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[PATCH v2 0/6] cpufreq: schedutil: fixes for flags updates

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  [PATCH v2 0/6] cpufreq: schedutil: fixes for flags updates Patrick Bellasi <patrick.bellasi@arm.com> - 2017-07-04 19:40 +0200
    [PATCH v2 2/6] cpufreq: schedutil: reset sg_cpus's flags at IDLE enter Patrick Bellasi <patrick.bellasi@arm.com> - 2017-07-04 19:40 +0200
      Re: [PATCH v2 2/6] cpufreq: schedutil: reset sg_cpus's flags at IDLE  enter Viresh Kumar <viresh.kumar@linaro.org> - 2017-07-05 07:00 +0200
        Re: [PATCH v2 2/6] cpufreq: schedutil: reset sg_cpus's flags at IDLE  enter Patrick Bellasi <patrick.bellasi@arm.com> - 2017-07-05 15:10 +0200
          Re: [PATCH v2 2/6] cpufreq: schedutil: reset sg_cpus's flags at IDLE  enter Viresh Kumar <viresh.kumar@linaro.org> - 2017-07-06 07:50 +0200
      Re: [PATCH v2 2/6] cpufreq: schedutil: reset sg_cpus's flags at IDLE enter Joel Fernandes <joelaf@google.com> - 2017-07-07 06:50 +0200
        Re: [PATCH v2 2/6] cpufreq: schedutil: reset sg_cpus's flags at IDLE  enter Juri Lelli <juri.lelli@arm.com> - 2017-07-07 12:20 +0200
          Re: [PATCH v2 2/6] cpufreq: schedutil: reset sg_cpus's flags at IDLE  enter Saravana Kannan <skannan@codeaurora.org> - 2017-07-11 21:20 +0200
    [PATCH v2 5/6] sched/rt: fast switch to maximum frequency when RT tasks are scheduled Patrick Bellasi <patrick.bellasi@arm.com> - 2017-07-04 19:40 +0200
    [PATCH v2 6/6] cpufreq: schedutil: relax rate-limiting while running RT/DL tasks Patrick Bellasi <patrick.bellasi@arm.com> - 2017-07-04 19:40 +0200
    Re: [PATCH v2 0/6] cpufreq: schedutil: fixes for flags updates "Rafael J. Wysocki" <rjw@rjwysocki.net> - 2017-07-07 00:40 +0200

#1681107 — [PATCH v2 0/6] cpufreq: schedutil: fixes for flags updates

Each time a CPU utilisation update is issued by the scheduler a flag, which
mainly defines which scheduling class is asking for the update, is used by the
frequency selection policy to support the selection of the most appropriate
OPP.
 
In the current implementation, CPU flags are overridden each time the scheduler
calls schedutil for an update. Such a behavior seems to be sub-optimal,
especially on systems where frequency domains span across multiple CPUs.
 
Indeed, assuming CPU1 and CPU2 share the same frequency domain, there can be
the following issues:
 
 A) Small FAIR task running at MAX OPP.
    A RT task, which just executed on CPU1, can keep the domain at the
    max frequency for a prolonged period of time after its completion,
    even if there are no longer RT tasks running on CPUs of its domain.
 
 B) FAIR wakeup reducing the OPP of the current RT task.
    A FAIR task enqueued in a CPU where a RT task is running overrides the flag
    configured by the RT task thus potentially causing an unwanted frequency
    drop.
 
 C) RT wakeup not running at max OPP.
    An RT task waking up on a CPU which has recently updated its OPP can
    be forced to run at a lower frequency because of the throttling
    enforced by schedutil, even if there are not OPP transitions
    currently in progress.

.:: Patches organization
========================
 
This series proposes a set of fixes for the aforementioned issues and it's an
update addressing all the main comments collected from the previous posting
[1].
 
Patches have been re-ordered to have the "less controversial" bits at the
beginning and also to better match the order of the three main issues described
above. These are the relative patches:
 
 A) Fix small FAIR task running at MAX OPP:
    cpufreq: schedutil: ignore the sugov kthread for frequencies selections
    cpufreq: schedutil: reset sg_cpus's flags at IDLE enter
 
 B) FAIR wakeup reducing the OPP of the current RT task.
    cpufreq: schedutil: ensure max frequency while running RT/DL tasks
 
 C) RT wakeup not running at max OPP.
    sched/rt: fast switch to maximum frequency when RT tasks are scheduled
    cpufreq: schedutil: relax rate-limiting while running RT/DL tasks
    cpufreq: schedutil: avoid utilisation update when not necessary

.:: Experimental Results
========================
 
The misbehavior have been verified using a set of simple rt-app based synthetic
workloads, running on a ARM's Juno R2 board where the CPUs of the big cluster
(CPU1 and CPU2) have been reserved to run the workload tasks in isolation from
other system tasks.
 
A detailed description of the experiments executed, and the corresponding
collected results, is available [2] online.
 
