Re: [PATCH v2 2/2] [RFC] CPUFreq: Add support for cpu-perf-dependencies

[Lukasz Luba]

On 10/12/20 11:59 AM, Ionela Voinescu wrote:
> On Monday 12 Oct 2020 at 11:22:57 (+0100), Lukasz Luba wrote:
> [..]
> > > > I thought about it and looked for other platforms' DT to see if can reuse
> > > > existing opp information. Unfortunately I don't think it is optimal. The reason
> > > > being that, because cpus have the same opp table it does not necessarily mean
> > > > that they share a clock wire. It just tells us that they have the same
> > > > capabilities (literally just tells us they have the same V/f op points).
> > > > Unless I am missing something?
> > > >
> > > > When comparing with ACPI/_PSD it becomes more intuitive that there is no
> > > > equivalent way to reveal "perf-dependencies" in DT.
> > >
> > > You should be able to by examining the clock tree. But perhaps SCMI
> > > abstracts all that and just presents virtual clocks without parent
> > > clocks available to determine what clocks are shared? Fix SCMI if that's
> > > the case.
> >
> > True, the SCMI clock does not support discovery of clock tree:
> > (from 4.6.1 Clock management protocol background)
> > 'The protocol does not cover discovery of the clock tree, which must be
> > described through firmware tables instead.' [1]
> >
> > In this situation, would it make sense, instead of this binding from
> > patch 1/2, create a binding for internal firmware/scmi node?
> >
> > Something like:
> >
> > firmware {
> > 	scmi {
> > 	...		
> > 		scmi-perf-dep {
> > 			compatible = "arm,scmi-perf-dependencies";
> > 			cpu-perf-dep0 {
> > 				cpu-perf-affinity = <&CPU0>, <&CPU1>;
> > 			};
> > 			cpu-perf-dep1 {
> > 				cpu-perf-affinity = <&CPU3>, <&CPU4>;
> > 			};
> > 			cpu-perf-dep2 {
> > 				cpu-perf-affinity = <&CPU7>;
> > 			};
> > 		};
> > 	};
> > };
> >
> > The code which is going to parse the binding would be inside the
> > scmi perf protocol code and used via API by scmi-cpufreq.c.
>
> While SCMI cpufreq would be able to benefit from the functionality that
> Nicola is trying to introduce, it's not the only driver, and more
> importantly, it's not *going* to be the only driver benefiting from
> this.
>
> Currently there is also qcom-cpufreq-hw.c and the future
> mediatek-cpufreq-hw.c that is currently under review [1]. They both do
> their frequency setting by interacting with HW/FW, and could either take
> or update their OPP tables from there. Therefore, if the platform would
> require it, they could also expose different controls for frequency
> setting and could benefit from additional information about clock
> domains (either through opp-shared or the new entries in Nicola's patch),
> without driver changes.
>
> Another point to be made is that I strongly believe this is going to be
> the norm in the future. Directly setting PLLs and regulator voltages
> has been proven unsafe and unsecure.
>
> Therefore, I see this as support for a generic cpufreq feature (a
> hardware coordination type), rather than support for a specific driver.
>
> [1] https://lkml.org/lkml/2020/9/10/11
>
> > Now regarding the 'dependent_cpus' mask.
> >
> > We could avoid adding a new field 'dependent_cpus' in policy
> > struct, but I am not sure of one bit - Frequency Invariant Engine,
> > (which is also not fixed by just adding a new cpumask).
>    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>    Let's take it step by step..
> > We have 3 subsystems to fix:
> > 1. EAS - EM has API function which takes custom cpumask, so no issue,
>             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> 	   keep in mind that EAS it's using the max aggregation method
> 	   that schedutil is using. So if we are to describe the
> 	   functionality correctly, it needs both a cpumask describing
> 	   the frequency domains and an aggregation method.

EAS does not use schedutil max agregation, it calculates max_util
internally.

The compute_energy() loops through the CPUs in the domain and
takes the utilization from them via schedutil_cpu_util(cpu_rq(cpu)).
It figures out max_util and then em_cpu_energy() maps it to next
frequency for the cluster. It just needs proper utilization from
CPUs, which is taken from run-queues, which is a sum of utilization
of tasks being there. This leads to problem how we account utilization
of a task. This is the place where the FIE is involved. EAS assumes the
utilization is calculated properly.

> >    fix would be to use it via the scmi-cpufreq.c
>
> > 2. IPA (for calculating the power of a cluster, not whole thermal needs
> >    this knowledge about 'dependent cpus') - this can be fixed internally
>
> > 3. Frequency Invariant Engine (FIE) - currently it relies on schedutil
> >    filtering and providing max freq of all cpus in the cluster into the
> >    FIE; this info is then populated to all 'related_cpus' which will
> >    have this freq (we know, because there is no other freq requests);
> >    Issues:
> > 3.1. Schedutil is not going to check all cpus in the cluster to take
> >    max freq, which is then passed into the cpufreq driver and FIE
> > 3.2. FIE would have to (or maybe we would drop it) have a logic similar
> >    to what schedutil does (max freq search and set, then filter next
> >    freq requests from other cpus in the next period e.g. 10ms)
> > 3.3. Schedutil is going to invoke freq change for each cpu independently
> >    and the current code just calls arch_set_freq_scale() - adding just
> >    'dependent_cpus' won't help
>
> I don't believe these are issues. As we need changes for EAS and IPA, we'd
> need changes for FIE. We don't need more than the cpumask that shows
> frequency domains as we already already have the aggregation method that
> schedutil uses to propagate the max frequency in a domain across CPUs.

Schedutil is going to work in !policy_is_shared() mode, which leads to
sugov_update_single() being the 'main' function. We won't have
schedutil goodness which is handling related_cpus use case.

Then in software FIE would you just change the call from:
	arch_set_freq_scale(policy->related_cpus,...)
to:
	arch_set_freq_scale(policy->dependent_cpus,...)
?

This code would be called from any CPU (without filtering) and it
would loop through cpumask updating freq_scale, which is wrong IMO.
You need some 'logic', which is not currently in there.

Leaving the 'related_cpus' would also be wrong (because real CPU
frequency is different, so we would account task utilization wrongly).

> This would be the default method if cycle counters are not present. It
> might not reflect the frequency the cores actually get from HW, but for
> that cycle counters should be used.

IMHO the configurations with per-cpu freq requests while there are CPUs
'dependent' and there are no HW counters to use for tasks
utilization accounting - should be blocked. Then we don't need
'dependent_cpus' in software FIE. Then one less from your requirements
list for new cpumask.

> > 3.4 What would be the real frequency of these cpus and what would be
> >    set to FIE
> > 3.5 FIE is going to filter to soon requests from other dependent cpus?
> >
> > IMHO the FIE needs more bits than just a new cpumask.
> > Maybe we should consider to move FIE arch_set_freq_scale() call into the
> > cpufreq driver, which will know better how to aggregate/filter requests
> > and then call FIE update?
>
> I'm quite strongly against this :). As described before, this is not a
> feature that a single driver needs, and even if it was, the aggregation
> method for FIE is not a driver policy.

Software version of FIE has issues in this case, schedutil or EAS won't
help (different code path).

Regards,
Lukasz