linux-kernel - Re: [PATCH] cpufreq/schedutil: Only bind threads if needed

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Message-ID: <ac4c9060-e447-46da-9f37-167864a7906f@arm.com>
Date: Thu, 12 Sep 2024 17:01:33 +0100
From: Christian Loehle <christian.loehle@....com>
To: "Rafael J. Wysocki" <rafael@...nel.org>
Cc: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 linux-pm <linux-pm@...r.kernel.org>, Qais Yousef <qyousef@...alina.io>,
 Juri Lelli <juri.lelli@...hat.com>, Ingo Molnar <mingo@...hat.com>,
 Viresh Kumar <viresh.kumar@...aro.org>,
 Dietmar Eggemann <dietmar.eggemann@....com>,
 Pierre Gondois <pierre.gondois@....com>
Subject: Re: [PATCH] cpufreq/schedutil: Only bind threads if needed

On 9/12/24 16:41, Rafael J. Wysocki wrote:
> On Thu, Sep 12, 2024 at 3:53 PM Christian Loehle
> <christian.loehle@....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@....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
>> --