linux-kernel - Re: [Update][PATCH v3 3/4] cpufreq: schedutil: Simplify iowait boosting

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Date:   Mon, 01 Apr 2019 11:27:25 +0200
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     Linux PM <linux-pm@...r.kernel.org>,
        Quentin Perret <quentin.perret@....com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Chen Yu <yu.c.chen@...el.com>,
        Gabriele Mazzotta <gabriele.mzt@...il.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [Update][PATCH v3 3/4] cpufreq: schedutil: Simplify iowait boosting

On Thursday, March 28, 2019 11:33:21 AM CEST Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> 
> There is not reason for the minimum iowait boost value in the
> schedutil cpufreq governor to depend on the available range of CPU
> frequencies.  In fact, that dependency is generally confusing,
> because it causes the iowait boost to behave somewhat differently
> on CPUs with the same maximum frequency and different minimum
> frequencies, for example.
> 
> For this reason, replace the min field in struct sugov_cpu
> with a constant and choose its values to be 1/8 of
> SCHED_CAPACITY_SCALE (for consistency with the intel_pstate
> driver's internal governor).
> 
> [Note that policy->cpuinfo.max_freq will not be a constant any more
>  after a subsequent change, so this change is depended on by it.]
> 
> Link: https://lore.kernel.org/lkml/20190305083202.GU32494@hirez.programming.kicks-ass.net/T/#ee20bdc98b7d89f6110c0d00e5c3ee8c2ced93c3d
> Suggested-by: Peter Zijlstra <peterz@...radead.org>
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> ---
> 
> As pointed out by Quentin, the original patch overlooked two kerneldoc
> comments that needed to be updated along with the code, so do that here.
> 
> No other changes with respect to the original.
> 
> ---
>  kernel/sched/cpufreq_schedutil.c |   21 ++++++++++-----------
>  1 file changed, 10 insertions(+), 11 deletions(-)
> 
> Index: linux-pm/kernel/sched/cpufreq_schedutil.c
> ===================================================================
> --- linux-pm.orig/kernel/sched/cpufreq_schedutil.c
> +++ linux-pm/kernel/sched/cpufreq_schedutil.c
> @@ -13,6 +13,8 @@
>  #include <linux/sched/cpufreq.h>
>  #include <trace/events/power.h>
>  
> +#define IOWAIT_BOOST_MIN	(SCHED_CAPACITY_SCALE / 8)
> +
>  struct sugov_tunables {
>  	struct gov_attr_set	attr_set;
>  	unsigned int		rate_limit_us;
> @@ -51,7 +53,6 @@ struct sugov_cpu {
>  	u64			last_update;
>  
>  	unsigned long		bw_dl;
> -	unsigned long		min;
>  	unsigned long		max;
>  
>  	/* The field below is for single-CPU policies only: */
> @@ -291,8 +292,8 @@ static unsigned long sugov_get_util(stru
>   *
>   * The IO wait boost of a task is disabled after a tick since the last update
>   * of a CPU. If a new IO wait boost is requested after more then a tick, then
> - * we enable the boost starting from the minimum frequency, which improves
> - * energy efficiency by ignoring sporadic wakeups from IO.
> + * we enable the boost starting from IOWAIT_BOOST_MIN, which improves energy
> + * efficiency by ignoring sporadic wakeups from IO.
>   */
>  static bool sugov_iowait_reset(struct sugov_cpu *sg_cpu, u64 time,
>  			       bool set_iowait_boost)
> @@ -303,7 +304,7 @@ static bool sugov_iowait_reset(struct su
>  	if (delta_ns <= TICK_NSEC)
>  		return false;
>  
> -	sg_cpu->iowait_boost = set_iowait_boost ? sg_cpu->min : 0;
> +	sg_cpu->iowait_boost = set_iowait_boost ? IOWAIT_BOOST_MIN : 0;
>  	sg_cpu->iowait_boost_pending = set_iowait_boost;
>  
>  	return true;
> @@ -317,8 +318,9 @@ static bool sugov_iowait_reset(struct su
>   *
>   * Each time a task wakes up after an IO operation, the CPU utilization can be
>   * boosted to a certain utilization which doubles at each "frequent and
> - * successive" wakeup from IO, ranging from the utilization of the minimum
> - * OPP to the utilization of the maximum OPP.
> + * successive" wakeup from IO, ranging from IOWAIT_BOOST_MIN to the utilization
> + * of the maximum OPP.
> + *
>   * To keep doubling, an IO boost has to be requested at least once per tick,
>   * otherwise we restart from the utilization of the minimum OPP.
>   */
> @@ -349,7 +351,7 @@ static void sugov_iowait_boost(struct su
>  	}
>  
>  	/* First wakeup after IO: start with minimum boost */
> -	sg_cpu->iowait_boost = sg_cpu->min;
> +	sg_cpu->iowait_boost = IOWAIT_BOOST_MIN;
>  }
>  
>  /**
> @@ -389,7 +391,7 @@ static unsigned long sugov_iowait_apply(
>  		 * No boost pending; reduce the boost value.
>  		 */
>  		sg_cpu->iowait_boost >>= 1;
> -		if (sg_cpu->iowait_boost < sg_cpu->min) {
> +		if (sg_cpu->iowait_boost < IOWAIT_BOOST_MIN) {
>  			sg_cpu->iowait_boost = 0;
>  			return util;
>  		}
> @@ -826,9 +828,6 @@ static int sugov_start(struct cpufreq_po
>  		memset(sg_cpu, 0, sizeof(*sg_cpu));
>  		sg_cpu->cpu			= cpu;
>  		sg_cpu->sg_policy		= sg_policy;
> -		sg_cpu->min			=
> -			(SCHED_CAPACITY_SCALE * policy->cpuinfo.min_freq) /
> -			policy->cpuinfo.max_freq;
>  	}
>  
>  	for_each_cpu(cpu, policy->cpus) {
> 
> 

Any more comments on this patch?

If not, I'll queue it up along with the rest of the series.

Thanks!