Remove the unconditional binding of sugov kthreads to the affected CPUs
if the cpufreq driver indicates that updates can happen from any CPU.
This allows userspace to set affinities to either save power (waking up
bigger CPUs on HMP can be expensive) or increasing performance (by
letting the utilized CPUs run without preemption of the sugov kthread).

Without this patch the behavior of sugov threads will basically be a
boot-time dice roll on which CPU of the PD has to handle all the
cpufreq updates. With the recent decreases of update filtering these
two basic problems become more and more apparent:
1. The wake_cpu might be idle and we are waking it up from another
CPU just for the cpufreq update. Apart from wasting power, the exit
latency of it's idle state might be longer than the sugov threads
running time, essentially delaying the cpufreq update unnecessarily.
2. We are preempting either the requesting or another busy CPU of the
PD, while the update could be done from a CPU that we deem less
important and pay the price of an IPI and two context-switches.

The change is essentially not setting PF_NO_SETAFFINITY on
dvfs_possible_from_any_cpu, no behavior change if userspace doesn't
touch affinities.

Signed-off-by: Christian Loehle <christian.loehle@arm.com>
---
 kernel/sched/cpufreq_schedutil.c | 6 +++++-
 kernel/sched/syscalls.c          | 3 +++
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
index 43111a515a28..466fb79e0b81 100644
--- a/kernel/sched/cpufreq_schedutil.c
+++ b/kernel/sched/cpufreq_schedutil.c
@@ -683,7 +683,11 @@ static int sugov_kthread_create(struct sugov_policy *sg_policy)
 	}
 
 	sg_policy->thread = thread;
-	kthread_bind_mask(thread, policy->related_cpus);
+	if (policy->dvfs_possible_from_any_cpu)
+		set_cpus_allowed_ptr(thread, policy->related_cpus);
+	else
+		kthread_bind_mask(thread, policy->related_cpus);
+
 	init_irq_work(&sg_policy->irq_work, sugov_irq_work);
 	mutex_init(&sg_policy->work_lock);
 
diff --git a/kernel/sched/syscalls.c b/kernel/sched/syscalls.c
index c62acf509b74..7d4a4edfcfb9 100644
--- a/kernel/sched/syscalls.c
+++ b/kernel/sched/syscalls.c
@@ -1159,6 +1159,9 @@ int dl_task_check_affinity(struct task_struct *p, const struct cpumask *mask)
 	if (!task_has_dl_policy(p) || !dl_bandwidth_enabled())
 		return 0;
 
+	if (dl_entity_is_special(&p->dl))
+		return 0;
+
 	/*
 	 * Since bandwidth control happens on root_domain basis,
 	 * if admission test is enabled, we only admit -deadline
-- 
2.34.1

On 9/12/24 14:53, Christian Loehle wrote:
> Remove the unconditional binding of sugov kthreads to the affected CPUs
> if the cpufreq driver indicates that updates can happen from any CPU.
> This allows userspace to set affinities to either save power (waking up
> bigger CPUs on HMP can be expensive) or increasing performance (by
> letting the utilized CPUs run without preemption of the sugov kthread).
> 
> Without this patch the behavior of sugov threads will basically be a
> boot-time dice roll on which CPU of the PD has to handle all the
> cpufreq updates. With the recent decreases of update filtering these
> two basic problems become more and more apparent:
> 1. The wake_cpu might be idle and we are waking it up from another
> CPU just for the cpufreq update. Apart from wasting power, the exit
> latency of it's idle state might be longer than the sugov threads
> running time, essentially delaying the cpufreq update unnecessarily.
> 2. We are preempting either the requesting or another busy CPU of the
> PD, while the update could be done from a CPU that we deem less
> important and pay the price of an IPI and two context-switches.
> 
> The change is essentially not setting PF_NO_SETAFFINITY on
> dvfs_possible_from_any_cpu, no behavior change if userspace doesn't
> touch affinities.
> 
> Signed-off-by: Christian Loehle <christian.loehle@arm.com>

I'll add some numbers to illustrate, although the example might not be
particularly realistic.
The classic fio workload will trigger cpufreq update very often, so
I used that on the Pixel6, affinity set to CPU7 (bitmask 80)(Big PD is
[6,7]).
Without this patch we have either all sugov enqueues on CPU6 or CPU7,
depending on where the first CPU frequency request (since boot) was
issued from (the deadline select_task_rq is rather simple, so it
will just wake_cpu if that is still valid, which here it always is).

