Subject: Re: [RFC V2 1/2] topology: Allow multiple entities to provide sched_freq_tick() callback

From: Ionela Voinescu <[email-masked]>
Date: Tue, 19 Jan 2021 19:05:27 +0000

Hi,

On Friday 15 Jan 2021 at 13:18:47 (+0530), Viresh Kumar wrote:
> On 13-01-21, 16:18, Ionela Voinescu wrote:
> > On Tuesday 15 Dec 2020 at 16:46:35 (+0530), Viresh Kumar wrote:
> > > +void topology_scale_freq_tick(void)
> > > +{
> > > +	struct scale_freq_tick_data *sftd = *this_cpu_ptr(&sft_data);
> > > +
> > > +	if (sftd)
> > > +		sftd->scale_freq();
> > > +}
> > 
> > What do you think about having a single topology function that handles
> > all sources of invariance (cpufreq, arch counters, platform counters)?
> 
> I think keeping them separate is better, both of these are called from
> scheduler's context (hotpath) and adding any more unnecessary
> conditionals there should be rather avoided. We could have given a

It would be a single added condition on the hotpath for !cpus and I
think CPUs nowadays do a very good job optimising those.
Also, you lose a branch and a cpumask operation on the
arch_set_freq_scale path.

> though to merging them if they were going to share some code, but that
> is not the case here clearly. They are quite different.
> 

I think it would improve clarity a lot by having a single topology
function, with clear comments on which path is taken when.

In regards to them sharing code, there's not much code they could be
sharing, as one just does some simple math on provided values.
I attempted to have a single function for frequency invariance at some
point [1], as the logic for the computation is the same, but I could
never convince myself the clarity gained was worth the changes. But
I really like this version of your code for this purpose as well.

But I'll leave that to your judgement.

[1] https://gitlab.arm.com/linux-arm/linux-power/-/commit/b9277295475307051c34ca31aef28eac50115e1a

> This is how it looks now:
> 
> diff --git a/arch/arm64/include/asm/topology.h b/arch/arm64/include/asm/topology.h
> index 3b8dca4eb08d..be6a53ba3e2d 100644
> --- a/arch/arm64/include/asm/topology.h
> +++ b/arch/arm64/include/asm/topology.h
> @@ -17,15 +17,9 @@ int pcibus_to_node(struct pci_bus *bus);
>  #include <linux/arch_topology.h>
>  
>  void update_freq_counters_refs(void);
> -void topology_scale_freq_tick(void);
>  
> -#ifdef CONFIG_ARM64_AMU_EXTN
> -/*
> - * Replace task scheduler's default counter-based
> - * frequency-invariance scale factor setting.
> - */
> +/* Replace task scheduler's default frequency-invariance scale factor setting */
>  #define arch_scale_freq_tick topology_scale_freq_tick

You could probably move this define under the comment below, as both
arch functions do frequency-invariance accounting, but from different
sources.

