Date:   Tue, 19 Apr 2022 09:14:29 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Xuewen Yan <xuewen.yan94@...il.com>
Cc:     Lukasz Luba <Lukasz.Luba@....com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Xuewen Yan <xuewen.yan@...soc.com>, rafael@...nel.org,
        viresh.kumar@...aro.org, mingo@...hat.com, peterz@...radead.org,
        rostedt@...dmis.org, linux-kernel@...r.kernel.org,
        di.shen@...soc.com
Subject: Re: [PATCH] sched: Take thermal pressure into account when determine
 rt fits capacity

On Sat, 16 Apr 2022 at 04:47, Xuewen Yan <xuewen.yan94@...il.com> wrote:
>
> Hi Luba  / Dietmar
>
> On Wed, Apr 13, 2022 at 9:26 PM Lukasz Luba <lukasz.luba@....com> wrote:
> >
> >
> >
> > On 4/11/22 15:07, Dietmar Eggemann wrote:
> > > On 11/04/2022 10:52, Xuewen Yan wrote:
> > >> HI Dietmar
> > >>
> > >> On Mon, Apr 11, 2022 at 4:21 PM Dietmar Eggemann
> > >> <dietmar.eggemann@....com> wrote:
> > >>>
> > >>> On 07/04/2022 07:19, Xuewen Yan wrote:
> > >>>> There are cases when the cpu max capacity might be reduced due to thermal.
> > >>>> Take into the thermal pressure into account when judge whether the rt task
> > >>>> fits the cpu. And when schedutil govnor get cpu util, the thermal pressure
> > >>>> also should be considered.
> > >>>>
> > >>>> Signed-off-by: Xuewen Yan <xuewen.yan@...soc.com>
> > >>>> ---
> > >>>>   kernel/sched/cpufreq_schedutil.c | 1 +
> > >>>>   kernel/sched/rt.c                | 1 +
> > >>>>   2 files changed, 2 insertions(+)
> > >>>>
> > >>>> diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> > >>>> index 3dbf351d12d5..285ad51caf0f 100644
> > >>>> --- a/kernel/sched/cpufreq_schedutil.c
> > >>>> +++ b/kernel/sched/cpufreq_schedutil.c
> > >>>> @@ -159,6 +159,7 @@ static void sugov_get_util(struct sugov_cpu *sg_cpu)
> > >>>>        struct rq *rq = cpu_rq(sg_cpu->cpu);
> > >>>>        unsigned long max = arch_scale_cpu_capacity(sg_cpu->cpu);
> > >>>>
> > >>>> +     max -= arch_scale_thermal_pressure(sg_cpu->cpu);
> > >>>
> > >>> max' = arch_scale_cpu_capacity() - arch_scale_thermal_pressure()
> > >>>
> > >>> For the energy part (A) we use max' in compute_energy() to cap sum_util
> > >>> and max_util at max' and to call em_cpu_energy(..., max_util, sum_util,
> > >>> max'). This was done to match (B)'s `policy->max` capping.
> > >>>
> > >>> For the frequency part (B) we have freq_qos_update_request() in:
> > >>>
> > >>> power_actor_set_power()
> > >>>    ...
> > >>>    cdev->ops->set_cur_state()
> > >>>
> > >>>      cpufreq_set_cur_state()
> > >>>        freq_qos_update_request()      <-- !
> > >>>        arch_update_thermal_pressure()
> > >>>
> > >>> restricting `policy->max` which then clamps `target_freq` in:
> > >>>
> > >>>    cpufreq_update_util()
> > >>>      ...
> > >>>      get_next_freq()
> > >>>        cpufreq_driver_resolve_freq()
> > >>>          __resolve_freq()
> > >>>
> > >>
> > >> Do you mean that the "max" here will not affect the frequency
> > >> conversion, so there is no need to change it?
> > >> But is it better to reflect the influence of thermal here?
> > >
> > > I guess your point is that even though max' has no effect on frequency
> > > since QOS caps policy->max anyway, it is still easier to understand the
> > > dependency between schedutil and EAS/EM when it comes to the use of
> > > thermal pressure.
> > >
> > > I agree. The way how the "hidden" policy->max capping guarantees that
> > > schedutil and EAS/EM are doing the same even when only the latter uses
> > > max' is not obvious.
> >
> > +1 here, IMO we shouldn't rely on hidden stuff. There are two which set
> > the thermal pressure, but one is not setting the freq_qos which causes
> > the update of the 'policy->max'. So the schedutil will send that high
> > frequency but that driver would just ignore and clamp internally. In the
> > end we might argue it still works, but is it clean and visible from the
> > code? Funny thing might start to happen then the driver, which might be
> > the last safety net cannot capture this.

