Re: [RFC][PATCH 2/2] cpufreq: schedutil: Force max frequency on busy CPUs

[Viresh Kumar]

On 19-03-17, 14:34, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
> 
> The PELT metric used by the schedutil governor underestimates the
> CPU utilization in some cases.  The reason for that may be time spent
> in interrupt handlers and similar which is not accounted for by PELT.
> 
> That can be easily demonstrated by running kernel compilation on
> a Sandy Bridge Intel processor, running turbostat in parallel with
> it and looking at the values written to the MSR_IA32_PERF_CTL
> register.  Namely, the expected result would be that when all CPUs
> were 100% busy, all of them would be requested to run in the maximum
> P-state, but observation shows that this clearly isn't the case.
> The CPUs run in the maximum P-state for a while and then are
> requested to run slower and go back to the maximum P-state after
> a while again.  That causes the actual frequency of the processor to
> visibly oscillate below the sustainable maximum in a jittery fashion
> which clearly is not desirable.
> 
> To work around this issue use the observation that, from the
> schedutil governor's perspective, CPUs that are never idle should
> always run at the maximum frequency and make that happen.
> 
> To that end, add a counter of idle calls to struct sugov_cpu and
> modify cpuidle_idle_call() to increment that counter every time it
> is about to put the given CPU into an idle state.  Next, make the
> schedutil governor look at that counter for the current CPU every
> time before it is about to start heavy computations.  If the counter
> has not changed for over SUGOV_BUSY_THRESHOLD time (equal to 50 ms),
> the CPU has not been idle for at least that long and the governor
> will choose the maximum frequency for it without looking at the PELT
> metric at all.

Looks like we are fixing a PELT problem with a schedutil Hack :)

> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
> ---
>  include/linux/sched/cpufreq.h    |    6 ++++++
>  kernel/sched/cpufreq_schedutil.c |   38 ++++++++++++++++++++++++++++++++++++--
>  kernel/sched/idle.c              |    3 +++
>  3 files changed, 45 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
> @@ -20,6 +20,7 @@
>  #include "sched.h"
>  
>  #define SUGOV_KTHREAD_PRIORITY	50
> +#define SUGOV_BUSY_THRESHOLD	(50 * NSEC_PER_MSEC)
>  
>  struct sugov_tunables {
>  	struct gov_attr_set attr_set;
> @@ -55,6 +56,9 @@ struct sugov_cpu {
>  
>  	unsigned long iowait_boost;
>  	unsigned long iowait_boost_max;
> +	unsigned long idle_calls;
> +	unsigned long saved_idle_calls;
> +	u64 busy_start;
>  	u64 last_update;
>  
>  	/* The fields below are only needed when sharing a policy. */
> @@ -192,6 +196,34 @@ static void sugov_iowait_boost(struct su
>  	sg_cpu->iowait_boost >>= 1;
>  }
>  
> +void cpufreq_schedutil_idle_call(void)
> +{
> +	struct sugov_cpu *sg_cpu = this_cpu_ptr(&sugov_cpu);
> +
> +	sg_cpu->idle_calls++;
> +}
> +
> +static bool sugov_cpu_is_busy(struct sugov_cpu *sg_cpu)
> +{
> +	if (sg_cpu->idle_calls != sg_cpu->saved_idle_calls) {
> +		sg_cpu->busy_start = 0;
> +		return false;
> +	}
> +
> +	if (!sg_cpu->busy_start) {
> +		sg_cpu->busy_start = sg_cpu->last_update;
> +		return false;
> +	}
> +
> +	return sg_cpu->last_update - sg_cpu->busy_start > SUGOV_BUSY_THRESHOLD;
> +}
> +
> +static void sugov_save_idle_calls(struct sugov_cpu *sg_cpu)
> +{
> +	if (!sg_cpu->busy_start)
> +		sg_cpu->saved_idle_calls = sg_cpu->idle_calls;

Why aren't we doing this in sugov_cpu_is_busy() itself ? And isn't it possible
for idle_calls to get incremented by this time?

> +}
> +
>  static void sugov_update_single(struct update_util_data *hook, u64 time,
>  				unsigned int flags)
>  {
> @@ -207,7 +239,7 @@ static void sugov_update_single(struct u
>  	if (!sugov_should_update_freq(sg_policy, time))
>  		return;
>  
> -	if (flags & SCHED_CPUFREQ_RT_DL) {
> +	if ((flags & SCHED_CPUFREQ_RT_DL) || sugov_cpu_is_busy(sg_cpu)) {
>  		next_f = policy->cpuinfo.max_freq;
>  	} else {
>  		sugov_get_util(&util, &max);
> @@ -215,6 +247,7 @@ static void sugov_update_single(struct u
>  		next_f = get_next_freq(sg_policy, util, max);
>  	}
>  	sugov_update_commit(sg_policy, time, next_f);
> +	sugov_save_idle_calls(sg_cpu);
>  }
>  
>  static unsigned int sugov_next_freq_shared(struct sugov_cpu *sg_cpu)
> @@ -278,12 +311,13 @@ static void sugov_update_shared(struct u
>  	sg_cpu->last_update = time;
>  
>  	if (sugov_should_update_freq(sg_policy, time)) {
> -		if (flags & SCHED_CPUFREQ_RT_DL)
> +		if ((flags & SCHED_CPUFREQ_RT_DL) || sugov_cpu_is_busy(sg_cpu))

What about others CPUs in this policy?

>  			next_f = sg_policy->policy->cpuinfo.max_freq;
>  		else
>  			next_f = sugov_next_freq_shared(sg_cpu);
>  
>  		sugov_update_commit(sg_policy, time, next_f);
> +		sugov_save_idle_calls(sg_cpu);
>  	}
>  
>  	raw_spin_unlock(&sg_policy->update_lock);

-- 
viresh