linux-kernel - Re: [PATCH] cpufreq: intel_pstate: Fix EPP setting via sysfs in active mode

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Date:   Tue, 28 Jul 2020 22:47:52 -0700
From:   Francisco Jerez <currojerez@...eup.net>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Linux PM <linux-pm@...r.kernel.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>
Cc:     LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] cpufreq: intel_pstate: Fix EPP setting via sysfs in active mode

"Rafael J. Wysocki" <rjw@...ysocki.net> writes:

> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
>
> Because intel_pstate_set_energy_pref_index() reads and writes the
> MSR_HWP_REQUEST register without using the cached value of it used by
> intel_pstate_hwp_boost_up() and intel_pstate_hwp_boost_down(), those
> functions may overwrite the value written by it and so the EPP value
> set via sysfs may be lost.
>
> To avoid that, make intel_pstate_set_energy_pref_index() take the
> cached value of MSR_HWP_REQUEST just like the other two routines
> mentioned above and update it with the new EPP value coming from
> user space in addition to updating the MSR.
>
> Note that the MSR itself still needs to be updated too in case
> hwp_boost is unset or the boosting mechanism is not active at the
> EPP change time.
>
> Fixes: e0efd5be63e8 ("cpufreq: intel_pstate: Add HWP boost utility and sched util hooks")
> Reported-by: Francisco Jerez <currojerez@...eup.net>
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>

Reviewed-by: Francisco Jerez <currojerez@...eup.net>

> ---
>
> This patch is on top of https://patchwork.kernel.org/patch/11689347/
>
> ---
>  drivers/cpufreq/intel_pstate.c |   17 ++++++++++++-----
>  1 file changed, 12 insertions(+), 5 deletions(-)
>
> Index: linux-pm/drivers/cpufreq/intel_pstate.c
> ===================================================================
> --- linux-pm.orig/drivers/cpufreq/intel_pstate.c
> +++ linux-pm/drivers/cpufreq/intel_pstate.c
> @@ -653,11 +653,12 @@ static int intel_pstate_set_energy_pref_
>  		epp = cpu_data->epp_default;
>  
>  	if (boot_cpu_has(X86_FEATURE_HWP_EPP)) {
> -		u64 value;
> -
> -		ret = rdmsrl_on_cpu(cpu_data->cpu, MSR_HWP_REQUEST, &value);
> -		if (ret)
> -			return ret;
> +		/*
> +		 * Use the cached HWP Request MSR value, because the register
> +		 * itself may be updated by intel_pstate_hwp_boost_up() or
> +		 * intel_pstate_hwp_boost_down() at any time.
> +		 */
> +		u64 value = READ_ONCE(cpu_data->hwp_req_cached);
>  
>  		value &= ~GENMASK_ULL(31, 24);
>  
> @@ -667,6 +668,12 @@ static int intel_pstate_set_energy_pref_
>  			epp = epp_values[pref_index - 1];
>  
>  		value |= (u64)epp << 24;
> +		/*
> +		 * The only other updater of hwp_req_cached in the active mode,
> +		 * intel_pstate_hwp_set(), is called under the same lock as this
> +		 * function, so it cannot run in parallel with the update below.
> +		 */
> +		WRITE_ONCE(cpu_data->hwp_req_cached, value);
>  		ret = wrmsrl_on_cpu(cpu_data->cpu, MSR_HWP_REQUEST, value);
>  	} else {
>  		if (epp == -EINVAL)


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