Subject: Re: [PATCH 2/8] cpufreq: move invariance setter calls in cpufreq core

From: Ionela Voinescu <[email-masked]>
Date: Thu, 2 Jul 2020 12:45:24 +0100

Hi Rafael,

On Wednesday 01 Jul 2020 at 17:51:26 (+0200), Rafael J. Wysocki wrote:
> On Wed, Jul 1, 2020 at 5:28 PM Ionela Voinescu <[email-masked]> wrote:
> >
> > Hey,
> >
> > On Wednesday 01 Jul 2020 at 16:16:19 (+0530), Viresh Kumar wrote:
> > > On 01-07-20, 10:07, Ionela Voinescu wrote:
> > > > From: Valentin Schneider <[email-masked]>
> > > >
> > > > To properly scale its per-entity load-tracking signals, the task scheduler
> > > > needs to be given a frequency scale factor, i.e. some image of the current
> > > > frequency the CPU is running at. Currently, this scale can be computed
> > > > either by using counters (APERF/MPERF on x86, AMU on arm64), or by
> > > > piggy-backing on the frequency selection done by cpufreq.
> > > >
> > > > For the latter, drivers have to explicitly set the scale factor
> > > > themselves, despite it being purely boiler-plate code: the required
> > > > information depends entirely on the kind of frequency switch callback
> > > > implemented by the driver, i.e. either of: target_index(), target(),
> > > > fast_switch() and setpolicy().
> > > >
> > > > The fitness of those callbacks with regard to driving the Frequency
> > > > Invariance Engine (FIE) is studied below:
> > > >
> > > > target_index()
> > > > ==============
> > > > Documentation states that the chosen frequency "must be determined by
> > > > freq_table[index].frequency". It isn't clear if it *has* to be that
> > > > frequency, or if it can use that frequency value to do some computation
> > > > that ultimately leads to a different frequency selection. All drivers
> > > > go for the former, while the vexpress-spc-cpufreq has an atypical
> > > > implementation.
> > > >
> > > > Thefore, the hook works on the asusmption the core can use
> > > > freq_table[index].frequency.
> > > >
> > > > target()
> > > > =======
> > > > This has been flagged as deprecated since:
> > > >
> > > >   commit 9c0ebcf78fde ("cpufreq: Implement light weight ->target_index() routine")
> > > >
> > > > It also doesn't have that many users:
> > > >
> > > >   cpufreq-nforce2.c:371:2:  .target = nforce2_target,
> > > >   cppc_cpufreq.c:416:2:             .target = cppc_cpufreq_set_target,
> > > >   pcc-cpufreq.c:573:2:              .target = pcc_cpufreq_target,
> > > >
> > > > Should we care about drivers using this hook, we may be able to exploit
> > > > cpufreq_freq_transition_{being, end}(). Otherwise, if FIE support is
> > > > desired in their current state, arch_set_freq_scale() could still be
> > > > called directly by the driver, while CPUFREQ_CUSTOM_SET_FREQ_SCALE
> > > > could be used to mark support for it.
> > > >
> > > > fast_switch()
> > > > =============
> > > > This callback *has* to return the frequency that was selected.
> > > >
> > > > setpolicy()
> > > > ===========
> > > > This callback does not have any designated way of informing what was the
> > > > end choice. But there are only two drivers using setpolicy(), and none
> > > > of them have current FIE support:
> > > >
> > > >   drivers/cpufreq/longrun.c:281:    .setpolicy      = longrun_set_policy,
> > > >   drivers/cpufreq/intel_pstate.c:2215:      .setpolicy      = intel_pstate_set_policy,
> > > >
> > > > The intel_pstate is known to use counter-driven frequency invariance.
> > >
> > > Same for acpi-cpufreq driver as well ?
> > >
> >
> > The acpi-cpufreq driver defines target_index() and fast_switch() so it
> > should go through the setting in cpufreq core. But x86 does not actually
> > define arch_set_freq_scale() so when called it won't do anything (won't
> > set any frequency scale factor), but rely on counters to set it through
> > the arch_scale_freq_tick().
> 
> Right.
> 
> So on x86 (Intel flavor of it at least), cpufreq has nothing to do
> with this regardless of what driver is in use.
> 
> > But this cpufreq functionality could potentially be used.
> 
> How so?
> 

I was thinking of a scenario in which counters were not available and
cpufreq could give a rough indication of the current performance, if
arch_set_freq_scale() would be defined to pass that information.
It's improbable, but the implementation would allow it.

