linux-kernel - Re: [V2] sched/schedutil: Don't set next_freq to UINT

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Date:   Fri, 11 May 2018 13:47:12 -0700
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     Rafael Wysocki <rjw@...ysocki.net>, Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-pm@...r.kernel.org,
        Vincent Guittot <vincent.guittot@...aro.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [V2] sched/schedutil: Don't set next_freq to UINT_MAX

On Wed, May 09, 2018 at 04:05:24PM +0530, Viresh Kumar wrote:
> The schedutil driver sets sg_policy->next_freq to UINT_MAX on certain
> occasions to discard the cached value of next freq:
> - In sugov_start(), when the schedutil governor is started for a group
>   of CPUs.
> - And whenever we need to force a freq update before rate-limit
>   duration, which happens when:
>   - there is an update in cpufreq policy limits.
>   - Or when the utilization of DL scheduling class increases.
> 
> In return, get_next_freq() doesn't return a cached next_freq value but
> recalculates the next frequency instead.
> 
> But having special meaning for a particular value of frequency makes the
> code less readable and error prone. We recently fixed a bug where the
> UINT_MAX value was considered as valid frequency in
> sugov_update_single().
> 
> All we need is a flag which can be used to discard the value of
> sg_policy->next_freq and we already have need_freq_update for that. Lets
> reuse it instead of setting next_freq to UINT_MAX.
> 
> Signed-off-by: Viresh Kumar <viresh.kumar@...aro.org>
> ---
> V2:
> - Rebased over the fix sent by Rafael
> 
>   lkml.kernel.org/r/2276196.ev9rMjHTR0@...ire.rjw.lan
> 
> - Remove the additional check from sugov_update_single() as well.
> - This is for 4.18 now instead of stable kernels.

Reviewed-by: Joel Fernandes (Google) <joel@...lfernandes.org>

(please note my email address change as well in your contact/address-book).

thanks,

- Joel


> 
>  kernel/sched/cpufreq_schedutil.c | 18 ++++++------------
>  1 file changed, 6 insertions(+), 12 deletions(-)
> 
> diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> index e23e84724f39..daaca23697dc 100644
> --- a/kernel/sched/cpufreq_schedutil.c
> +++ b/kernel/sched/cpufreq_schedutil.c
> @@ -95,15 +95,8 @@ static bool sugov_should_update_freq(struct sugov_policy *sg_policy, u64 time)
>  	if (sg_policy->work_in_progress)
>  		return false;
>  
> -	if (unlikely(sg_policy->need_freq_update)) {
> -		sg_policy->need_freq_update = false;
> -		/*
> -		 * This happens when limits change, so forget the previous
> -		 * next_freq value and force an update.
> -		 */
> -		sg_policy->next_freq = UINT_MAX;
> +	if (unlikely(sg_policy->need_freq_update))
>  		return true;
> -	}
>  
>  	delta_ns = time - sg_policy->last_freq_update_time;
>  
> @@ -165,8 +158,10 @@ static unsigned int get_next_freq(struct sugov_policy *sg_policy,
>  
>  	freq = (freq + (freq >> 2)) * util / max;
>  
> -	if (freq == sg_policy->cached_raw_freq && sg_policy->next_freq != UINT_MAX)
> +	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);
>  }
> @@ -305,8 +300,7 @@ static void sugov_update_single(struct update_util_data *hook, u64 time,
>  	 * Do not reduce the frequency if the CPU has not been idle
>  	 * recently, as the reduction is likely to be premature then.
>  	 */
> -	if (busy && next_f < sg_policy->next_freq &&
> -	    sg_policy->next_freq != UINT_MAX) {
> +	if (busy && next_f < sg_policy->next_freq) {
>  		next_f = sg_policy->next_freq;
>  
>  		/* Reset cached freq as next_freq has changed */
> @@ -671,7 +665,7 @@ static int sugov_start(struct cpufreq_policy *policy)
>  
>  	sg_policy->freq_update_delay_ns	= sg_policy->tunables->rate_limit_us * NSEC_PER_USEC;
>  	sg_policy->last_freq_update_time	= 0;
> -	sg_policy->next_freq			= UINT_MAX;
> +	sg_policy->next_freq			= 0;
>  	sg_policy->work_in_progress		= false;
>  	sg_policy->need_freq_update		= false;
>  	sg_policy->cached_raw_freq		= 0;