linux-kernel - Re: [PATCH v2 2/3] x86/sched: Add basic support for CPU capacity scaling

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Message-ID: <20240826220849.GA7696@ranerica-svr.sc.intel.com>
Date: Mon, 26 Aug 2024 15:08:49 -0700
From: Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
To: "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc: x86 Maintainers <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
	Linux PM <linux-pm@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
	"Rafael J. Wysocki" <rafael@...nel.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Ricardo Neri <ricardo.neri@...el.com>,
	Tim Chen <tim.c.chen@...el.com>
Subject: Re: [PATCH v2 2/3] x86/sched: Add basic support for CPU capacity
 scaling

On Mon, Aug 12, 2024 at 02:42:26PM +0200, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>

[...]

> +bool arch_enable_hybrid_capacity_scale(void)
> +{
> +	int cpu;
> +
> +	if (static_branch_unlikely(&arch_hybrid_cap_scale_key)) {
> +		WARN_ONCE(1, "Hybrid CPU capacity scaling already enabled");
> +		return true;
> +	}

Maybe an empty line here for readability?

> +	arch_cpu_scale = alloc_percpu(struct arch_hybrid_cpu_scale);
> +	if (!arch_cpu_scale)
> +		return false;
> +
> +	for_each_possible_cpu(cpu) {
> +		per_cpu_ptr(arch_cpu_scale, cpu)->capacity = SCHED_CAPACITY_SCALE;
> +		per_cpu_ptr(arch_cpu_scale, cpu)->freq_ratio = arch_max_freq_ratio;
> +	}
> +
> +	static_branch_enable(&arch_hybrid_cap_scale_key);
> +
> +	pr_info("Hybrid CPU capacity scaling enabled\n");
> +
> +	return true;
> +}
> +
> +/**
> + * arch_set_cpu_capacity - Set scale-invariance parameters for a CPU
> + * @cpu: Target CPU.
> + * @cap: Capacity of @cpu, relative to @base_cap, at its maximum frequency.
> + * @base_cap: System-wide maximum CPU capacity.

It is confusing to e that @base_cap is the maximum capacity of the system.
Maybe @max_cap?

> + * @max_freq: Frequency of @cpu corresponding to @cap.
> + * @base_freq: Frequency of @cpu at which MPERF counts.
> + *
> + * The units in which @cap and @base_cap are expressed do not matter, so long
> + * as they are consistent, because the former is effectively divided by the
> + * latter.  Analogously for @max_freq and @base_freq.
> + *
> + * After calling this function for all CPUs, call arch_rebuild_sched_domains()
> + * to let the scheduler know that capacity-aware scheduling can be used going
> + * forward.
> + */
> +void arch_set_cpu_capacity(int cpu, unsigned long cap, unsigned long base_cap,
> +			   unsigned long max_freq, unsigned long base_freq)
> +{
> +	if (static_branch_likely(&arch_hybrid_cap_scale_key)) {
> +		WRITE_ONCE(per_cpu_ptr(arch_cpu_scale, cpu)->capacity,
> +			   div_u64(cap << SCHED_CAPACITY_SHIFT, base_cap));
> +		WRITE_ONCE(per_cpu_ptr(arch_cpu_scale, cpu)->freq_ratio,
> +			   div_u64(max_freq << SCHED_CAPACITY_SHIFT, base_freq));
> +	} else {
> +		WARN_ONCE(1, "Hybrid CPU capacity scaling not enabled");
> +	}
> +}
> +
> +unsigned long arch_scale_cpu_capacity(int cpu)
> +{
> +	if (static_branch_unlikely(&arch_hybrid_cap_scale_key))
> +		return READ_ONCE(per_cpu_ptr(arch_cpu_scale, cpu)->capacity);
> +
> +	return SCHED_CAPACITY_SCALE;
> +}
> +EXPORT_SYMBOL_GPL(arch_scale_cpu_capacity);
> +
>  static void scale_freq_tick(u64 acnt, u64 mcnt)
>  {
>  	u64 freq_scale;
> +	u64 freq_ratio;

Why can't freq_ratio be declared on the same line as freq_scale?

>  
>  	if (!arch_scale_freq_invariant())
>  		return;
> @@ -359,7 +439,12 @@ static void scale_freq_tick(u64 acnt, u6
>  	if (check_shl_overflow(acnt, 2*SCHED_CAPACITY_SHIFT, &acnt))
>  		goto error;
>  
> -	if (check_mul_overflow(mcnt, arch_max_freq_ratio, &mcnt) || !mcnt)
> +	if (static_branch_unlikely(&arch_hybrid_cap_scale_key))
> +		freq_ratio = READ_ONCE(this_cpu_ptr(arch_cpu_scale)->freq_ratio);
> +	else
> +		freq_ratio = arch_max_freq_ratio;

It seems that arch_max_freq_ratio will never be used on hybrid processors
and computing arch_turbo_freq_ratio will be a waste of cycles.

Unfortunately, intel_set_max_freq_ratio() is called before the
arch_hybrid_cap_scale_key static key is set.

Maybe some rework is in order?

Thanks and BR,
Ricardo