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Date:   Mon, 24 Jul 2023 16:53:28 +0100
From:   Qais Yousef <qyousef@layalina.io>
To:     Xuewen Yan <xuewen.yan94@gmail.com>
Cc:     Xuewen Yan <xuewen.yan@unisoc.com>, rafael@kernel.org,
        viresh.kumar@linaro.org, mingo@redhat.com, peterz@infradead.org,
        vincent.guittot@linaro.org, dietmar.eggemann@arm.com,
        rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de,
        bristot@redhat.com, vschneid@redhat.com, guohua.yan@unisoc.com,
        linux-pm@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] cpufreq: schedutil: next_freq need update when
 cpufreq_limits changed
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On 07/24/23 11:36, Xuewen Yan wrote:
> On Sat, Jul 22, 2023 at 7:02 AM Qais Yousef <qyousef@layalina.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@unisoc.com>
> > > Co-developed-by: Guohua Yan <guohua.yan@unisoc.com>
> > > Signed-off-by: Guohua Yan <guohua.yan@unisoc.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..

Ah I see; we just use prev_util but the request will go through and the driver
should observe the new limit regardless of what util value we pass to it. Got
ya.

> 
> 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;
>  }

Sound reasonable in principle, but it could lead to overhead; for example when
the system is busy and maxed out, the last_freq_update_time will never be
updated and will end up continuously calling to the driver to change frequency
without any rate limit AFAICS. Which might not be an acceptable overhead,
I don't know. Logically this is wasted cycles preventing the tasks from doing
useful work. I think we need to look at such corner cases and treat them
appropriately to not call the driver if we go with this approach.


Cheers

--
Qais Yousef

> 
> 
> 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
> > >