Short highlights for these experiments are:
 
 - Patches in group A reduce energy consumption by ~50% by ensuring that
   a small task is always running at the minimum OPP even when the
   sugov's RT kthread is used to change frequencies in the same cluster.
 
 - Patches in group B increase from 4% to 98% the chances for a RT
   task to complete its activations while running at the max OPP.
 
 - Patches in group C do not show measurable differences mainly because of the
   slow OPP switching support available on the JUNO board used for testing.
   However, a trace inspection shows that the sequence of traced events is much
   more deterministic and it better matches the expected system behaviors.
   For example, as soon as a RT task wakeup the scheduler ask for an OPP switch
   to max frequency.
 
Cheers Patrick
 
.:: References
==============
 
[1] https://lkml.org/lkml/2017/3/2/385
[2] https://gist.github.com/derkling/0cd7210e4fa6f2ec3558073006e5ad70


Patrick Bellasi (6):
  cpufreq: schedutil: ignore sugov kthreads
  cpufreq: schedutil: reset sg_cpus's flags at IDLE enter
  cpufreq: schedutil: ensure max frequency while running RT/DL tasks
  cpufreq: schedutil: update CFS util only if used
  sched/rt: fast switch to maximum frequency when RT tasks are scheduled
  cpufreq: schedutil: relax rate-limiting while running RT/DL tasks

 include/linux/sched/cpufreq.h    |  1 +
 kernel/sched/cpufreq_schedutil.c | 61 ++++++++++++++++++++++++++++++++--------
 kernel/sched/idle_task.c         |  4 +++
 kernel/sched/rt.c                | 15 ++++++++--
 4 files changed, 67 insertions(+), 14 deletions(-)

--
2.7.4

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#1681109 — [PATCH v2 2/6] cpufreq: schedutil: reset sg_cpus's flags at IDLE enter

Currently, sg_cpu's flags are set to the value defined by the last call of
the cpufreq_update_util()/cpufreq_update_this_cpu(); for RT/DL classes
this corresponds to the SCHED_CPUFREQ_{RT/DL} flags always being set.

When multiple CPU shares the same frequency domain it might happen that a
CPU which executed a RT task, right before entering IDLE, has one of the
SCHED_CPUFREQ_RT_DL flags set, permanently, until it exits IDLE.

Although such an idle CPU is _going to be_ ignored by the
sugov_next_freq_shared():
  1. this kind of "useless RT requests" are ignored only if more then
     TICK_NSEC have elapsed since the last update
  2. we can still potentially trigger an already too late switch to
     MAX, which starts also a new throttling interval
  3. the internal state machine is not consistent with what the
     scheduler knows, i.e. the CPU is now actually idle

Thus, in sugov_next_freq_shared(), where utilisation and flags are
aggregated across all the CPUs of a frequency domain, it can turn out
that all the CPUs of that domain can run unnecessary at the maximum OPP
until another event happens in the idle CPU, which eventually clear the
SCHED_CPUFREQ_{RT/DL} flag, or the IDLE CPUs gets ignored after
TICK_NSEC since the CPU entering IDLE.

Such a behaviour can harm the energy efficiency of systems where RT
workloads are not so frequent and other CPUs in the same frequency
domain are running small utilisation workloads, which is a quite common
scenario in mobile embedded systems.

This patch proposes a solution which is aligned with the current principle
to update the flags each time a scheduling event happens. The scheduling
of the idle_task on a CPU is considered one of such meaningful events.
That's why when the idle_task is selected for execution we poke the
schedutil policy to reset the flags for that CPU.

No frequency transitions are activated at that point, which is fair in
case the RT workload should come back in the future. However, this still
allows other CPUs in the same frequency domain to scale down the
frequency in case that should be possible.

Signed-off-by: Patrick Bellasi <patrick.bellasi@arm.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Cc: Viresh Kumar <viresh.kumar@linaro.org>
Cc: linux-kernel@vger.kernel.org
Cc: linux-pm@vger.kernel.org

---
Changes from v1:
- added "unlikely()" around the statement (SteveR)
---
 include/linux/sched/cpufreq.h    | 1 +
 kernel/sched/cpufreq_schedutil.c | 7 +++++++
 kernel/sched/idle_task.c         | 4 ++++
 3 files changed, 12 insertions(+)

diff --git a/include/linux/sched/cpufreq.h b/include/linux/sched/cpufreq.h
index d2be2cc..36ac8d2 100644
--- a/include/linux/sched/cpufreq.h
+++ b/include/linux/sched/cpufreq.h
@@ -10,6 +10,7 @@
 #define SCHED_CPUFREQ_RT	(1U << 0)
 #define SCHED_CPUFREQ_DL	(1U << 1)
 #define SCHED_CPUFREQ_IOWAIT	(1U << 2)
+#define SCHED_CPUFREQ_IDLE	(1U << 3)
 
