linux-kernel - Re: [PATCH v2 4/4] cpufreq: schedutil: Always call drvier if need_freq

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Message-ID: <20201027042559.hang4fnpyfd6yqu4@vireshk-i7>
Date:   Tue, 27 Oct 2020 09:55:59 +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 4/4] cpufreq: schedutil: Always call drvier if
 need_freq_update is set

Spelling mistake in $subject (driver)

On 23-10-20, 17:36, 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 clear
> need_freq_update only if the CPUFREQ_NEED_UPDATE_LIMITS flag is
> not set for the cpufreq driver in use (and it should be set for all
> potentially affected drivers) and make sugov_update_next_freq()
> check need_freq_update and continue when it is set regardless of
> whether or not the new target frequency is equal to the old one.
> 
> Fixes: f6ebbcf08f37 ("cpufreq: intel_pstate: Implement passive mode with HWP enabled")
> Reported-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>
> ---
> 
> New patch in v2.
> 
> ---
>  kernel/sched/cpufreq_schedutil.c |    8 ++++++--
>  1 file changed, 6 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
> @@ -102,11 +102,12 @@ 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 && !sg_policy->need_freq_update)
>  		return false;
>  
>  	sg_policy->next_freq = next_freq;
>  	sg_policy->last_freq_update_time = time;
> +	sg_policy->need_freq_update = false;
>  
>  	return true;
>  }
> @@ -164,7 +165,10 @@ static unsigned int get_next_freq(struct
>  	if (freq == sg_policy->cached_raw_freq && !sg_policy->need_freq_update)
>  		return sg_policy->next_freq;
>  
> -	sg_policy->need_freq_update = false;
> +	if (sg_policy->need_freq_update)
> +		sg_policy->need_freq_update =
> +			cpufreq_driver_test_flags(CPUFREQ_NEED_UPDATE_LIMITS);
> +

The behavior here is a bit different from what we did in cpufreq.c. In cpufreq
core we are _always_ allowing the call to reach the driver's target() routine,
but here we do it only if limits have changed. Wonder if we should have similar
behavior here as well ?

Over that the code here can be rewritten a bit like:

	if (sg_policy->need_freq_update)
                sg_policy->need_freq_update = cpufreq_driver_test_flags(CPUFREQ_NEED_UPDATE_LIMITS);
        else if (freq == sg_policy->cached_raw_freq)
		return sg_policy->next_freq;

-- 
viresh