Date:   Mon, 10 Oct 2022 14:21:11 +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,
        "daniel.lezcano@...aro.org" <daniel.lezcano@...aro.org>
Subject: Re: [PATCH 2/2] cpufreq: Update CPU capacity reduction in store_scaling_max_freq()

On Mon, 10 Oct 2022 at 12:49, Lukasz Luba <lukasz.luba@....com> wrote:
>
>
> +CC Daniel
>
> On 10/10/22 11:22, Vincent Guittot wrote:
> > On Mon, 10 Oct 2022 at 12:12, Lukasz Luba <lukasz.luba@....com> wrote:
> >>
> >>
> >>
> >> On 10/10/22 10:32, Vincent Guittot wrote:
> >>> 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
> >>
> >> Well, I think this is a bit more complex. Thermal framework governor
> >> reduces the perf IDs from top in the freq asc table and keeps that
> >> in the statistics in sysfs. It also updates the thermal pressure signal.
> >> When we rebuild the capacity of CPUs and make the capacity_orig smaller,
> >> the capacity_of would still have the thermal framework reduced capacity
> >> in there. We would end up with too small CPU capacity due to this
> >> subtraction in capacity_of.
> >
> > That's why using user space interface should not be used to do dynamic scaling.
> > I still think that user space interface is not the right interface
> >
> >>
> >> Ideally, I would see a mechanism which is aware of this performance
> >> reduction reason:
> >> 1. thermal capping
> >> 2. power capping (from DTPM)
> >> 3. max freq reduction by user space
> >
> > Yes for thermal and power capping  but no for user space
> >
> >>
> >> That common place would figure and maintain the context for the
> >> requested capacity reduction.
> >>
> >> BTW, those Android user space max freq requests are not that long,
> >> mostly due to camera capturing (you can see a few in this file,
> >> e.g. [1]).
> >
> > Why are they doing this ?
> > This doesn't seem to be the correct interface to use. It seems to do
> > some power budget and they should use the right interface for this
>
> Yes, I agree. I have sent explanation with this to Peter's emails.
> Daniel tries to give them a better interface: DTPM, but also would
> suffer the same issue of capacity reduction for this short time.

The comments in this thread are only about using the userspace
interface scale_max_freq to dynamically scale max freq and then to try
to report these changes in the thermal_pressure, which is the purpose
of this patch.

As said at LPC, I'm fine to rename thermal_pressure for something more
generic but this is not the purpose of this patch. This patch is about
connecting userspace scale_max_freq to thermal_pressure and it's not
the right things to do

>
> We have a few discussions about it, also Daniel presented on a few
> LPC those issues.

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