linux-kernel - Re: [PATCH] cpufreq: schedutil: force frequency update when limits change

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Date:   Thu, 25 Jun 2020 15:53:05 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Wei Wang <wvw@...gle.com>
Cc:     wei.vince.wang@...il.com, dsmythies@...us.net,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        "Joel Fernandes (Google)" <joel@...lfernandes.org>,
        linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] cpufreq: schedutil: force frequency update when limits
 change

On 24-06-20, 23:46, Wei Wang wrote:
> To avoid reducing the frequency of a CPU prematurely, we skip reducing
> the frequency if the CPU had been busy recently.
> 
> This should not be done when the limits of the policy are changed, for
> example due to thermal throttling. We should always get the frequency
> within the new limits as soon as possible.
> 
> There was a fix in
> commit 600f5badb78c ("cpufreq: schedutil: Don't skip freq update when
> limits change") upstream which introduced another flag. However, the
> fix didn't address the case when next_freq is the same as previously
> voted, which is then checked in sugov_update_next_freq. As a result, the
> frequency would be stuck at low until the high demanding workload quits.
> 
> test trace:
>   kworker/u19:0-1872  ( 1872) [002] ....   347.878871: cpu_frequency_limits: min=600000 max=2348000 cpu_id=6
>          dhry64-11525 (11525) [007] d.h2   347.880012: sugov_should_update_freq: thermal limit on policy6
>          dhry64-11525 (11525) [007] d.h2   347.880012: sugov_deferred_update: policy6 skipped update
>          dhry64-11525 (11525) [007] d.h2   347.884040: sugov_deferred_update: policy6 skipped update

I am not sure these are helpful in the logs as the code which
generated them isn't there in the kernel.

> ...
> 
> This patch fixes this by skipping the check and forcing an update in
> this case. The second flag was kept as the limits_change flag could be
> updated in thermal kworker from another CPU.

I am sorry but I am not fully sure of what the problem is. Can you
describe that by giving an example with some random frequency, and
tell the expected and actual behavior ?

> Fixes: ecd288429126 ("cpufreq: schedutil: Don't set next_freq to UINT_MAX")
> Signed-off-by: Wei Wang <wvw@...gle.com>
> ---
>  kernel/sched/cpufreq_schedutil.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> index 7fbaee24c824..dc2cd768022e 100644
> --- a/kernel/sched/cpufreq_schedutil.c
> +++ b/kernel/sched/cpufreq_schedutil.c
> @@ -102,11 +102,12 @@ static bool sugov_should_update_freq(struct sugov_policy *sg_policy, u64 time)
>  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->need_freq_update && sg_policy->next_freq == next_freq)

AFAIU, if the next freq is same as currently programmed one, there is
no need to force update it.

>  		return false;
>  
>  	sg_policy->next_freq = next_freq;
>  	sg_policy->last_freq_update_time = time;
> +	sg_policy->need_freq_update = false;
>  
>  	return true;
>  }
> @@ -178,7 +179,6 @@ static unsigned int get_next_freq(struct sugov_policy *sg_policy,
>  	if (freq == sg_policy->cached_raw_freq && !sg_policy->need_freq_update)
>  		return sg_policy->next_freq;
>  
> -	sg_policy->need_freq_update = false;
>  	sg_policy->cached_raw_freq = freq;
>  	return cpufreq_driver_resolve_freq(policy, freq);
>  }
> -- 
> 2.27.0.212.ge8ba1cc988-goog

-- 
viresh