Message-ID: <1844525.jBn1oKmyb6@aspire.rjw.lan>
Date:   Tue, 21 Mar 2017 15:46:07 +0100
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     Patrick Bellasi <patrick.bellasi@....com>
Cc:     Vincent Guittot <vincent.guittot@...aro.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Juri Lelli <juri.lelli@....com>,
        Joel Fernandes <joelaf@...gle.com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: [RFC][PATCH v2 2/2] cpufreq: schedutil: Avoid decreasing frequency of busy CPUs

On Tuesday, March 21, 2017 02:38:42 PM Patrick Bellasi wrote:
> On 21-Mar 15:26, Rafael J. Wysocki wrote:
> > On Tuesday, March 21, 2017 02:37:08 PM Vincent Guittot wrote:
> > > On 21 March 2017 at 14:22, Peter Zijlstra <peterz@...radead.org> wrote:
> > > > On Tue, Mar 21, 2017 at 09:50:28AM +0100, Vincent Guittot wrote:
> > > >> On 20 March 2017 at 22:46, Rafael J. Wysocki <rjw@...ysocki.net> wrote:
> > > >
> > > >> > To work around this issue use the observation that, from the
> > > >> > schedutil governor's perspective, it does not make sense to decrease
> > > >> > the frequency of a CPU that doesn't enter idle and avoid decreasing
> > > >> > the frequency of busy CPUs.
> > > >>
> > > >> I don't fully agree with that statement.
> > > >> If there are 2 runnable tasks on CPU A and scheduler migrates the
> > > >> waiting task to another CPU B so CPU A is less loaded now, it makes
> > > >> sense to reduce the OPP. That's even for that purpose that we have
> > > >> decided to use scheduler metrics in cpufreq governor so we can adjust
> > > >> OPP immediately when tasks migrate.
> > > >> That being said, i probably know why you see such OPP switches in your
> > > >> use case. When we migrate a task, we also migrate/remove its
> > > >> utilization from CPU.
> > > >> If the CPU is not overloaded, it means that runnable tasks have all
> > > >> computation that they need and don't have any reason to use more when
> > > >> a task migrates to another CPU. so decreasing the OPP makes sense
> > > >> because the utilzation is decreasing
> > > >> If the CPU is overloaded, it means that runnable tasks have to share
> > > >> CPU time and probably don't have all computations that they would like
> > > >> so when a task migrate, the remaining tasks on the CPU will increase
> > > >> their utilization and fill space left by the task that has just
> > > >> migrated. So the CPU's utilization will decrease when a task migrates
> > > >> (and as a result the OPP) but then its utilization will increase with
> > > >> remaining tasks running more time as well as the OPP
> > > >>
> > > >> So you need to make the difference between this 2 cases: Is a CPU
> > > >> overloaded or not. You can't really rely on the utilization to detect
> > > >> that but you could take advantage of the load which take into account
> > > >> the waiting time of tasks
> > > >
> > > > I'm confused. What two cases? You only list the overloaded case, but he
> > > 
> > > overloaded vs not overloaded use case.
> > > For the not overloaded case, it makes sense to immediately update to
> > > OPP to be aligned with the new utilization of the CPU even if it was
> > > not idle in the past couple of ticks
> > 
> > Yes, if the OPP (or P-state if you will) can be changed immediately.  If it can't,
> > conditions may change by the time we actually update it and in that case It'd
> > be better to wait and see IMO.
> > 
> > In any case, the theory about migrating tasks made sense to me, so below is
> > what I tested.  It works, and besides it has a nice feature that I don't need
> > to fetch for the timekeeping data. :-)
> > 
> > I only wonder if we want to do this or only prevent the frequency from
> > decreasing in the overloaded case?
> > 
> > ---
> >  kernel/sched/cpufreq_schedutil.c |    8 +++++---
> >  1 file changed, 5 insertions(+), 3 deletions(-)
> > 
> > Index: linux-pm/kernel/sched/cpufreq_schedutil.c
> > ===================================================================
> > --- linux-pm.orig/kernel/sched/cpufreq_schedutil.c
> > +++ linux-pm/kernel/sched/cpufreq_schedutil.c
> > @@ -61,6 +61,7 @@ struct sugov_cpu {
> >  	unsigned long util;
> >  	unsigned long max;
> >  	unsigned int flags;
> > +	bool overload;
> >  };
> >  
> >  static DEFINE_PER_CPU(struct sugov_cpu, sugov_cpu);
> > @@ -207,7 +208,7 @@ 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) || this_rq()->rd->overload) {
> >  		next_f = policy->cpuinfo.max_freq;
> 
> Isn't this going to max OPP every time we have more than 1 task in
> that CPU?
> 
> In that case it will not fit the case: we have two 10% tasks on that CPU.