> >
> > > And I think we should do the freq-invariance thing for all the above categories
> > > nevertheless.
> > >
> >
> > I'm not sure what you mean by this. You mean we should also (try to) set
> > the frequency scale factor for drivers defining setpolicy() and target()?
> 
> No, we shouldn't.
> 
> The sched tick potentially does that already and nothing more needs to
> be done unless we know it for the fact that the scale factor is not
> set by the tick.
> 
> > > > If FIE support is desired in their current state, arch_set_freq_scale()
> > > > could still be called directly by the driver, while
> > > > CPUFREQ_CUSTOM_SET_FREQ_SCALE could be used to mark support for it.
> > > >
> > > > Conclusion
> > > > ==========
> > > >
> > > > Given that the significant majority of current FIE enabled drivers use
> > > > callbacks that lend themselves to triggering the setting of the FIE scale
> > > > factor in a generic way, move the invariance setter calls to cpufreq core,
> > > > while filtering drivers that flag custom support using
> > > > CPUFREQ_CUSTOM_SET_FREQ_SCALE.
> > > >
> > > > Signed-off-by: Valentin Schneider <[email-masked]>
> > > > Signed-off-by: Ionela Voinescu <[email-masked]>
> > > > Cc: Rafael J. Wysocki <[email-masked]>
> > > > Cc: Viresh Kumar <[email-masked]>
> > > > ---
> > > >  drivers/cpufreq/cpufreq.c | 20 +++++++++++++++++---
> > > >  1 file changed, 17 insertions(+), 3 deletions(-)
> > > >
> > > > diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> > > > index 0128de3603df..83b58483a39b 100644
> > > > --- a/drivers/cpufreq/cpufreq.c
> > > > +++ b/drivers/cpufreq/cpufreq.c
> > > > @@ -2046,9 +2046,16 @@ EXPORT_SYMBOL(cpufreq_unregister_notifier);
> > > >  unsigned int cpufreq_driver_fast_switch(struct cpufreq_policy *policy,
> > > >                                     unsigned int target_freq)
> > > >  {
> > > > +   unsigned int freq;
> > > > +
> > > >     target_freq = clamp_val(target_freq, policy->min, policy->max);
> > > > +   freq = cpufreq_driver->fast_switch(policy, target_freq);
> > > > +
> > >
> > > > +   if (freq && !(cpufreq_driver->flags & CPUFREQ_CUSTOM_SET_FREQ_SCALE))
> > > > +           arch_set_freq_scale(policy->related_cpus, freq,
> > > > +                               policy->cpuinfo.max_freq);
> 
> policy->cpuinfo.max_freq need not be the one to use in all cases when
> boost is supported.
> 
> policy->cpuinfo.max_freq may be the max boost freq and you may want to
> scale with respect to the max sustainable one anyway.
> 
> > > This needs to be a separate function.
> > >
> >
> > Yes, that would be nicer.
> >
> > > >
> > > > -   return cpufreq_driver->fast_switch(policy, target_freq);
> > > > +   return freq;
> > > >  }
> > > >  EXPORT_SYMBOL_GPL(cpufreq_driver_fast_switch);
> > > >
> > > > @@ -2140,7 +2147,7 @@ int __cpufreq_driver_target(struct cpufreq_policy *policy,
> > > >                         unsigned int relation)
> > > >  {
> > > >     unsigned int old_target_freq = target_freq;
> > > > -   int index;
> > > > +   int index, retval;
> > > >
> > > >     if (cpufreq_disabled())
> > > >             return -ENODEV;
> > > > @@ -2171,7 +2178,14 @@ int __cpufreq_driver_target(struct cpufreq_policy *policy,
> > > >
> > > >     index = cpufreq_frequency_table_target(policy, target_freq, relation);
> > > >
> > > > -   return __target_index(policy, index);
> > > > +   retval = __target_index(policy, index);
> > > > +
> > > > +   if (!retval && !(cpufreq_driver->flags & CPUFREQ_CUSTOM_SET_FREQ_SCALE))
> > > > +           arch_set_freq_scale(policy->related_cpus,
> > > > +                               policy->freq_table[index].frequency,
> > >
> > > policy->cur gets updated for both target and target_index type drivers. You can
> > > use that safely. It gets updated after the postchange notification.
> > >
> >
> > This would allow us to cover the drivers that define target() as well (not
> > only target_index() and fast_switch()). Looking over the code we only take
> > that path (calling cpufreq_freq_transition_end()), for
> > !CPUFREQ_ASYNC_NOTIFICATION. But again, that's only used for
> > powernow-k8 which is deprecated.
> >
> > I'll attempt a nice way to use this.
> 
> On arches like x86, policy->cur may not be the current frequency of
> the CPU, though.  On relatively recent systems it actually isn't that
> frequency most of the time.
> 

Yes, as discussed on the other patches my reasoning was that
arch_set_freq_scale() would filter less accurate information from
cpufreq and give priority to counter use.

But I understand your reasoning on this, and that both you and Viresh
would prefer a more strict 'opt in' policy for which drivers are
appropriate for use with frequency invariance.

So I'll make the suggested changes.

Kind regards,
Ionela.

> Thanks!

_______________________________________________
linux-arm-kernel mailing list
[email-masked]
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel