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

[Ionela Voinescu <ionela.voinescu@arm.com>]

Hi Lukasz,

On Monday 12 Oct 2020 at 14:48:20 (+0100), Lukasz Luba wrote:
> 
> 
> 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.
> 

But isn't it the same logic mechanism that schedutil uses?

> 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
  ^^^^^^^^^^^^^^^^^^^^^^^

Same for schedutil: sugov_next_freq_shared() calls sugov_get_util()
which then calls schedutil_cpu_util().

If your point is that one is applying the max function in compute_energy()
while the other is doing it in sugov_next_freq_shared(), I'll re-enforce
my argument that they are logically doing the same *type* of
aggregation. EAS is relying on it and schedutil was purposely modified
for this purpose:

938e5e4b0d15  sched/cpufreq: Prepare schedutil for Energy Aware
Scheduling

> 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.

This is separate. Above we were discussing the aggregation method and
what CPUs this is applied on. I'll continue on FIE below.

> > 
> > >    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.
> 

Agreed! I did not mean that I'd rely on schedutil to do the aggregation
and hand me the answer. But my suggestion is to use the same logical
method - maximum, for cases where counters are not present.

> 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.
> 

Definitely! But that's because the FIE changes above are incomplete.
That's why whomever does these changes should go beyond:
s/related_cpus/dependent_cpus.

We don't need more information from DT additional to this dependent_cpus
maks, but it does not mean the end solution for making use of it will be
a simple "s/related_cpus/dependent_cpus".

> 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.
> 

I'd go for a default.. better have something than removing it
altogether, but we'll see.

I'll stop this here as I think we're distracting a bit from the main
purpose of this RFC. I don't believe FIE brings an additional
requirement. "Software" FIE will need fixing/optimizing/bypassing
(we'll agree later on the implementation) but it does not need anything
else from DT/ACPI.

Thank you,
Ionela.

> > 
> > > 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