I set different affinities for the sugov:6 worker and annotate
IOPS (throughput) and power (mW average), the test is for 30s each.
cpumask IOPS
80	7477
	888.3 mW	{'CPU-Big': 742.5668631414397, 'CPU-Mid': 11.919500003712095, 'CPU-Little': 133.79554163073317}
40	7378
	942.8 mW	{'CPU-Big': 797.4037245094225, 'CPU-Mid': 12.440698878099667, 'CPU-Little': 132.98899390286172}
f	7469
	873.7 mW	{'CPU-Big': 718.2933574826287, 'CPU-Mid': 11.89176754939742, 'CPU-Little': 143.55634276873246}
2	7501
	872.8 mW	{'CPU-Big': 718.6036614909397, 'CPU-Mid': 11.711731623773632, 'CPU-Little': 142.4830173663132}
1	7392
	859.5 mW	{'CPU-Big': 704.926078017567, 'CPU-Mid': 12.196892652625284, 'CPU-Little': 142.4231989686622}

Throughput is somewhat comparable for all, anyway the frequency only
bounces from capacity 1024 to 512 because of the instability of
iowait boost.
For 40 (CPU6) we see significantly more power usage, as the sugov
will prevent CPU6 from power down, as it gets woken up by the sugov
worker often.
f,2 and 1 affinities have slightly higher power coming from the
littles (as expected), but significantly less power from the bigs,
since they don't have the double context-switch on preemption and
the actual sugov CPU cycles.
1 has by far the least power, as it's always in WFI anyway, it
handles the IO interrupts from fio on CPU7.

Granted that is a somewhat worst-case scenario just to illustrate
the problem.

I also have a patch for preferring wake_cpu = smp_processor_id() for
the sugov worker, which is somewhat adjacent to this, the numbers
above make a case for that even without touching affinities.
I thought this might be less controversial for now.

On Thu, Sep 12, 2024 at 3:53 PM Christian Loehle
<christian.loehle@arm.com> wrote:
>
> Remove the unconditional binding of sugov kthreads to the affected CPUs
> if the cpufreq driver indicates that updates can happen from any CPU.
> This allows userspace to set affinities to either save power (waking up
> bigger CPUs on HMP can be expensive) or increasing performance (by
> letting the utilized CPUs run without preemption of the sugov kthread).
>
> Without this patch the behavior of sugov threads will basically be a
> boot-time dice roll on which CPU of the PD has to handle all the
> cpufreq updates. With the recent decreases of update filtering these
> two basic problems become more and more apparent:
> 1. The wake_cpu might be idle and we are waking it up from another
> CPU just for the cpufreq update. Apart from wasting power, the exit
> latency of it's idle state might be longer than the sugov threads
> running time, essentially delaying the cpufreq update unnecessarily.
> 2. We are preempting either the requesting or another busy CPU of the
> PD, while the update could be done from a CPU that we deem less
> important and pay the price of an IPI and two context-switches.
>
> The change is essentially not setting PF_NO_SETAFFINITY on
> dvfs_possible_from_any_cpu, no behavior change if userspace doesn't
> touch affinities.

I'd like to hear from Viresh on this.

> Signed-off-by: Christian Loehle <christian.loehle@arm.com>
> ---
>  kernel/sched/cpufreq_schedutil.c | 6 +++++-
>  kernel/sched/syscalls.c          | 3 +++
>  2 files changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> index 43111a515a28..466fb79e0b81 100644
> --- a/kernel/sched/cpufreq_schedutil.c
> +++ b/kernel/sched/cpufreq_schedutil.c
> @@ -683,7 +683,11 @@ static int sugov_kthread_create(struct sugov_policy *sg_policy)
>         }
>
>         sg_policy->thread = thread;
> -       kthread_bind_mask(thread, policy->related_cpus);
> +       if (policy->dvfs_possible_from_any_cpu)
> +               set_cpus_allowed_ptr(thread, policy->related_cpus);
> +       else
> +               kthread_bind_mask(thread, policy->related_cpus);
> +
>         init_irq_work(&sg_policy->irq_work, sugov_irq_work);
>         mutex_init(&sg_policy->work_lock);
>
> diff --git a/kernel/sched/syscalls.c b/kernel/sched/syscalls.c
> index c62acf509b74..7d4a4edfcfb9 100644
> --- a/kernel/sched/syscalls.c
> +++ b/kernel/sched/syscalls.c
> @@ -1159,6 +1159,9 @@ int dl_task_check_affinity(struct task_struct *p, const struct cpumask *mask)
>         if (!task_has_dl_policy(p) || !dl_bandwidth_enabled())
>                 return 0;
>
> +       if (dl_entity_is_special(&p->dl))
> +               return 0;
> +

Care to explain this particular piece?