> -#endif /* CONFIG_ARM64_AMU_EXTN */
>  
>  /* Replace task scheduler's default frequency-invariant accounting */
>  #define arch_set_freq_scale topology_set_freq_scale
> diff --git a/arch/arm64/kernel/topology.c b/arch/arm64/kernel/topology.c
> index e08a4126453a..1e47dfd465f8 100644
> --- a/arch/arm64/kernel/topology.c
> +++ b/arch/arm64/kernel/topology.c
> @@ -199,8 +199,44 @@ static int freq_inv_set_max_ratio(int cpu, u64 max_rate, u64 ref_rate)
>  	return 0;
>  }
>  
> -static DEFINE_STATIC_KEY_FALSE(amu_fie_key);
> -#define amu_freq_invariant() static_branch_unlikely(&amu_fie_key)
> +static void amu_scale_freq_tick(void)
> +{
> +	u64 prev_core_cnt, prev_const_cnt;
> +	u64 core_cnt, const_cnt, scale;
> +
> +	prev_const_cnt = this_cpu_read(arch_const_cycles_prev);
> +	prev_core_cnt = this_cpu_read(arch_core_cycles_prev);
> +
> +	update_freq_counters_refs();
> +
> +	const_cnt = this_cpu_read(arch_const_cycles_prev);
> +	core_cnt = this_cpu_read(arch_core_cycles_prev);
> +
> +	if (unlikely(core_cnt <= prev_core_cnt ||
> +		     const_cnt <= prev_const_cnt))
> +		return;
> +
> +	/*
> +	 *	    /\core    arch_max_freq_scale
> +	 * scale =  ------- * --------------------
> +	 *	    /\const   SCHED_CAPACITY_SCALE
> +	 *
> +	 * See validate_cpu_freq_invariance_counters() for details on
> +	 * arch_max_freq_scale and the use of SCHED_CAPACITY_SHIFT.
> +	 */
> +	scale = core_cnt - prev_core_cnt;
> +	scale *= this_cpu_read(arch_max_freq_scale);
> +	scale = div64_u64(scale >> SCHED_CAPACITY_SHIFT,
> +			  const_cnt - prev_const_cnt);
> +
> +	scale = min_t(unsigned long, scale, SCHED_CAPACITY_SCALE);
> +	this_cpu_write(freq_scale, (unsigned long)scale);
> +}
> +
> +static struct scale_freq_data amu_sfd = {
> +	.source = SCALE_FREQ_SOURCE_ARCH,
> +	.set_freq_scale = amu_scale_freq_tick,
> +};
>  
>  static void amu_fie_setup(const struct cpumask *cpus)
>  {
> @@ -227,7 +263,7 @@ static void amu_fie_setup(const struct cpumask *cpus)
>  	if (!invariant && !cpumask_equal(amu_fie_cpus, cpu_present_mask))
>  		return;
>  
> -	static_branch_enable(&amu_fie_key);
> +	topology_set_scale_freq_source(&amu_sfd, amu_fie_cpus);
>  
>  	pr_debug("CPUs[%*pbl]: counters will be used for FIE.",
>  		 cpumask_pr_args(cpus));
> @@ -283,53 +319,6 @@ static int __init init_amu_fie(void)
>  }
>  core_initcall(init_amu_fie);
>  
> -bool arch_freq_counters_available(const struct cpumask *cpus)
> -{
> -	return amu_freq_invariant() &&
> -	       cpumask_subset(cpus, amu_fie_cpus);
> -}
> -
> -void topology_scale_freq_tick(void)
> -{
> -	u64 prev_core_cnt, prev_const_cnt;
> -	u64 core_cnt, const_cnt, scale;
> -	int cpu = smp_processor_id();
> -
> -	if (!amu_freq_invariant())
> -		return;
> -
> -	if (!cpumask_test_cpu(cpu, amu_fie_cpus))
> -		return;
> -
> -	prev_const_cnt = this_cpu_read(arch_const_cycles_prev);
> -	prev_core_cnt = this_cpu_read(arch_core_cycles_prev);
> -
> -	update_freq_counters_refs();
> -
> -	const_cnt = this_cpu_read(arch_const_cycles_prev);
> -	core_cnt = this_cpu_read(arch_core_cycles_prev);
> -
> -	if (unlikely(core_cnt <= prev_core_cnt ||
> -		     const_cnt <= prev_const_cnt))
> -		return;
> -
> -	/*
> -	 *	    /\core    arch_max_freq_scale
> -	 * scale =  ------- * --------------------
> -	 *	    /\const   SCHED_CAPACITY_SCALE
> -	 *
> -	 * See validate_cpu_freq_invariance_counters() for details on
> -	 * arch_max_freq_scale and the use of SCHED_CAPACITY_SHIFT.
> -	 */
> -	scale = core_cnt - prev_core_cnt;
> -	scale *= this_cpu_read(arch_max_freq_scale);
> -	scale = div64_u64(scale >> SCHED_CAPACITY_SHIFT,
> -			  const_cnt - prev_const_cnt);
> -
> -	scale = min_t(unsigned long, scale, SCHED_CAPACITY_SCALE);
> -	this_cpu_write(freq_scale, (unsigned long)scale);
> -}
> -
>  #ifdef CONFIG_ACPI_CPPC_LIB
>  #include <acpi/cppc_acpi.h>
>  
> diff --git a/drivers/base/arch_topology.c b/drivers/base/arch_topology.c
