Re: [PATCH] cpufreq: schedutil: Don't skip freq update when limits change

[Viresh Kumar <viresh.kumar@linaro.org>]

On 29-07-19, 00:55, Doug Smythies wrote:
> On 2019.07.25 23:58 Viresh Kumar wrote:
> > Hmm, so I tried to reproduce your setup on my ARM board.
> > - booted only with CPU0 so I hit the sugov_update_single() routine
> > - And applied below diff to make CPU look permanently busy:
> >
> > -------------------------8<-------------------------
> >diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> > index 2f382b0959e5..afb47490e5dc 100644
> > --- a/kernel/sched/cpufreq_schedutil.c
> > +++ b/kernel/sched/cpufreq_schedutil.c
> > @@ -121,6 +121,7 @@ static void sugov_fast_switch(struct sugov_policy *sg_policy, u64 time,
> >         if (!sugov_update_next_freq(sg_policy, time, next_freq))
> >                return;
> > 
> > +       pr_info("%s: %d: %u\n", __func__, __LINE__, freq);
> 
> ?? there is no "freq" variable here, and so this doesn't compile. However this works:
> 
> +       pr_info("%s: %d: %u\n", __func__, __LINE__, next_freq);

There are two paths we can take to change the frequency, normal
sleep-able path (sugov_work) or fast path. Only one of them is taken
by any driver ever. In your case it is the fast path always and in
mine it was the slow path.

I only tested the diff with slow-path and copy pasted to fast path
while giving out to you and so the build issue. Sorry about that.

Also make sure that the print is added after sugov_update_next_freq()
is called, not before it.

> >         next_freq = cpufreq_driver_fast_switch(policy, next_freq);
> >        if (!next_freq)
> >                return;
> > @@ -424,14 +425,10 @@ static unsigned long sugov_iowait_apply(struct sugov_cpu *sg_cpu, u64 time,
> > #ifdef CONFIG_NO_HZ_COMMON
> > static bool sugov_cpu_is_busy(struct sugov_cpu *sg_cpu)
> > {
> > -       unsigned long idle_calls = tick_nohz_get_idle_calls_cpu(sg_cpu->cpu);
> > -       bool ret = idle_calls == sg_cpu->saved_idle_calls;
> > -
> > -       sg_cpu->saved_idle_calls = idle_calls;
> > -       return ret;
> > +       return true;
> >  }
> >  #else
> > -static inline bool sugov_cpu_is_busy(struct sugov_cpu *sg_cpu) { return false; }
> > +static inline bool sugov_cpu_is_busy(struct sugov_cpu *sg_cpu) { return true; }
> > #endif /* CONFIG_NO_HZ_COMMON */
> > 
> >  /*
> > @@ -565,6 +562,7 @@ static void sugov_work(struct kthread_work *work)
> >         sg_policy->work_in_progress = false;
> >         raw_spin_unlock_irqrestore(&sg_policy->update_lock, flags);
> > 
> > +       pr_info("%s: %d: %u\n", __func__, __LINE__, freq);
> >         mutex_lock(&sg_policy->work_lock);
> >         __cpufreq_driver_target(sg_policy->policy, freq, CPUFREQ_RELATION_L);
> >         mutex_unlock(&sg_policy->work_lock);
> > 
> > -------------------------8<-------------------------
> >
> > Now, the frequency never gets down and so gets set to the maximum
> > possible after a bit.
> >
> > - Then I did:
> >
> > echo <any-low-freq-value> > /sys/devices/system/cpu/cpufreq/policy0/scaling_max_freq
> >
> > Without my patch applied:
> >        The print never gets printed and so frequency doesn't go down.
> >
> > With my patch applied:
> >        The print gets printed immediately from sugov_work() and so
> >        the frequency reduces.
> > 
> > Can you try with this diff along with my Patch2 ? I suspect there may
> > be something wrong with the intel_cpufreq driver as the patch fixes
> > the only path we have in the schedutil governor which takes busyness
> > of a CPU into account.
> 
> With this diff along with your patch2 There is never a print message
> from sugov_work. There are from sugov_fast_switch.

Which is okay. sugov_work won't get hit in your case as I explained
above.

> Note that for the intel_cpufreq CPU scaling driver and the schedutil
> governor I adjust the maximum clock frequency this way:
> 
> echo <any-low-percent> > /sys/devices/system/cpu/intel_pstate/max_perf_pct

This should eventually call sugov_limits() in schedutil governor, this
can be easily checked with another print message.

> I also applied the pr_info messages to the reverted kernel, and re-did
> my tests (where everything works as expected). There is never a print
> message from sugov_work. There are from sugov_fast_switch.

that's fine.

> Notes:
> 
> I do not know if:
> /sys/devices/system/cpu/cpufreq/policy*/scaling_max_freq
> /sys/devices/system/cpu/cpufreq/policy*/scaling_min_freq
> Need to be accurate when using the intel_pstate driver in passive mode.
> They are not.
> The commit comment for 9083e4986124389e2a7c0ffca95630a4983887f0
> suggests that they might need to be representative.
> I wonder if something similar to that commit is needed
> for other global changes, such as max_perf_pct and min_perf_pct?

We are already calling intel_pstate_update_policies() in that case, so
it should be fine I believe.

> intel_cpufreq/ondemand doesn't work properly on the reverted kernel.

reverted kernel ? The patch you reverted was only for schedutil and it
shouldn't have anything to do with ondemand.

> (just discovered, not investigated)
> I don't know about other governors.

When you do:

echo <any-low-percent> > /sys/devices/system/cpu/intel_pstate/max_perf_pct

How soon does the print from sugov_fast_switch() gets printed ?
Immediately ? Check with both the kernels, with my patch and with the
reverted patch.

Also see if there is any difference in the next_freq value in both the
kernels when you change max_perf_pct.

FWIW, we now know the difference between intel-pstate and
acpi-cpufreq/my testcase and why we see differences here. In the cases
where my patch fixed the issue (acpi/ARM), we were really changing the
limits, i.e. policy->min/max. This happened because we touched
scaling_max_freq directly.

For the case of intel-pstate, you are changing max_perf_pct which
doesn't change policy->max directly. I am not very sure how all of it
work really, but at least schedutil will not see policy->max changing.

@Rafael: Do you understand why things don't work properly with
intel_cpufreq driver ?

-- 
viresh