Re: [PATCH] cpufreq: schedutil: next_freq need update when cpufreq_limits changed

[Xuewen Yan]

Hi Pierre,

On Mon, Jul 24, 2023 at 11:33 PM Pierre Gondois <pierre.gondois@xxxxxxx> wrote:
>
>
>
> On 7/24/23 05:36, Xuewen Yan wrote:
> > On Sat, Jul 22, 2023 at 7:02 AM Qais Yousef <qyousef@xxxxxxxxxxx> 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@xxxxxxxxxx>
> >>> Co-developed-by: Guohua Yan <guohua.yan@xxxxxxxxxx>
> >>> Signed-off-by: Guohua Yan <guohua.yan@xxxxxxxxxx>
> >>> ---
> >>>   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;
> >   }
> >
>
> Hello Xuewen,
> FWIW, the patch and explanation for sugov_update_single_perf() seem sensible to
> me. Just a comment about cpufreq_driver_adjust_perf() and
> (struct cpufreq_driver)->adjust_perf(): wouldn't their prototype need to be
> updated (i.e. not return void) to do the change suggested above ?

Yes, their function type should be changed from void to bool or init.
For this patch, I just raise a question for everyone to discuss. If
this is a problem, the official patch needs to be revised later.

BR
xuewen

>
> Regards,
> Pierre
>
> >
> > 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
> >>>
> >