 #define SCHED_CPUFREQ_RT_DL	(SCHED_CPUFREQ_RT | SCHED_CPUFREQ_DL)
 
diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
index eaba6d6..004ae18 100644
--- a/kernel/sched/cpufreq_schedutil.c
+++ b/kernel/sched/cpufreq_schedutil.c
@@ -304,6 +304,12 @@ static void sugov_update_shared(struct update_util_data *hook, u64 time,
 
 	sg_cpu->util = util;
 	sg_cpu->max = max;
+
+	/* CPU is entering IDLE, reset flags without triggering an update */
+	if (unlikely(flags & SCHED_CPUFREQ_IDLE)) {
+		sg_cpu->flags = 0;
+		goto done;
+	}
 	sg_cpu->flags = flags;
 
 	sugov_set_iowait_boost(sg_cpu, time, flags);
@@ -318,6 +324,7 @@ static void sugov_update_shared(struct update_util_data *hook, u64 time,
 		sugov_update_commit(sg_policy, time, next_f);
 	}
 
+done:
 	raw_spin_unlock(&sg_policy->update_lock);
 }
 
diff --git a/kernel/sched/idle_task.c b/kernel/sched/idle_task.c
index 0c00172..a844c91 100644
--- a/kernel/sched/idle_task.c
+++ b/kernel/sched/idle_task.c
@@ -29,6 +29,10 @@ pick_next_task_idle(struct rq *rq, struct task_struct *prev, struct rq_flags *rf
 	put_prev_task(rq, prev);
 	update_idle_core(rq);
 	schedstat_inc(rq->sched_goidle);
+
+	/* kick cpufreq (see the comment in kernel/sched/sched.h). */
+	cpufreq_update_this_cpu(rq, SCHED_CPUFREQ_IDLE);
+
 	return rq->idle;
 }
 
-- 
2.7.4

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#1681273 — Re: [PATCH v2 2/6] cpufreq: schedutil: reset sg_cpus's flags at IDLE enter

On 04-07-17, 18:34, Patrick Bellasi wrote:
> diff --git a/include/linux/sched/cpufreq.h b/include/linux/sched/cpufreq.h
> index d2be2cc..36ac8d2 100644
> --- a/include/linux/sched/cpufreq.h
> +++ b/include/linux/sched/cpufreq.h
> @@ -10,6 +10,7 @@
>  #define SCHED_CPUFREQ_RT	(1U << 0)
>  #define SCHED_CPUFREQ_DL	(1U << 1)
>  #define SCHED_CPUFREQ_IOWAIT	(1U << 2)
> +#define SCHED_CPUFREQ_IDLE	(1U << 3)
>  
>  #define SCHED_CPUFREQ_RT_DL	(SCHED_CPUFREQ_RT | SCHED_CPUFREQ_DL)
>  
> diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> index eaba6d6..004ae18 100644
> --- a/kernel/sched/cpufreq_schedutil.c
> +++ b/kernel/sched/cpufreq_schedutil.c
> @@ -304,6 +304,12 @@ static void sugov_update_shared(struct update_util_data *hook, u64 time,
>  
>  	sg_cpu->util = util;
>  	sg_cpu->max = max;
> +
> +	/* CPU is entering IDLE, reset flags without triggering an update */
> +	if (unlikely(flags & SCHED_CPUFREQ_IDLE)) {
> +		sg_cpu->flags = 0;
> +		goto done;
> +	}

Why is it important to have the above diff at all ? For example we aren't doing
similar stuff in sugov_update_single() and that will go on and try to change the
frequency if rate_limit_us time is over since last update.

And also why is it important to write 0 to sg_cpu->flags ? What wouldn't work if
we set sg_cpu->flags to SCHED_CPUFREQ_IDLE in this case ? i.e. Just the below
statement should be good for us.

>  	sg_cpu->flags = flags;
>  
>  	sugov_set_iowait_boost(sg_cpu, time, flags);
> @@ -318,6 +324,7 @@ static void sugov_update_shared(struct update_util_data *hook, u64 time,
>  		sugov_update_commit(sg_policy, time, next_f);
>  	}
>  
> +done:
>  	raw_spin_unlock(&sg_policy->update_lock);
>  }
>  
> diff --git a/kernel/sched/idle_task.c b/kernel/sched/idle_task.c
> index 0c00172..a844c91 100644
> --- a/kernel/sched/idle_task.c
> +++ b/kernel/sched/idle_task.c
> @@ -29,6 +29,10 @@ pick_next_task_idle(struct rq *rq, struct task_struct *prev, struct rq_flags *rf
>  	put_prev_task(rq, prev);
>  	update_idle_core(rq);
>  	schedstat_inc(rq->sched_goidle);
> +
> +	/* kick cpufreq (see the comment in kernel/sched/sched.h). */
> +	cpufreq_update_this_cpu(rq, SCHED_CPUFREQ_IDLE);
> +

This looks correct.

Can we completely avoid the utilization contribution of the CPUs which have gone
idle? Right now we avoid them with help of (delta_ns > TICK_NSEC). Can we
instead check this SCHED_CPUFREQ_IDLE flag ?