schedutil reflects what scheduler wants not what HW can do. If you
start to cap the freq with arch_scale_thermal_pressure() in schedutil,
you will loose some opportunity to run at higher frequency because
arch_scale_thermal_pressure() is transient and might change just after
using it. This means that you will stay at lower freq after mitigation
stops until a new cpufreq_update_util() happens. ANd I don't vene
mentioned when thermal mitigation is managed by HW at a much higher
frequency than what Linux can handle

arch_scale_thermal_pressure() must not be used but thermal_load_avg()
like scale_rt_capacity() what Dietmar suggested

> >
> > We also should be OK with energy estimation and the CPU capacity vs.
> > task utilization comparisons, since the thermal pressure is accounted
> > there* (until the thermal is controlled in kernel not in FW, which is
> > something where we are heading with scmi-cpufreq mentioned in this
> > cover letter [1]).
>
> IMO, If so, we don't want to modify the original code, but also need to
> consider the impact of thermal, maybe it is possible to add a new
> macro definition
> like this:
>
> #define arch_scale_cpu_capacity_except_thermal()
> (arch_scale_cpu_capacity() - arch_scale_thermal_pressure())
>
> >
> > >
> > > I just wanted to mention the historical reason why the code looks like
> > > it does today.
> > >
> > >>> [...]
> > >>>
> > >>>> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> > >>>> index a32c46889af8..d9982ebd4821 100644
> > >>>> --- a/kernel/sched/rt.c
> > >>>> +++ b/kernel/sched/rt.c
> > >>>> @@ -466,6 +466,7 @@ static inline bool rt_task_fits_capacity(struct task_struct *p, int cpu)
> > >>>>        max_cap = uclamp_eff_value(p, UCLAMP_MAX);
> > >>>>
> > >>>>        cpu_cap = capacity_orig_of(cpu);
> > >>>> +     cpu_cap -= arch_scale_thermal_pressure(cpu);
> > >>>>
> > >>>>        return cpu_cap >= min(min_cap, max_cap);
> > >>>>   }
> > >>>
> > >>> IMHO, this should follow what we do with rq->cpu_capacity
> > >>> (capacity_of(), the remaining capacity for CFS). E.g. we use
> > >>> capacity_of() in find_energy_efficient_cpu() and select_idle_capacity()
> > >>> to compare capacities. So we would need a function like
> > >>> scale_rt_capacity() for RT (minus the rq->avg_rt.util_avg) but then also
> > >>> one for DL (minus rq->avg_dl.util_avg and rq->avg_rt.util_avg).
> > >>
> > >> It's a really good idea. And do you already have the corresponding patch?
> > >> If there is, can you tell me the corresponding link?
> > >
> > > No, I don't have any code for this. Should be trivial though. But the
> > > issue is that the update would probably have to be decoupled from CFS
> > > load_balance (update_group_capacity()) and the use cases in RT/DL are
> > > only valid for asymmetric CPU capacity systems.
> >
> > Having in mind those I would vote for fixing it incrementally.
> > Thus, IMHO this patch is good for taking it. Later we might think how
> > to better estimate the real CPU capacity visible from RT and DL classes.
> > In this shape it is good for many systems which only use RT,
> > but not DL class. Those systems w/ RT and w/o DL might suffer on some
> > asymmetric CPU platforms where medium cores have capacity e.g. 850 and
> > thermal pressure reduced the big cores capacity by 250 making them 774.
> >
>
> Your mean is that before there is better way to handle RT capacity, we
> can take this patch temporarily?
> If so, I can update the patch which will just fix the rt.c.
>
> In fact, in the mobile phone usage scenario where the cpu contains 3
> clusters (small/middle/big),
> the capacity of the big core's capacity will be smaller than that of
> the middle core due to the thermal effect.
> At this time, we do not want the big core CPU to be used as an RT
> task's alternative cpu.
>
> > Regards,
> > Lukasz
> >
> > [1]
> > https://lore.kernel.org/linux-pm/20211007080729.8262-1-lukasz.luba@arm.com/
>
>
> BR
> xuewen.yan

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