Good point.

> Previous solution was better IMO, apart from using overloaded instead
> of overutilized (which is not yet there) :-/

OK, so the one below works too.

---
 kernel/sched/cpufreq_schedutil.c |   11 +++++++++++
 1 file changed, 11 insertions(+)

Index: linux-pm/kernel/sched/cpufreq_schedutil.c
===================================================================
--- linux-pm.orig/kernel/sched/cpufreq_schedutil.c
+++ linux-pm/kernel/sched/cpufreq_schedutil.c
@@ -37,6 +37,7 @@ struct sugov_policy {
 	s64 freq_update_delay_ns;
 	unsigned int next_freq;
 	unsigned int cached_raw_freq;
+	bool overload;
 
 	/* The next fields are only needed if fast switch cannot be used. */
 	struct irq_work irq_work;
@@ -61,6 +62,7 @@ struct sugov_cpu {
 	unsigned long util;
 	unsigned long max;
 	unsigned int flags;
+	bool overload;
 };
 
 static DEFINE_PER_CPU(struct sugov_cpu, sugov_cpu);
@@ -93,6 +95,9 @@ static void sugov_update_commit(struct s
 {
 	struct cpufreq_policy *policy = sg_policy->policy;
 
+	if (sg_policy->overload && next_freq < sg_policy->next_freq)
+		next_freq = sg_policy->next_freq;
+
 	if (policy->fast_switch_enabled) {
 		if (sg_policy->next_freq == next_freq) {
 			trace_cpu_frequency(policy->cur, smp_processor_id());
@@ -207,6 +212,8 @@ static void sugov_update_single(struct u
 	if (!sugov_should_update_freq(sg_policy, time))
 		return;
 
+	sg_policy->overload = this_rq()->rd->overload;
+
 	if (flags & SCHED_CPUFREQ_RT_DL) {
 		next_f = policy->cpuinfo.max_freq;
 	} else {
@@ -225,6 +232,8 @@ static unsigned int sugov_next_freq_shar
 	unsigned long util = 0, max = 1;
 	unsigned int j;
 
+	sg_policy->overload = false;
+
 	for_each_cpu(j, policy->cpus) {
 		struct sugov_cpu *j_sg_cpu = &per_cpu(sugov_cpu, j);
 		unsigned long j_util, j_max;
@@ -253,6 +262,7 @@ static unsigned int sugov_next_freq_shar
 		}
 
 		sugov_iowait_boost(j_sg_cpu, &util, &max);
+		sg_policy->overload = sg_policy->overload || sg_cpu->overload;
 	}
 
 	return get_next_freq(sg_policy, util, max);
@@ -273,6 +283,7 @@ static void sugov_update_shared(struct u
 	sg_cpu->util = util;
 	sg_cpu->max = max;
 	sg_cpu->flags = flags;
+	sg_cpu->overload = this_rq()->rd->overload;
 
 	sugov_set_iowait_boost(sg_cpu, time, flags);
 	sg_cpu->last_update = time;

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