-- 
viresh

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#1681525 — Re: [PATCH v2 2/6] cpufreq: schedutil: reset sg_cpus's flags at IDLE enter

On 05-Jul 10:20, Viresh Kumar wrote:
> On 04-07-17, 18:34, Patrick Bellasi wrote:
> > diff --git a/include/linux/sched/cpufreq.h b/include/linux/sched/cpufreq.h
> > index d2be2cc..36ac8d2 100644
> > --- a/include/linux/sched/cpufreq.h
> > +++ b/include/linux/sched/cpufreq.h
> > @@ -10,6 +10,7 @@
> >  #define SCHED_CPUFREQ_RT	(1U << 0)
> >  #define SCHED_CPUFREQ_DL	(1U << 1)
> >  #define SCHED_CPUFREQ_IOWAIT	(1U << 2)
> > +#define SCHED_CPUFREQ_IDLE	(1U << 3)
> >  
> >  #define SCHED_CPUFREQ_RT_DL	(SCHED_CPUFREQ_RT | SCHED_CPUFREQ_DL)
> >  
> > diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> > index eaba6d6..004ae18 100644
> > --- a/kernel/sched/cpufreq_schedutil.c
> > +++ b/kernel/sched/cpufreq_schedutil.c
> > @@ -304,6 +304,12 @@ static void sugov_update_shared(struct update_util_data *hook, u64 time,
> >  
> >  	sg_cpu->util = util;
> >  	sg_cpu->max = max;
> > +
> > +	/* CPU is entering IDLE, reset flags without triggering an update */
> > +	if (unlikely(flags & SCHED_CPUFREQ_IDLE)) {
> > +		sg_cpu->flags = 0;
> > +		goto done;
> > +	}
> 
> Why is it important to have the above diff at all ? For example we aren't doing
> similar stuff in sugov_update_single() and that will go on and try to change the
> frequency if rate_limit_us time is over since last update.

The p repose here is mainly to avoid interference of IDLE CPUs on
other CPUs in the same frequency domain, by just resetting their
"requests".

In the single case, it's completely up to the policy to decide what to
do when we enter IDLE without risking to affect other CPUs.
But perhaps you are right, maybe we should use the same heuristics in
both cases. Entering idle just reset the flags and do not enforce for
example a frequency drop.

> And also why is it important to write 0 to sg_cpu->flags ? What wouldn't work if
> we set sg_cpu->flags to SCHED_CPUFREQ_IDLE in this case ? i.e. Just the below
> statement should be good for us.

Let say flags have the RT/DL flag set when the RT task sleep, is there
any specific reason to keep this flag up while the CPU is IDLE?
IOW, why should we care about an information related to an even which
is now over?

The proposal of this patch is just meant to make sure that the flags,
being a state variable, always describe the current status of the
sugov "state machine".
If a CPU is IDLE there are not sensible events going on and thus flags
should better be reset.

> 
> >  	sg_cpu->flags = flags;
> >  
> >  	sugov_set_iowait_boost(sg_cpu, time, flags);
> > @@ -318,6 +324,7 @@ static void sugov_update_shared(struct update_util_data *hook, u64 time,
> >  		sugov_update_commit(sg_policy, time, next_f);
> >  	}
> >  
> > +done:
> >  	raw_spin_unlock(&sg_policy->update_lock);
> >  }
> >  
> > diff --git a/kernel/sched/idle_task.c b/kernel/sched/idle_task.c
> > index 0c00172..a844c91 100644
> > --- a/kernel/sched/idle_task.c
> > +++ b/kernel/sched/idle_task.c
> > @@ -29,6 +29,10 @@ pick_next_task_idle(struct rq *rq, struct task_struct *prev, struct rq_flags *rf
> >  	put_prev_task(rq, prev);
> >  	update_idle_core(rq);
> >  	schedstat_inc(rq->sched_goidle);
> > +
> > +	/* kick cpufreq (see the comment in kernel/sched/sched.h). */
> > +	cpufreq_update_this_cpu(rq, SCHED_CPUFREQ_IDLE);
> > +
> 
> This looks correct.
> 
> Can we completely avoid the utilization contribution of the CPUs which have gone
> idle? Right now we avoid them with help of (delta_ns > TICK_NSEC). Can we
> instead check this SCHED_CPUFREQ_IDLE flag ?

I would say that the blocked utilization of an IDLE CPU is still worth
to be considered, at least for a limited amount of time, for few main
reasons:

1. it represents CPU bandwidth that is likely to be required by a task
   which can wakeup in a short while. Consider for example an 80% task
   activated every 16ms: even if it's not running right now it's
   likely to wakeup in the next ~3ms to run for the following ~13ms.
   Thus, we should probably better consider that CPU utilization.

2. we already have policies to gratefully reduce the current OPP if
   its utilization decrease. This means that we are interested in a
   sort of policy which favors higher OPPs to avoid impacting
   performance of tasks which suddenly wakeup.
 
3. A CPU entering IDLE is not a great source of new information
   for OPP selection, I would not strictly bind an OPP change to this
   event. That's also why this patch propose to clear the flags
   without actually triggering an OPP change.

