Re: [PATCH] cpufreq: schedutil: Don't use the limits_changed flag any more

[Yue Hu]

On Fri, 12 Feb 2021 17:14:03 +0100
"Rafael J. Wysocki" <rafael@xxxxxxxxxx> wrote:

> On Mon, Feb 8, 2021 at 4:08 AM Yue Hu <zbestahu@xxxxxxxxx> wrote:
> >
> > From: Yue Hu <huyue2@xxxxxxxxxx>
> >
> > The limits_changed flag was introduced by commit 600f5badb78c
> > ("cpufreq: schedutil: Don't skip freq update when limits change")
> > due to race condition where need_freq_update is cleared in
> > get_next_freq() which causes reducing the CPU frequency is
> > ineffective while busy.
> >
> > But now, the race condition above is gone because get_next_freq()
> > doesn't clear the flag any more after commit 23a881852f3e ("cpufreq:
> > schedutil: Don't skip freq update if need_freq_update is set").
> >
> > Moreover, need_freq_update currently will be set to true only in
> > sugov_should_update_freq() if CPUFREQ_NEED_UPDATE_LIMITS is not set
> > for the driver. However, limits may have changed at any time.  
> 
> Yes, they may change at any time.
> 
> > And subsequent frequence update is depending on need_freq_update.  
> 
> I'm not following, sorry.
> 
> need_freq_update is set in sugov_should_update_freq() when
> limits_changed is cleared and it cannot be modified until
> sugov_update_next_freq() runs on the same CPU.

get_next_freq() will check if need to update freq not by
limits_changed but by need_freq_update.

And sugov_update_next_freq() is also checking need_freq_update to
update freq or not.

> 
> > So, we may skip this update.  
> 
> I'm not sure why?

As mentioned above, need_freq_update may will not be set when
limits_changed is set since set need_freq_update happens only in
sugov_should_update_freq(), so i understand we may skip this update
or not update in time.

> 
> > Hence, let's remove it to avoid above issue and make code more
> > simple.
> >
> > Signed-off-by: Yue Hu <huyue2@xxxxxxxxxx>
> > ---
> >  kernel/sched/cpufreq_schedutil.c | 11 +++--------
> >  1 file changed, 3 insertions(+), 8 deletions(-)
> >
> > diff --git a/kernel/sched/cpufreq_schedutil.c
> > b/kernel/sched/cpufreq_schedutil.c index 41e498b..7dd85fb 100644
> > --- a/kernel/sched/cpufreq_schedutil.c
> > +++ b/kernel/sched/cpufreq_schedutil.c
> > @@ -40,7 +40,6 @@ struct sugov_policy {
> >         struct task_struct      *thread;
> >         bool                    work_in_progress;
> >
> > -       bool                    limits_changed;
> >         bool                    need_freq_update;
> >  };
> >
> > @@ -89,11 +88,8 @@ static bool sugov_should_update_freq(struct
> > sugov_policy *sg_policy, u64 time) if
> > (!cpufreq_this_cpu_can_update(sg_policy->policy)) return false;
> >
> > -       if (unlikely(sg_policy->limits_changed)) {
> > -               sg_policy->limits_changed = false;
> > -               sg_policy->need_freq_update = true;
> > +       if (unlikely(sg_policy->need_freq_update))
> >                 return true;
> > -       }
> >
> >         delta_ns = time - sg_policy->last_freq_update_time;
> >
> > @@ -323,7 +319,7 @@ static bool sugov_cpu_is_busy(struct sugov_cpu
> > *sg_cpu) static inline void ignore_dl_rate_limit(struct sugov_cpu
> > *sg_cpu, struct sugov_policy *sg_policy) {
> >         if (cpu_bw_dl(cpu_rq(sg_cpu->cpu)) > sg_cpu->bw_dl)
> > -               sg_policy->limits_changed = true;
> > +               sg_policy->need_freq_update = true;
> >  }
> >
> >  static inline bool sugov_update_single_common(struct sugov_cpu
> > *sg_cpu, @@ -759,7 +755,6 @@ static int sugov_start(struct
> > cpufreq_policy *policy) sg_policy->last_freq_update_time        = 0;
> >         sg_policy->next_freq                    = 0;
> >         sg_policy->work_in_progress             = false;
> > -       sg_policy->limits_changed               = false;
> >         sg_policy->cached_raw_freq              = 0;
> >
> >         sg_policy->need_freq_update =
> > cpufreq_driver_test_flags(CPUFREQ_NEED_UPDATE_LIMITS); @@ -813,7
> > +808,7 @@ static void sugov_limits(struct cpufreq_policy *policy)
> > mutex_unlock(&sg_policy->work_lock); }
> >
> > -       sg_policy->limits_changed = true;
> > +       sg_policy->need_freq_update = true;  
> 
> This may be running in parallel with sugov_update_next_freq() on a
> different CPU, so the latter may clear need_freq_update right after it
> has been set here unless I'm overlooking something.

Whether this logic is also happening for limits_changed in
sugo_should_update_freq() or not?

> 
> >  }
> >
> >  struct cpufreq_governor schedutil_gov = {
> > --
> > 1.9.1
> >