> index de8587cc119e..e2115ea348dc 100644
> --- a/drivers/base/arch_topology.c
> +++ b/drivers/base/arch_topology.c
> @@ -21,17 +21,65 @@
>  #include <linux/sched.h>
>  #include <linux/smp.h>
>  
> +static DEFINE_PER_CPU(struct scale_freq_data *, sft_data);
> +static struct cpumask scale_freq_counters_mask;
> +
> +static bool supports_scale_freq_counters(const struct cpumask *cpus)
> +{
> +	return cpumask_subset(cpus, &scale_freq_counters_mask);
> +}
> +
>  bool topology_scale_freq_invariant(void)
>  {
>  	return cpufreq_supports_freq_invariance() ||
> -	       arch_freq_counters_available(cpu_online_mask);
> +	       supports_scale_freq_counters(cpu_online_mask);
> +}
> +
> +void topology_set_scale_freq_source(struct scale_freq_data *data,
> +				  const struct cpumask *cpus)
> +{
> +	struct scale_freq_data *sfd;
> +	int cpu;
> +
> +	for_each_cpu(cpu, cpus) {
> +		sfd = per_cpu(sft_data, cpu);
> +
> +		/* Use ARCH provided counters whenever possible */
> +		if (!sfd || sfd->source != SCALE_FREQ_SOURCE_ARCH) {
> +			per_cpu(sft_data, cpu) = data;
> +			cpumask_set_cpu(cpu, &scale_freq_counters_mask);
> +		}
> +	}
>  }
> +EXPORT_SYMBOL_GPL(topology_set_scale_freq_source);
>  
> -__weak bool arch_freq_counters_available(const struct cpumask *cpus)
> +void topology_clear_scale_freq_source(enum scale_freq_source source,
> +				    const struct cpumask *cpus)
>  {
> -	return false;
> +	struct scale_freq_data *sfd;
> +	int cpu;
> +
> +	for_each_cpu(cpu, cpus) {
> +		sfd = per_cpu(sft_data, cpu);
> +
> +		if (sfd && sfd->source == source) {
> +			per_cpu(sft_data, cpu) = NULL;
> +			cpumask_clear_cpu(cpu, &scale_freq_counters_mask);
> +		}
> +	}
>  }
> +EXPORT_SYMBOL_GPL(topology_clear_scale_freq_source);
> +
> +void topology_scale_freq_tick(void)
> +{
> +	struct scale_freq_data *sfd = *this_cpu_ptr(&sft_data);
> +
> +	if (sfd)
> +		sfd->set_freq_scale();
> +}
> +
>  DEFINE_PER_CPU(unsigned long, freq_scale) = SCHED_CAPACITY_SCALE;
> +EXPORT_SYMBOL_GPL(freq_scale);
>  
>  void topology_set_freq_scale(const struct cpumask *cpus, unsigned long cur_freq,
>  			     unsigned long max_freq)
> @@ -47,7 +95,7 @@ void topology_set_freq_scale(const struct cpumask *cpus, unsigned long cur_freq,
>  	 * want to update the scale factor with information from CPUFREQ.
>  	 * Instead the scale factor will be updated from arch_scale_freq_tick.
>  	 */
> -	if (arch_freq_counters_available(cpus))
> +	if (supports_scale_freq_counters(cpus))
>  		return;
>  
>  	scale = (cur_freq << SCHED_CAPACITY_SHIFT) / max_freq;
> diff --git a/include/linux/arch_topology.h b/include/linux/arch_topology.h
> index 0f6cd6b73a61..1bcbf8eff991 100644
> --- a/include/linux/arch_topology.h
> +++ b/include/linux/arch_topology.h
> @@ -34,7 +34,18 @@ void topology_set_freq_scale(const struct cpumask *cpus, unsigned long cur_freq,
>  			     unsigned long max_freq);
>  bool topology_scale_freq_invariant(void);
>  
> -bool arch_freq_counters_available(const struct cpumask *cpus);
> +enum scale_freq_source {

Maybe you can add SCALE_FREQ_SOURCE_CPUFREQ as the first value (=0).
That will give a hint of CPUFREQ being a source of information for FIE
and the default when counters are not supported, even if nothing gets
done with that value (for now at least).

> +	SCALE_FREQ_SOURCE_ARCH,
> +};
> +
> +struct scale_freq_data {
> +	enum scale_freq_source source;
> +	void (*set_freq_scale)(void);
> +};
> +
> +void topology_scale_freq_tick(void);
> +void topology_set_scale_freq_source(struct scale_freq_data *data, const struct cpumask *cpus);
> +void topology_clear_scale_freq_source(enum scale_freq_source source, const struct cpumask *cpus);
>  
>  DECLARE_PER_CPU(unsigned long, thermal_pressure);
>  

+1 on the rewrite!

Many thanks,
Ionela.

> -- 
> viresh

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