Moreover, maybe the issue you are trying to solve it's more related to
having a stale utilization for an IDLE CPUs?
In that case we should fix the real source of the issue, which is the
utilization of an IDLE CPU not being updated over time. But that's
outside of the scope of this series.

Cheers Patrick

-- 
#include <best/regards.h>

Patrick Bellasi

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#1682057 — Re: [PATCH v2 2/6] cpufreq: schedutil: reset sg_cpus's flags at IDLE enter

On 05-07-17, 14:04, Patrick Bellasi wrote:
> On 05-Jul 10:20, Viresh Kumar wrote:
> > And also why is it important to write 0 to sg_cpu->flags ? What wouldn't work if
> > we set sg_cpu->flags to SCHED_CPUFREQ_IDLE in this case ? i.e. Just the below
> > statement should be good for us.
> 
> Let say flags have the RT/DL flag set when the RT task sleep, is there
> any specific reason to keep this flag up while the CPU is IDLE?
> IOW, why should we care about an information related to an even which
> is now over?

Maybe I wasn't able to communicate what I wanted to say, but I am not asking you
to keep RT/DL flags as is, but rather set the flags variable to
SCHED_CPUFREQ_IDLE (1 << 3). My concerns were about adding an additional
conditional statement here, while we can live without one.

> The proposal of this patch is just meant to make sure that the flags,
> being a state variable, always describe the current status of the
> sugov "state machine".
> If a CPU is IDLE there are not sensible events going on and thus flags
> should better be reset.

or set to SCHED_CPUFREQ_IDLE.

> > This looks correct.
> > 
> > Can we completely avoid the utilization contribution of the CPUs which have gone
> > idle? Right now we avoid them with help of (delta_ns > TICK_NSEC). Can we
> > instead check this SCHED_CPUFREQ_IDLE flag ?
> 
> I would say that the blocked utilization of an IDLE CPU is still worth
> to be considered, at least for a limited amount of time, for few main
> reasons:
> 
> 1. it represents CPU bandwidth that is likely to be required by a task
>    which can wakeup in a short while. Consider for example an 80% task
>    activated every 16ms: even if it's not running right now it's
>    likely to wakeup in the next ~3ms to run for the following ~13ms.
>    Thus, we should probably better consider that CPU utilization.
> 
> 2. we already have policies to gratefully reduce the current OPP if
>    its utilization decrease. This means that we are interested in a
>    sort of policy which favors higher OPPs to avoid impacting
>    performance of tasks which suddenly wakeup.
>  
> 3. A CPU entering IDLE is not a great source of new information
>    for OPP selection, I would not strictly bind an OPP change to this
>    event. That's also why this patch propose to clear the flags
>    without actually triggering an OPP change.
> 
> Moreover, maybe the issue you are trying to solve it's more related to
> having a stale utilization for an IDLE CPUs?

I wasn't trying to solve any issue here, but just discussing about what should
we do here. Yeah it seems fair to keep the utilization of the idle CPU for
another TICK, after which we are ignoring it anyway.

-- 
viresh

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#1682907 — Re: [PATCH v2 2/6] cpufreq: schedutil: reset sg_cpus's flags at IDLE enter

