Re: [PATCH v2 1/2] arm64: Provide an AMU-based version of arch_freq_get_on_cpu

[Beata Michalska <beata.michalska@arm.com>]

On Tue, Nov 28, 2023 at 03:13:51PM +0000, Ionela Voinescu wrote:
> Hi Beata,
> 
> On Monday 27 Nov 2023 at 16:08:37 (+0000), Beata Michalska wrote:
> > With the Frequency Invariance Engine (FIE) being already wired up with
> > sched tick and making use of relevant (core counter and constant
> > counter) AMU counters, getting the current frequency for a given CPU
> > on supported platforms, can be achieved by utilizing the frequency scale
> > factor which reflects an average CPU frequency for the last tick period
> > length.
> > 
> > Suggested-by: Ionela Voinescu <ionela.voinescu@arm.com>
> > Signed-off-by: Beata Michalska <beata.michalska@arm.com>
> > Reviewed-by: Sudeep Holla <sudeep.holla@arm.com>
> > ---
> > 
> > Notes:
> >     Due to [1], if merged, there might be a need to modify the patch to
> >     accommodate changes [1] introduces:
> >     
> >     	freq = cpufreq_get_hw_max_freq(cpu) >> SCHED_CAPACITY_SHIFT
> >     	to
> >     	freq = per_cpu(capacity_freq_ref, cpu); >> SCHED_CAPACITY_SHIFT
> >     [1]
> >     https://lore.kernel.org/linux-arm-kernel/20231121154349.GA1938@willie-the-truck/T/#mcb018d076dbce6f60ed2779634a9b6ffe622641e
> > 
> >  arch/arm64/kernel/topology.c | 39 ++++++++++++++++++++++++++++++++++++
> >  1 file changed, 39 insertions(+)
> > 
> > diff --git a/arch/arm64/kernel/topology.c b/arch/arm64/kernel/topology.c
> > index 615c1a20129f..ae2445f6e7da 100644
> > --- a/arch/arm64/kernel/topology.c
> > +++ b/arch/arm64/kernel/topology.c
> > @@ -17,6 +17,7 @@
> >  #include <linux/cpufreq.h>
> >  #include <linux/init.h>
> >  #include <linux/percpu.h>
> > +#include <linux/sched/isolation.h>
> >  
> >  #include <asm/cpu.h>
> >  #include <asm/cputype.h>
> > @@ -186,6 +187,44 @@ static void amu_scale_freq_tick(void)
> >  	this_cpu_write(arch_freq_scale, (unsigned long)scale);
> >  }
> >  
> > +unsigned int arch_freq_get_on_cpu(int cpu)
> > +{
> > +	unsigned int freq;
> > +	u64 scale;
> > +
> > +	if (!cpumask_test_cpu(cpu, amu_fie_cpus))
> > +		return 0;
> > +
> > +	/*
> > +	 * For those CPUs that are in full dynticks mode, try an alternative
> > +	 * source for the counters (and thus freq scale),
> > +	 * if available for given policy
> > +	 */
> > +	if (!housekeeping_cpu(cpu, HK_TYPE_TICK)) {
> > +		struct cpufreq_policy *policy = cpufreq_cpu_get(cpu);
> > +		int ref_cpu = nr_cpu_ids;
> > +
> > +		if (cpumask_intersects(housekeeping_cpumask(HK_TYPE_TICK),
> > +				       policy->cpus))
> > +			ref_cpu = cpumask_nth_and(cpu, policy->cpus,
> > +						  housekeeping_cpumask(HK_TYPE_TICK));
> > +		cpufreq_cpu_put(policy);
> > +		if (ref_cpu >= nr_cpu_ids)
> > +			return 0;
> > +		cpu = ref_cpu;
> > +	}
> > +
> > +	/*
> > +	 * Reversed computation to the one used to determine
> > +	 * the arch_freq_scale value
> > +	 * (see amu_scale_freq_tick for details)
> > +	 */
> > +	scale = per_cpu(arch_freq_scale, cpu);
> 
> Any reason for not using arch_scale_freq_capacity() here?
> 
> To me it seems a bit nicer to use the "official" function to return the
> frequency scale factor.
> 
Noted.
> > +	freq = cpufreq_get_hw_max_freq(cpu) >> SCHED_CAPACITY_SHIFT;
> 
> Given Vincent's patch at [1] I think here might be best to call
> arch_scale_freq_ref() instead. That's because the frequency scale factor
> will use that frequency as the maximum frequency in its calculations and
> we'd not want to use a different one here.
> 
OK.
> The annoyance is coping with capacity_freq_ref not having been set
> yet, and that would be easy if capacity_freq_ref was initialized to 0.
> Luckily with Vincent's changes it can now be 0. I'll comments on his
> patches and ask him to make this change.
> 
> So I think you can safely use arch_scale_freq_ref() here. If
> arch_scale_freq_ref() returns 0, arch_freq_get_on_cpu() will just return
> 0 as well.
> 
Will do.
> [1] https://lore.kernel.org/lkml/20231109101438.1139696-8-vincent.guittot@linaro.org/
> 
> > +	freq *= scale;
> 
> In some scenarios the frequencies visible to cpufreq might not look like
> actual frequencies, but some scaled abstract performance values. One
> example is cppc_cpufreq when one does not provide the optional frequency
> information in the CPC objects but just the performance information.
> 
> Therefore the maximum frequency seen here can be quite a small value, so
> it might be best to do the multiplication first and the shift after that.
> 
Right, that was in v1! Must have mixed up things ending with stale data.
Will address that in the next version - if one is out.

---
BR
Beata
> Thanks,
> Ionela.
> 
> > +	return freq;
> > +}
> > +
> >  static struct scale_freq_data amu_sfd = {
> >  	.source = SCALE_FREQ_SOURCE_ARCH,
> >  	.set_freq_scale = amu_scale_freq_tick,
> > -- 
> > 2.25.1
> >