>         /*
>          * Since bandwidth control happens on root_domain basis,
>          * if admission test is enabled, we only admit -deadline
> --

On 12-09-24, 17:41, Rafael J. Wysocki wrote:
> On Thu, Sep 12, 2024 at 3:53 PM Christian Loehle
> <christian.loehle@arm.com> wrote:
> >
> > Remove the unconditional binding of sugov kthreads to the affected CPUs
> > if the cpufreq driver indicates that updates can happen from any CPU.
> > This allows userspace to set affinities to either save power (waking up
> > bigger CPUs on HMP can be expensive) or increasing performance (by
> > letting the utilized CPUs run without preemption of the sugov kthread).
> >
> > Without this patch the behavior of sugov threads will basically be a
> > boot-time dice roll on which CPU of the PD has to handle all the
> > cpufreq updates. With the recent decreases of update filtering these
> > two basic problems become more and more apparent:
> > 1. The wake_cpu might be idle and we are waking it up from another
> > CPU just for the cpufreq update. Apart from wasting power, the exit
> > latency of it's idle state might be longer than the sugov threads
> > running time, essentially delaying the cpufreq update unnecessarily.
> > 2. We are preempting either the requesting or another busy CPU of the
> > PD, while the update could be done from a CPU that we deem less
> > important and pay the price of an IPI and two context-switches.
> >
> > The change is essentially not setting PF_NO_SETAFFINITY on
> > dvfs_possible_from_any_cpu, no behavior change if userspace doesn't
> > touch affinities.
> 
> I'd like to hear from Viresh on this.

Looks good to me.

Acked-by: Viresh Kumar <viresh.kumar@linaro.org>

-- 
viresh

On 9/12/24 16:41, Rafael J. Wysocki wrote:
> On Thu, Sep 12, 2024 at 3:53 PM Christian Loehle
> <christian.loehle@arm.com> wrote:
>>
>> Remove the unconditional binding of sugov kthreads to the affected CPUs
>> if the cpufreq driver indicates that updates can happen from any CPU.
>> This allows userspace to set affinities to either save power (waking up
>> bigger CPUs on HMP can be expensive) or increasing performance (by
>> letting the utilized CPUs run without preemption of the sugov kthread).
>>
>> Without this patch the behavior of sugov threads will basically be a
>> boot-time dice roll on which CPU of the PD has to handle all the
>> cpufreq updates. With the recent decreases of update filtering these
>> two basic problems become more and more apparent:
>> 1. The wake_cpu might be idle and we are waking it up from another
>> CPU just for the cpufreq update. Apart from wasting power, the exit
>> latency of it's idle state might be longer than the sugov threads
>> running time, essentially delaying the cpufreq update unnecessarily.
>> 2. We are preempting either the requesting or another busy CPU of the
>> PD, while the update could be done from a CPU that we deem less
>> important and pay the price of an IPI and two context-switches.
>>
>> The change is essentially not setting PF_NO_SETAFFINITY on
>> dvfs_possible_from_any_cpu, no behavior change if userspace doesn't
>> touch affinities.
> 
> I'd like to hear from Viresh on this.
> 
>> Signed-off-by: Christian Loehle <christian.loehle@arm.com>
>> ---
>>  kernel/sched/cpufreq_schedutil.c | 6 +++++-
>>  kernel/sched/syscalls.c          | 3 +++
>>  2 files changed, 8 insertions(+), 1 deletion(-)
>>
>> diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
>> index 43111a515a28..466fb79e0b81 100644
>> --- a/kernel/sched/cpufreq_schedutil.c
>> +++ b/kernel/sched/cpufreq_schedutil.c
>> @@ -683,7 +683,11 @@ static int sugov_kthread_create(struct sugov_policy *sg_policy)
>>         }
>>
>>         sg_policy->thread = thread;
>> -       kthread_bind_mask(thread, policy->related_cpus);
>> +       if (policy->dvfs_possible_from_any_cpu)
>> +               set_cpus_allowed_ptr(thread, policy->related_cpus);
>> +       else
>> +               kthread_bind_mask(thread, policy->related_cpus);
>> +
>>         init_irq_work(&sg_policy->irq_work, sugov_irq_work);
>>         mutex_init(&sg_policy->work_lock);
>>
>> diff --git a/kernel/sched/syscalls.c b/kernel/sched/syscalls.c
>> index c62acf509b74..7d4a4edfcfb9 100644
>> --- a/kernel/sched/syscalls.c
>> +++ b/kernel/sched/syscalls.c
>> @@ -1159,6 +1159,9 @@ int dl_task_check_affinity(struct task_struct *p, const struct cpumask *mask)
>>         if (!task_has_dl_policy(p) || !dl_bandwidth_enabled())
>>                 return 0;
>>
>> +       if (dl_entity_is_special(&p->dl))
>> +               return 0;
>> +
> 
> Care to explain this particular piece?