On Tue, Jul 4, 2017 at 10:34 AM, Patrick Bellasi
<patrick.bellasi@arm.com> wrote:
> Currently, sg_cpu's flags are set to the value defined by the last call of
> the cpufreq_update_util()/cpufreq_update_this_cpu(); for RT/DL classes
> this corresponds to the SCHED_CPUFREQ_{RT/DL} flags always being set.
>
> When multiple CPU shares the same frequency domain it might happen that a
> CPU which executed a RT task, right before entering IDLE, has one of the
> SCHED_CPUFREQ_RT_DL flags set, permanently, until it exits IDLE.
>
> Although such an idle CPU is _going to be_ ignored by the
> sugov_next_freq_shared():
>   1. this kind of "useless RT requests" are ignored only if more then
>      TICK_NSEC have elapsed since the last update
>   2. we can still potentially trigger an already too late switch to
>      MAX, which starts also a new throttling interval
>   3. the internal state machine is not consistent with what the
>      scheduler knows, i.e. the CPU is now actually idle
>
> Thus, in sugov_next_freq_shared(), where utilisation and flags are
> aggregated across all the CPUs of a frequency domain, it can turn out
> that all the CPUs of that domain can run unnecessary at the maximum OPP
> until another event happens in the idle CPU, which eventually clear the
> SCHED_CPUFREQ_{RT/DL} flag, or the IDLE CPUs gets ignored after
> TICK_NSEC since the CPU entering IDLE.
>
> Such a behaviour can harm the energy efficiency of systems where RT
> workloads are not so frequent and other CPUs in the same frequency
> domain are running small utilisation workloads, which is a quite common
> scenario in mobile embedded systems.
>
> This patch proposes a solution which is aligned with the current principle
> to update the flags each time a scheduling event happens. The scheduling
> of the idle_task on a CPU is considered one of such meaningful events.
> That's why when the idle_task is selected for execution we poke the
> schedutil policy to reset the flags for that CPU.
>
> No frequency transitions are activated at that point, which is fair in
> case the RT workload should come back in the future. However, this still
> allows other CPUs in the same frequency domain to scale down the
> frequency in case that should be possible.
>
> Signed-off-by: Patrick Bellasi <patrick.bellasi@arm.com>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> Cc: Viresh Kumar <viresh.kumar@linaro.org>
> Cc: linux-kernel@vger.kernel.org
> Cc: linux-pm@vger.kernel.org
>
> ---
> Changes from v1:
> - added "unlikely()" around the statement (SteveR)
> ---
>  include/linux/sched/cpufreq.h    | 1 +
>  kernel/sched/cpufreq_schedutil.c | 7 +++++++
>  kernel/sched/idle_task.c         | 4 ++++
>  3 files changed, 12 insertions(+)
>
> diff --git a/include/linux/sched/cpufreq.h b/include/linux/sched/cpufreq.h
> index d2be2cc..36ac8d2 100644
> --- a/include/linux/sched/cpufreq.h
> +++ b/include/linux/sched/cpufreq.h
> @@ -10,6 +10,7 @@
>  #define SCHED_CPUFREQ_RT       (1U << 0)
>  #define SCHED_CPUFREQ_DL       (1U << 1)
>  #define SCHED_CPUFREQ_IOWAIT   (1U << 2)
> +#define SCHED_CPUFREQ_IDLE     (1U << 3)
>
>  #define SCHED_CPUFREQ_RT_DL    (SCHED_CPUFREQ_RT | SCHED_CPUFREQ_DL)
>
> diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> index eaba6d6..004ae18 100644
> --- a/kernel/sched/cpufreq_schedutil.c
> +++ b/kernel/sched/cpufreq_schedutil.c
> @@ -304,6 +304,12 @@ static void sugov_update_shared(struct update_util_data *hook, u64 time,
>
>         sg_cpu->util = util;
>         sg_cpu->max = max;
> +
> +       /* CPU is entering IDLE, reset flags without triggering an update */
> +       if (unlikely(flags & SCHED_CPUFREQ_IDLE)) {
> +               sg_cpu->flags = 0;
> +               goto done;
> +       }

Instead of defining a new flag for idle, wouldn't another way be to
just clear the flag from the RT scheduling class with an extra call to
cpufreq_update_util with flags = 0 during dequeue_rt_entity? That
seems to me to be also the right place to clear the flag since the
flag is set in the corresponding class to begin with.

thanks,

-Joel

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#1683105 — Re: [PATCH v2 2/6] cpufreq: schedutil: reset sg_cpus's flags at IDLE enter

On 06/07/17 21:43, Joel Fernandes wrote:
> On Tue, Jul 4, 2017 at 10:34 AM, Patrick Bellasi
> <patrick.bellasi@arm.com> wrote:

[...]

> > @@ -304,6 +304,12 @@ static void sugov_update_shared(struct update_util_data *hook, u64 time,
> >
> >         sg_cpu->util = util;
> >         sg_cpu->max = max;
> > +
> > +       /* CPU is entering IDLE, reset flags without triggering an update */
> > +       if (unlikely(flags & SCHED_CPUFREQ_IDLE)) {
> > +               sg_cpu->flags = 0;
> > +               goto done;
> > +       }
> 
> Instead of defining a new flag for idle, wouldn't another way be to
> just clear the flag from the RT scheduling class with an extra call to
> cpufreq_update_util with flags = 0 during dequeue_rt_entity? That
> seems to me to be also the right place to clear the flag since the
> flag is set in the corresponding class to begin with.
> 

Make sense to me too. Also considering that for DL (with my patches) we
don't generally want to clear the flag at dequeue time, but only when
the 0-lag timer fires.

Best,

- Juri

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#1685305 — Re: [PATCH v2 2/6] cpufreq: schedutil: reset sg_cpus's flags at IDLE enter

On 07/07/2017 03:17 AM, Juri Lelli wrote:
> On 06/07/17 21:43, Joel Fernandes wrote:
>> On Tue, Jul 4, 2017 at 10:34 AM, Patrick Bellasi
>> <patrick.bellasi@arm.com> wrote:
>
> [...]
>
>>> @@ -304,6 +304,12 @@ static void sugov_update_shared(struct update_util_data *hook, u64 time,
>>>
>>>          sg_cpu->util = util;
>>>          sg_cpu->max = max;
>>> +
>>> +       /* CPU is entering IDLE, reset flags without triggering an update */
>>> +       if (unlikely(flags & SCHED_CPUFREQ_IDLE)) {
>>> +               sg_cpu->flags = 0;
>>> +               goto done;
>>> +       }
>>
>> Instead of defining a new flag for idle, wouldn't another way be to
>> just clear the flag from the RT scheduling class with an extra call to
>> cpufreq_update_util with flags = 0 during dequeue_rt_entity? That
>> seems to me to be also the right place to clear the flag since the
>> flag is set in the corresponding class to begin with.
>>
>
> Make sense to me too. Also considering that for DL (with my patches) we
> don't generally want to clear the flag at dequeue time, but only when
> the 0-lag timer fires.
>

Makes sense to me too.

