linux-kernel - Re: [PATCH v2 3/3] cpufreq: intel_pstate: Set asymmetric CPU capacity on hybrid systems

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Message-ID: <20240826230742.GA7773@ranerica-svr.sc.intel.com>
Date: Mon, 26 Aug 2024 16:07:42 -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 3/3] cpufreq: intel_pstate: Set asymmetric CPU
 capacity on hybrid systems

On Mon, Aug 12, 2024 at 02:44:30PM +0200, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
[...]
> @@ -3143,6 +3341,20 @@ static int intel_pstate_register_driver(
>  
>  	global.min_perf_pct = min_perf_pct_min();
>  
> +	/*
> +	 * On hybrid systems, use asym capacity instead of ITMT, but because
> +	 * the capacity of SMT threads is not deterministic even approximately,
> +	 * do not do that when SMT is in use.
> +	 */
> +	if (hwp_is_hybrid && !sched_smt_active() &&
> +	    arch_enable_hybrid_capacity_scale()) {
> +		sched_clear_itmt_support();
> +
> +		hybrid_init_cpu_scaling();
> +
> +		arch_rebuild_sched_domains();

sched_clear_itmt_support() also calls arch_rebuild_sched_domains(). The
latter is also called earlier via sched_set_itmt_support(), totaling 3
calls, two of which are wasted.

Perhaps at minimum hybrid_init_cpu_scaling() can be before
sched_clear_itmt_support(). The changes made by these two functions will
be picked up in a single call of arch_rebuild_sched_domains().

Moreover, ITMT can be not enabled at all if so we wish. By the time
intel_pstate_set_itmt_prio() is called, the value of hwp_is_hybrid is
already known.