> >
> > When the cpu's util is always max, the cpufreq will be max,
> > and then if we change the policy's scaling_max_freq to be a
> > lower freq, indeed, the sg_policy's next_freq need change to
> > be the lower freq, however, because the cpu_is_busy, the next_freq
> > would keep the max_freq.
> >
> > For example:
> > The cpu7 is single cpu:
> >
> > unisoc:/sys/devices/system/cpu/cpufreq/policy7 # while true;do done&
> > [1] 4737
> > unisoc:/sys/devices/system/cpu/cpufreq/policy7 # taskset -p 80 4737
> > pid 4737's current affinity mask: ff
> > pid 4737's new affinity mask: 80
> > unisoc:/sys/devices/system/cpu/cpufreq/policy7 # cat scaling_max_freq
> > 2301000
> > unisoc:/sys/devices/system/cpu/cpufreq/policy7 # cat scaling_cur_freq
> > 2301000
> > unisoc:/sys/devices/system/cpu/cpufreq/policy7 # echo 2171000 > scaling_max_freq
> > unisoc:/sys/devices/system/cpu/cpufreq/policy7 # cat scaling_max_freq
> > 2171000
> >
> > At this time, the sg_policy's next_freq would keep 2301000.
> >
> > To prevent the case happen, add the judgment of the need_freq_update flag.
> >
> > Signed-off-by: Xuewen Yan <xuewen.yan@...soc.com>
> > Co-developed-by: Guohua Yan <guohua.yan@...soc.com>
> > Signed-off-by: Guohua Yan <guohua.yan@...soc.com>
> > ---
> >  kernel/sched/cpufreq_schedutil.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> > index 4492608b7d7f..458d359f5991 100644
> > --- a/kernel/sched/cpufreq_schedutil.c
> > +++ b/kernel/sched/cpufreq_schedutil.c
> > @@ -350,7 +350,8 @@ static void sugov_update_single_freq(struct update_util_data *hook, u64 time,
> >        * Except when the rq is capped by uclamp_max.
> >        */
> >       if (!uclamp_rq_is_capped(cpu_rq(sg_cpu->cpu)) &&
> > -         sugov_cpu_is_busy(sg_cpu) && next_f < sg_policy->next_freq) {
> > +         sugov_cpu_is_busy(sg_cpu) && next_f < sg_policy->next_freq &&
> > +         !sg_policy->need_freq_update) {
>
> What about sugov_update_single_perf()? It seems to have the same problem, no?

There is no problem in sugov_update_single_perf, because the next_freq
is updated by drivers, maybe the next_freq is not used when using
sugov_update_single_perf..

But  for the last_freq_update_time, I think there are some problems
when using sugov_update_single_perf:
Now, there is no judgment condition for the update of the
last_freq_update_time. That means the last_freq_update_time is always
updated in sugov_update_single_perf.
And in sugov_should_update_freq: it would judge the
freq_update_delay_ns. As a result, If we use the
sugov_update_single_perf, the cpu frequency would only be periodically
updated according to freq_update_delay_ns.
Maybe we should judge the cpufreq_driver_adjust_perf's return value,
if the freq is not updated, the last_freq_update_time also does not
have to update.

Just like:
---
diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
index 458d359f5991..10f18b054f01 100644
--- a/kernel/sched/cpufreq_schedutil.c
+++ b/kernel/sched/cpufreq_schedutil.c
@@ -381,6 +381,7 @@ static void sugov_update_single_perf(struct
update_util_data *hook, u64 time,
        struct sugov_cpu *sg_cpu = container_of(hook, struct
sugov_cpu, update_util);
        unsigned long prev_util = sg_cpu->util;
        unsigned long max_cap;
+       bool freq_updated;

        /*
         * Fall back to the "frequency" path if frequency invariance is not
@@ -407,10 +408,11 @@ static void sugov_update_single_perf(struct
update_util_data *hook, u64 time,
            sugov_cpu_is_busy(sg_cpu) && sg_cpu->util < prev_util)
                sg_cpu->util = prev_util;

-       cpufreq_driver_adjust_perf(sg_cpu->cpu, map_util_perf(sg_cpu->bw_dl),
+       freq_updated = cpufreq_driver_adjust_perf(sg_cpu->cpu,
map_util_perf(sg_cpu->bw_dl),
                                   map_util_perf(sg_cpu->util), max_cap);

-       sg_cpu->sg_policy->last_freq_update_time = time;
+       if (freq_updated)
+               sg_cpu->sg_policy->last_freq_update_time = time;
 }


BR
Thanks!

---
xuewen
>
> LGTM otherwise.
>
>
> Cheers
>
> --
> Qais Yousef
>
> >               next_f = sg_policy->next_freq;
> >
> >               /* Restore cached freq as next_freq has changed */
> > --
> > 2.25.1
> >