-Saravana


-- 
Qualcomm Innovation Center, Inc.
The Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
a Linux Foundation Collaborative Project

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#1681110 — [PATCH v2 5/6] sched/rt: fast switch to maximum frequency when RT tasks are scheduled

Currently schedutil updates are triggered for the RT class using a single
call place, which is part of the rt::update_curr_rt() used in:

- dequeue_task_rt:
  but it does not make sense to set the schedutil's SCHED_CPUFREQ_RT in
  case the next task should not be an RT one

- put_prev_task_rt:
  likewise, we set the SCHED_CPUFREQ_RT flag without knowing if required
  by the next task

- pick_next_task_rt:
  likewise, the schedutil's SCHED_CPUFREQ_RT is set in case the prev task
  was RT, while we don't yet know if the next will be RT

- task_tick_rt:
  that's the only really useful call, which can ramp up the frequency in
  case a RT task started its execution without a chance to order a
  frequency switch (e.g. because of the schedutil ratelimit)

Apart from the last call in task_tick_rt, the others are at least useless.
Thus, although being a simple solution, not all the call sites of that
update_curr_rt() are interesting to trigger a frequency switch as well as
some of the most interesting points are not covered by that call.
For example, a task set to RT has to wait the next tick to get the
frequency boost.

This patch fixes these issues by placing explicitly the schedutils
update calls in the only sensible places, which are:
- when an RT task wakeups and it's enqueued in a CPU
- when we actually pick a RT task for execution
- at each tick time
- when a task is set to be RT

Signed-off-by: Patrick Bellasi <patrick.bellasi@arm.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Cc: Viresh Kumar <viresh.kumar@linaro.org>
Cc: linux-kernel@vger.kernel.org
Cc: linux-pm@vger.kernel.org
---
 kernel/sched/rt.c | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index 45caf93..8c25e95 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -969,9 +969,6 @@ static void update_curr_rt(struct rq *rq)
 	if (unlikely((s64)delta_exec <= 0))
 		return;
 
-	/* Kick cpufreq (see the comment in kernel/sched/sched.h). */
-	cpufreq_update_this_cpu(rq, SCHED_CPUFREQ_RT);
-
 	schedstat_set(curr->se.statistics.exec_max,
 		      max(curr->se.statistics.exec_max, delta_exec));
 
@@ -1337,6 +1334,9 @@ enqueue_task_rt(struct rq *rq, struct task_struct *p, int flags)
 
 	if (!task_current(rq, p) && p->nr_cpus_allowed > 1)
 		enqueue_pushable_task(rq, p);
+
+	/* Kick cpufreq (see the comment in kernel/sched/sched.h). */
+	cpufreq_update_this_cpu(rq, SCHED_CPUFREQ_RT);
 }
 
 static void dequeue_task_rt(struct rq *rq, struct task_struct *p, int flags)
@@ -1574,6 +1574,9 @@ pick_next_task_rt(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
 
 	p = _pick_next_task_rt(rq);
 
+	/* Kick cpufreq (see the comment in kernel/sched/sched.h). */
+	cpufreq_update_this_cpu(rq, SCHED_CPUFREQ_RT);
+
 	/* The running task is never eligible for pushing */
 	dequeue_pushable_task(rq, p);
 
@@ -2367,6 +2370,9 @@ static void task_tick_rt(struct rq *rq, struct task_struct *p, int queued)
 {
 	struct sched_rt_entity *rt_se = &p->rt;
 
+	/* Kick cpufreq (see the comment in kernel/sched/sched.h). */
+	cpufreq_update_this_cpu(rq, SCHED_CPUFREQ_RT);
+
 	update_curr_rt(rq);
 
 	watchdog(rq, p);
@@ -2402,6 +2408,9 @@ static void set_curr_task_rt(struct rq *rq)
 
 	p->se.exec_start = rq_clock_task(rq);
 
+	/* Kick cpufreq (see the comment in kernel/sched/sched.h). */
+	cpufreq_update_this_cpu(rq, SCHED_CPUFREQ_RT);
+
 	/* The running task is never eligible for pushing */
 	dequeue_pushable_task(rq, p);
 }
-- 
2.7.4

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#1681111 — [PATCH v2 6/6] cpufreq: schedutil: relax rate-limiting while running RT/DL tasks

The policy in use for RT/DL tasks sets the maximum frequency when a task
in these classes calls for a cpufreq_update_this_cpu().  However, the
current implementation is still enforcing a frequency switch rate
limiting when these tasks are running.
This is potentially working against the goal to switch to the maximum OPP
when RT tasks are running. In certain unfortunate cases it can also happen
that a RT task almost completes its activation at a lower OPP.