Looks suspicious but the truncated comment below explains it:
	/*
	 * Since bandwidth control happens on root_domain basis,
	 * if admission test is enabled, we only admit -deadline
	 * tasks allowed to run on all the CPUs in the task's
	 * root_domain.
	 */
So that would only allow setting it to all CPUs for the relevant
platforms unfortunately.

That should be fine though since the sugov task is pretty much
a dummy in terms of bandwidth / admission control internally, so
no harm done to not enforce this when userspace wants to set
affinities.
...Unless Juri disagrees.

> 
>>         /*
>>          * Since bandwidth control happens on root_domain basis,
>>          * if admission test is enabled, we only admit -deadline
>> --

Hi,

On 12/09/24 17:01, Christian Loehle wrote:
> On 9/12/24 16:41, Rafael J. Wysocki wrote:
> > On Thu, Sep 12, 2024 at 3:53 PM Christian Loehle

...

> >> diff --git a/kernel/sched/syscalls.c b/kernel/sched/syscalls.c
> >> index c62acf509b74..7d4a4edfcfb9 100644
> >> --- a/kernel/sched/syscalls.c
> >> +++ b/kernel/sched/syscalls.c
> >> @@ -1159,6 +1159,9 @@ int dl_task_check_affinity(struct task_struct *p, const struct cpumask *mask)
> >>         if (!task_has_dl_policy(p) || !dl_bandwidth_enabled())
> >>                 return 0;
> >>
> >> +       if (dl_entity_is_special(&p->dl))
> >> +               return 0;
> >> +
> > 
> > Care to explain this particular piece?
> 
> Looks suspicious but the truncated comment below explains it:
> 	/*
> 	 * Since bandwidth control happens on root_domain basis,
> 	 * if admission test is enabled, we only admit -deadline
> 	 * tasks allowed to run on all the CPUs in the task's
> 	 * root_domain.
> 	 */
> So that would only allow setting it to all CPUs for the relevant
> platforms unfortunately.
> 
> That should be fine though since the sugov task is pretty much
> a dummy in terms of bandwidth / admission control internally, so
> no harm done to not enforce this when userspace wants to set
> affinities.
> ...Unless Juri disagrees.

Nope, I agree. :)

Wonder if we should put a comment along the lines of what you said above
right above the new conditions (so that people will not need to wonder
about it in the future). But not a strict requirement for me.

Thanks! (and apologies for the delay in replying)
Juri

On 9/25/24 09:14, Juri Lelli wrote:
> Hi,
> 
> On 12/09/24 17:01, Christian Loehle wrote:
>> On 9/12/24 16:41, Rafael J. Wysocki wrote:
>>> On Thu, Sep 12, 2024 at 3:53 PM Christian Loehle
> 
> ...
> 
>>>> diff --git a/kernel/sched/syscalls.c b/kernel/sched/syscalls.c
>>>> index c62acf509b74..7d4a4edfcfb9 100644
>>>> --- a/kernel/sched/syscalls.c
>>>> +++ b/kernel/sched/syscalls.c
>>>> @@ -1159,6 +1159,9 @@ int dl_task_check_affinity(struct task_struct *p, const struct cpumask *mask)
>>>>         if (!task_has_dl_policy(p) || !dl_bandwidth_enabled())
>>>>                 return 0;
>>>>
>>>> +       if (dl_entity_is_special(&p->dl))
>>>> +               return 0;
>>>> +
>>>
>>> Care to explain this particular piece?
>>
>> Looks suspicious but the truncated comment below explains it:
>> 	/*
>> 	 * Since bandwidth control happens on root_domain basis,
>> 	 * if admission test is enabled, we only admit -deadline
>> 	 * tasks allowed to run on all the CPUs in the task's
>> 	 * root_domain.
>> 	 */
>> So that would only allow setting it to all CPUs for the relevant
>> platforms unfortunately.
>>
>> That should be fine though since the sugov task is pretty much
>> a dummy in terms of bandwidth / admission control internally, so
>> no harm done to not enforce this when userspace wants to set
>> affinities.
>> ...Unless Juri disagrees.
> 
> Nope, I agree. :)
> 
> Wonder if we should put a comment along the lines of what you said above
> right above the new conditions (so that people will not need to wonder
> about it in the future). But not a strict requirement for me.
> 
> Thanks! (and apologies for the delay in replying)
> Juri

Thank you Juri for taking a look. I agree with the comment, will
do a v2.
Regards,
Christian