linux-kernel - Re: [PATCH v1] cpufreq: intel_pstate: Use HWP to initialize ITMT if CPPC is missing

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Date: Thu, 20 Jun 2024 12:57:18 -0500
From: Mario Limonciello <mario.limonciello@....com>
To: "Rafael J. Wysocki" <rjw@...ysocki.net>,
 Linux PM <linux-pm@...r.kernel.org>
Cc: Aaron Rainbolt <arainbolt@...cus.org>, LKML
 <linux-kernel@...r.kernel.org>, Linux ACPI <linux-acpi@...r.kernel.org>,
 Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>
Subject: Re: [PATCH v1] cpufreq: intel_pstate: Use HWP to initialize ITMT if
 CPPC is missing

On 6/20/2024 11:14, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> 
> It is reported that single-thread performance on some hybrid systems
> dropped significantly after commit 7feec7430edd ("ACPI: CPPC: Only probe
> for _CPC if CPPC v2 is acked") which prevented _CPC from being used if
> the support for it had not been confirmed by the platform firmware.
> 
> The problem is that if the platform firmware does not confirm CPPC v2
> support, cppc_get_perf_caps() returns an error which prevents the
> intel_pstate driver from enabling ITMT.  Consequently, the scheduler
> does not get any hints on CPU performance differences, so in a hybrid
> system some tasks may run on CPUs with lower capacity even though they
> should be running on high-capacity CPUs.
> 
> To address this, modify intel_pstate to use the information from
> MSR_HWP_CAPABILITIES to enable ITMT if CPPC is not available (which is
> done already if the highest performance number coming from CPPC is not
> realistic).
> 
> Fixes: 7feec7430edd ("ACPI: CPPC: Only probe for _CPC if CPPC v2 is acked")
> Closes: https://lore.kernel.org/linux-acpi/d01b0a1f-bd33-47fe-ab41-43843d8a374f@kfocus.org
> Link: https://lore.kernel.org/linux-acpi/ZnD22b3Br1ng7alf@kf-XE
> Reported-by: Aaron Rainbolt <arainbolt@...cus.org>
> Tested-by: Aaron Rainbolt <arainbolt@...cus.org>
> Cc: 5.19+ <stable@...r.kernel.org> # 5.19+ > Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com

Reviewed-by: Mario Limonciello <mario.limonciello@....com>

> ---
>   drivers/cpufreq/intel_pstate.c |   13 ++++++-------
>   1 file changed, 6 insertions(+), 7 deletions(-)
> 
> Index: linux-pm/drivers/cpufreq/intel_pstate.c
> ===================================================================
> --- linux-pm.orig/drivers/cpufreq/intel_pstate.c
> +++ linux-pm/drivers/cpufreq/intel_pstate.c
> @@ -355,15 +355,14 @@ static void intel_pstate_set_itmt_prio(i
>   	int ret;
>   
>   	ret = cppc_get_perf_caps(cpu, &cppc_perf);
> -	if (ret)
> -		return;
> -
>   	/*
> -	 * On some systems with overclocking enabled, CPPC.highest_perf is hardcoded to 0xff.
> -	 * In this case we can't use CPPC.highest_perf to enable ITMT.
> -	 * In this case we can look at MSR_HWP_CAPABILITIES bits [8:0] to decide.
> +	 * If CPPC is not available, fall back to MSR_HWP_CAPABILITIES bits [8:0].
> +	 *
> +	 * Also, on some systems with overclocking enabled, CPPC.highest_perf is
> +	 * hardcoded to 0xff, so CPPC.highest_perf cannot be used to enable ITMT.
> +	 * Fall back to MSR_HWP_CAPABILITIES then too.
>   	 */
> -	if (cppc_perf.highest_perf == CPPC_MAX_PERF)
> +	if (ret || cppc_perf.highest_perf == CPPC_MAX_PERF)
>   		cppc_perf.highest_perf = HWP_HIGHEST_PERF(READ_ONCE(all_cpu_data[cpu]->hwp_cap_cached));
>   
>   	/*
> 
> 
>