This patch overrides on purpose the rate limiting configuration
to better serve RT/DL tasks. As long as a frequency scaling operation
is not in progress, a frequency switch is always authorized when
running in "rt_mode", i.e. the current task in a CPU belongs to the
RT/DL class.

Signed-off-by: Patrick Bellasi <patrick.bellasi@arm.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Cc: Viresh Kumar <viresh.kumar@linaro.org>
Cc: linux-kernel@vger.kernel.org
Cc: linux-pm@vger.kernel.org
---
 kernel/sched/cpufreq_schedutil.c | 19 ++++++++++++-------
 1 file changed, 12 insertions(+), 7 deletions(-)

diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
index df433f1..7b1dc7e 100644
--- a/kernel/sched/cpufreq_schedutil.c
+++ b/kernel/sched/cpufreq_schedutil.c
@@ -72,7 +72,8 @@ static DEFINE_PER_CPU(struct sugov_cpu, sugov_cpu);
 
 /************************ Governor internals ***********************/
 
-static bool sugov_should_update_freq(struct sugov_policy *sg_policy, u64 time)
+static bool sugov_should_update_freq(struct sugov_policy *sg_policy,
+				     u64 time, bool rt_mode)
 {
 	s64 delta_ns;
 
@@ -89,6 +90,10 @@ static bool sugov_should_update_freq(struct sugov_policy *sg_policy, u64 time)
 		return true;
 	}
 
+	/* Always update if a RT/DL task is running */
+	if (rt_mode)
+		return true;
+
 	delta_ns = time - sg_policy->last_freq_update_time;
 	return delta_ns >= sg_policy->freq_update_delay_ns;
 }
@@ -226,11 +231,6 @@ static void sugov_update_single(struct update_util_data *hook, u64 time,
 	sugov_set_iowait_boost(sg_cpu, time, flags);
 	sg_cpu->last_update = time;
 
-	if (!sugov_should_update_freq(sg_policy, time))
-		return;
-
-	busy = sugov_cpu_is_busy(sg_cpu);
-
 	/*
 	 * While RT/DL tasks are running we do not want FAIR tasks to
 	 * overvrite this CPU's flags, still we can update utilization and
@@ -239,6 +239,11 @@ static void sugov_update_single(struct update_util_data *hook, u64 time,
 	rt_mode = task_has_dl_policy(current) ||
 		  task_has_rt_policy(current) ||
 		  (flags & SCHED_CPUFREQ_RT_DL);
+	if (!sugov_should_update_freq(sg_policy, time, rt_mode))
+		return;
+
+	busy = sugov_cpu_is_busy(sg_cpu);
+
 	if (rt_mode) {
 		next_f = policy->cpuinfo.max_freq;
 	} else {
@@ -336,7 +341,7 @@ static void sugov_update_shared(struct update_util_data *hook, u64 time,
 	sugov_set_iowait_boost(sg_cpu, time, flags);
 	sg_cpu->last_update = time;
 
-	if (sugov_should_update_freq(sg_policy, time)) {
+	if (sugov_should_update_freq(sg_policy, time, rt_mode)) {
 		next_f = rt_mode
 			? sg_policy->policy->cpuinfo.max_freq
 			: sugov_next_freq_shared(sg_cpu, time);
-- 
2.7.4

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#1682776

On Tuesday, July 04, 2017 06:34:05 PM Patrick Bellasi wrote:
> Each time a CPU utilisation update is issued by the scheduler a flag, which
> mainly defines which scheduling class is asking for the update, is used by the
> frequency selection policy to support the selection of the most appropriate
> OPP.
>  
> In the current implementation, CPU flags are overridden each time the scheduler
> calls schedutil for an update. Such a behavior seems to be sub-optimal,
> especially on systems where frequency domains span across multiple CPUs.
>  
> Indeed, assuming CPU1 and CPU2 share the same frequency domain, there can be
> the following issues:
>  
>  A) Small FAIR task running at MAX OPP.
>     A RT task, which just executed on CPU1, can keep the domain at the
>     max frequency for a prolonged period of time after its completion,
>     even if there are no longer RT tasks running on CPUs of its domain.
>  
>  B) FAIR wakeup reducing the OPP of the current RT task.
>     A FAIR task enqueued in a CPU where a RT task is running overrides the flag
>     configured by the RT task thus potentially causing an unwanted frequency
>     drop.
>  
>  C) RT wakeup not running at max OPP.
>     An RT task waking up on a CPU which has recently updated its OPP can
>     be forced to run at a lower frequency because of the throttling
>     enforced by schedutil, even if there are not OPP transitions
>     currently in progress.
> 
> .:: Patches organization
> ========================
>  
> This series proposes a set of fixes for the aforementioned issues and it's an
> update addressing all the main comments collected from the previous posting
> [1].

It seems to me that there is a nonzero overlap between this and the Juri's work.

If that's correct, I'd like this series to go on top of the Juri's one.

Thanks,
Rafael

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