linux-kernel - Re: [PATCH 2/2] cpufreq: Update CPU capacity reduction in store_scaling_max

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Date:   Mon, 10 Oct 2022 11:32:28 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Lukasz Luba <lukasz.luba@....com>
Cc:     Viresh Kumar <viresh.kumar@...aro.org>,
        linux-kernel@...r.kernel.org, rafael@...nel.org,
        linux-pm@...r.kernel.org, Dietmar.Eggemann@....com,
        peterz@...radead.org
Subject: Re: [PATCH 2/2] cpufreq: Update CPU capacity reduction in store_scaling_max_freq()

On Mon, 10 Oct 2022 at 11:30, Lukasz Luba <lukasz.luba@....com> wrote:
>
>
>
> On 10/10/22 10:15, Vincent Guittot wrote:
> > On Mon, 10 Oct 2022 at 11:02, Lukasz Luba <lukasz.luba@....com> wrote:
> >>
> >>
> >>
> >> On 10/10/22 06:39, Viresh Kumar wrote:
> >>> Would be good to always CC Scheduler maintainers for such a patch.
> >>
> >> Agree, I'll do that.
> >>
> >>>
> >>> On 30-09-22, 10:48, Lukasz Luba wrote:
> >>>> When the new max frequency value is stored, the task scheduler must
> >>>> know about it. The scheduler uses the CPUs capacity information in the
> >>>> task placement. Use the existing mechanism which provides information
> >>>> about reduced CPU capacity to the scheduler due to thermal capping.
> >>>>
> >>>> Signed-off-by: Lukasz Luba <lukasz.luba@....com>
> >>>> ---
> >>>>    drivers/cpufreq/cpufreq.c | 18 +++++++++++++++++-
> >>>>    1 file changed, 17 insertions(+), 1 deletion(-)
> >>>>
> >>>> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> >>>> index 1f8b93f42c76..205d9ea9c023 100644
> >>>> --- a/drivers/cpufreq/cpufreq.c
> >>>> +++ b/drivers/cpufreq/cpufreq.c
> >>>> @@ -27,6 +27,7 @@
> >>>>    #include <linux/slab.h>
> >>>>    #include <linux/suspend.h>
> >>>>    #include <linux/syscore_ops.h>
> >>>> +#include <linux/thermal.h>
> >>>>    #include <linux/tick.h>
> >>>>    #include <linux/units.h>
> >>>>    #include <trace/events/power.h>
> >>>> @@ -718,6 +719,8 @@ static ssize_t show_scaling_cur_freq(struct cpufreq_policy *policy, char *buf)
> >>>>    static ssize_t store_scaling_max_freq
> >>>>    (struct cpufreq_policy *policy, const char *buf, size_t count)
> >>>>    {
> >>>> +    unsigned int frequency;
> >>>> +    struct cpumask *cpus;
> >>>>       unsigned long val;
> >>>>       int ret;
> >>>>
> >>>> @@ -726,7 +729,20 @@ static ssize_t store_scaling_max_freq
> >>>>               return -EINVAL;
> >>>>
> >>>>       ret = freq_qos_update_request(policy->max_freq_req, val);
> >>>> -    return ret >= 0 ? count : ret;
> >>>> +    if (ret >= 0) {
> >>>> +            /*
> >>>> +             * Make sure that the task scheduler sees these CPUs
> >>>> +             * capacity reduction. Use the thermal pressure mechanism
> >>>> +             * to propagate this information to the scheduler.
> >>>> +             */
> >>>> +            cpus = policy->related_cpus;
> >>>
> >>> No need of this, just use related_cpus directly.
> >>>
> >>>> +            frequency = __resolve_freq(policy, val, CPUFREQ_RELATION_HE);
> >>>> +            arch_update_thermal_pressure(cpus, frequency);
> >>>
> >>> I wonder if using the thermal-pressure API here is the right thing to
> >>> do. It is a change coming from User, which may or may not be
> >>> thermal-related.
> >>
> >> Yes, I thought the same. Thermal-pressure name might be not the
> >> best for covering this use case. I have been thinking about this
> >> thermal pressure mechanism for a while, since there are other
> >> use cases like PowerCap DTPM which also reduces CPU capacity
> >> because of power policy from user-space. We don't notify
> >> the scheduler about it. There might be also an issue with virtual
> >> guest OS and how that kernel 'sees' the capacity of CPUs.
> >> We might try to use this 'thermal-pressure' in the guest kernel
> >> to notify about available CPU capacity (just a proposal, not
> >> even an RFC, since we are missing requirements, but issues where
> >> discussed on LPC 2022 on ChromeOS+Android_guest)
> >
> > The User space setting scaling_max_freq is a long scale event and it
> > should be considered as a new running environnement instead of a
> > transient event. I would suggest updating the EM is and capacity orig
> > of the system in this case. Similarly, we rebuild sched_domain with a
> > cpu hotplug. scaling_max_freq interface should not be used to do any
> > kind of dynamic scaling.
>
> I tend to agree, but the EM capacity would be only used in part of EAS
> code. The whole fair.c view to the capacity_of() (RT + DL + irq +
> thermal_pressure) would be still wrong in other parts, e.g.
> select_idle_sibling() and load balance.
>
> When we get this powerhint we might be already in overutilied state
> so EAS is disabled. IMO other mechanisms in the task scheduler
> should be also aware of that capacity reduction.

That's why I also mentioned the capacity_orig