linux-kernel - Re: [PATCH v2.2 4/4] cpufreq: schedutil: Always call driver if CPUFREQ_NEED_UPDATE

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Date:   Thu, 29 Oct 2020 16:53:47 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:     Linux PM <linux-pm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Zhang Rui <rui.zhang@...el.com>
Subject: Re: [PATCH v2.2 4/4] cpufreq: schedutil: Always call driver if
 CPUFREQ_NEED_UPDATE_LIMITS is set

On 29-10-20, 12:12, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> 
> Because sugov_update_next_freq() may skip a frequency update even if
> the need_freq_update flag has been set for the policy at hand, policy
> limits updates may not take effect as expected.
> 
> For example, if the intel_pstate driver operates in the passive mode
> with HWP enabled, it needs to update the HWP min and max limits when
> the policy min and max limits change, respectively, but that may not
> happen if the target frequency does not change along with the limit
> at hand.  In particular, if the policy min is changed first, causing
> the target frequency to be adjusted to it, and the policy max limit
> is changed later to the same value, the HWP max limit will not be
> updated to follow it as expected, because the target frequency is
> still equal to the policy min limit and it will not change until
> that limit is updated.
> 
> To address this issue, modify get_next_freq() to let the driver
> callback run if the CPUFREQ_NEED_UPDATE_LIMITS cpufreq driver flag
> is set regardless of whether or not the new frequency to set is
> equal to the previous one.
> 
> Fixes: f6ebbcf08f37 ("cpufreq: intel_pstate: Implement passive mode with HWP enabled")
> Reported-by: Zhang Rui <rui.zhang@...el.com>
> Tested-by: Zhang Rui <rui.zhang@...el.com>
> Cc: 5.9+ <stable@...r.kernel.org> # 5.9+
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> ---
> 
> v2.1 -> v2.2:
>    * Instead of updating need_freq_update if CPUFREQ_NEED_UPDATE_LIMITS is set
>      in get_next_freq() and checking it again in sugov_update_next_freq(),
>      check CPUFREQ_NEED_UPDATE_LIMITS directly in sugov_update_next_freq().
>    * Update the subject.
> 
> v2 -> v2.1:
>    * Fix typo in the subject.
>    * Make get_next_freq() and sugov_update_next_freq() ignore the
>      sg_policy->next_freq == next_freq case when CPUFREQ_NEED_UPDATE_LIMITS
>      is set for the driver.
>    * Add Tested-by from Rui (this version lets the driver callback run more
>      often than the v2, so the behavior in the Rui's case doesn't change).
> 
> ---
>  kernel/sched/cpufreq_schedutil.c |    6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> Index: linux-pm/kernel/sched/cpufreq_schedutil.c
> ===================================================================
> --- linux-pm.orig/kernel/sched/cpufreq_schedutil.c
> +++ linux-pm/kernel/sched/cpufreq_schedutil.c
> @@ -102,7 +102,8 @@ static bool sugov_should_update_freq(str
>  static bool sugov_update_next_freq(struct sugov_policy *sg_policy, u64 time,
>  				   unsigned int next_freq)
>  {
> -	if (sg_policy->next_freq == next_freq)
> +	if (sg_policy->next_freq == next_freq &&
> +	    !cpufreq_driver_test_flags(CPUFREQ_NEED_UPDATE_LIMITS))
>  		return false;

Since sg_policy->next_freq is used elsewhere as well, this is the best
we can do here.

>  	sg_policy->next_freq = next_freq;
> @@ -161,7 +162,8 @@ static unsigned int get_next_freq(struct
>  
>  	freq = map_util_freq(util, freq, max);
>  
> -	if (freq == sg_policy->cached_raw_freq && !sg_policy->need_freq_update)
> +	if (freq == sg_policy->cached_raw_freq && !sg_policy->need_freq_update &&
> +	    !cpufreq_driver_test_flags(CPUFREQ_NEED_UPDATE_LIMITS))
>  		return sg_policy->next_freq;
>  
>  	sg_policy->need_freq_update = false;

But I was wondering if instead of this we just do this here:

        if (!cpufreq_driver_test_flags(CPUFREQ_NEED_UPDATE_LIMITS))
                sg_policy->cached_raw_freq = freq;

And so the above check will always fail.

Acked-by: Viresh Kumar <viresh.kumar@...aro.org>

-- 
viresh