Date:   Thu, 17 Oct 2019 12:40:29 -0400
From:   Thara Gopinath <thara.gopinath@...aro.org>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ionela Voinescu <ionela.voinescu@....com>,
        Zhang Rui <rui.zhang@...el.com>,
        Eduardo Valentin <edubezval@...il.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Amit Kachhap <amit.kachhap@...il.com>,
        Javi Merino <javi.merino@...nel.org>,
        Daniel Lezcano <daniel.lezcano@...aro.org>
Subject: Re: [Patch v3 2/7] sched: Add infrastructure to store and update
 instantaneous thermal pressure

On 10/17/2019 04:44 AM, Vincent Guittot wrote:
> Hi Thara,
> 
> On Wed, 16 Oct 2019 at 23:22, Thara Gopinath <thara.gopinath@...aro.org> wrote:
>>
>> Hi Vincent,
>>
>> Thanks for the review
>> On 10/14/2019 11:50 AM, Vincent Guittot wrote:
>>> Hi Thara,
>>>
>>> On Mon, 14 Oct 2019 at 02:58, Thara Gopinath <thara.gopinath@...aro.org> wrote:
>>>>
>>>> Add thermal.c and thermal.h files that provides interface
>>>> APIs to initialize, update/average, track, accumulate and decay
>>>> thermal pressure per cpu basis. A per cpu structure max_capacity_info is
>>>> introduced to keep track of instantaneous per cpu thermal pressure.
>>>> Thermal pressure is the delta between max_capacity and cap_capacity.
>>>> API update_periodic_maxcap is called for periodic accumulate and decay
>>>> of the thermal pressure. It is to to be called from a periodic tick
>>>> function. This API calculates the delta between max_capacity and
>>>> cap_capacity and passes on the delta to update_thermal_avg to do the
>>>> necessary accumulate, decay and average. API update_maxcap_capacity is for
>>>> the system to update the thermal pressure by updating cap_capacity.
>>>> Considering, update_periodic_maxcap reads cap_capacity and
>>>> update_maxcap_capacity writes into cap_capacity, one can argue for
>>>> some sort of locking mechanism to avoid a stale value.
>>>> But considering update_periodic_maxcap can be called from a system
>>>> critical path like scheduler tick function, a locking mechanism is not
>>>> ideal. This means that it is possible the value used to
>>>> calculate average thermal pressure for a cpu can be stale for upto 1
>>>> tick period.
>>>>
>>>> Signed-off-by: Thara Gopinath <thara.gopinath@...aro.org>
>>>> ---
>>>>  include/linux/sched.h  | 14 +++++++++++
>>>>  kernel/sched/Makefile  |  2 +-
>>>>  kernel/sched/thermal.c | 66 ++++++++++++++++++++++++++++++++++++++++++++++++++
>>>>  kernel/sched/thermal.h | 13 ++++++++++
>>>>  4 files changed, 94 insertions(+), 1 deletion(-)
>>>>  create mode 100644 kernel/sched/thermal.c
>>>>  create mode 100644 kernel/sched/thermal.h
>>>>
>>>> diff --git a/include/linux/sched.h b/include/linux/sched.h
>>>> index 2c2e56b..875ce2b 100644
>>>> --- a/include/linux/sched.h
>>>> +++ b/include/linux/sched.h
>>>> @@ -1983,6 +1983,20 @@ static inline void rseq_syscall(struct pt_regs *regs)
>>>>
>>>>  #endif
>>>>
>>>> +#ifdef CONFIG_SMP
>>>> +void update_maxcap_capacity(int cpu, u64 capacity);
>>>> +
>>>> +void populate_max_capacity_info(void);
>>>> +#else
>>>> +static inline void update_maxcap_capacity(int cpu, u64 capacity)
>>>> +{
>>>> +}
>>>> +
>>>> +static inline void populate_max_capacity_info(void)
>>>> +{
>>>> +}
>>>> +#endif
>>>> +
>>>>  const struct sched_avg *sched_trace_cfs_rq_avg(struct cfs_rq *cfs_rq);
>>>>  char *sched_trace_cfs_rq_path(struct cfs_rq *cfs_rq, char *str, int len);
>>>>  int sched_trace_cfs_rq_cpu(struct cfs_rq *cfs_rq);
>>>> diff --git a/kernel/sched/Makefile b/kernel/sched/Makefile
>>>> index 21fb5a5..4d3b820 100644
>>>> --- a/kernel/sched/Makefile
>>>> +++ b/kernel/sched/Makefile
>>>> @@ -20,7 +20,7 @@ obj-y += core.o loadavg.o clock.o cputime.o
>>>>  obj-y += idle.o fair.o rt.o deadline.o
>>>>  obj-y += wait.o wait_bit.o swait.o completion.o
>>>>
>>>> -obj-$(CONFIG_SMP) += cpupri.o cpudeadline.o topology.o stop_task.o pelt.o
>>>> +obj-$(CONFIG_SMP) += cpupri.o cpudeadline.o topology.o stop_task.o pelt.o thermal.o
>>>>  obj-$(CONFIG_SCHED_AUTOGROUP) += autogroup.o
>>>>  obj-$(CONFIG_SCHEDSTATS) += stats.o
>>>>  obj-$(CONFIG_SCHED_DEBUG) += debug.o
>>>> diff --git a/kernel/sched/thermal.c b/kernel/sched/thermal.c
>>>> new file mode 100644
>>>> index 0000000..5f0b2d4
>>>> --- /dev/null
>>>> +++ b/kernel/sched/thermal.c
>>>> @@ -0,0 +1,66 @@
>>>> +// SPDX-License-Identifier: GPL-2.0
>>>> +/*
>>>> + * Sceduler Thermal Interactions
>>>> + *
>>>> + *  Copyright (C) 2018 Linaro, Inc., Thara Gopinath <thara.gopinath@...aro.org>
>>>> + */
>>>> +
>>>> +#include <linux/sched.h>
>>>> +#include "sched.h"
>>>> +#include "pelt.h"
>>>> +#include "thermal.h"
>>>> +
>>>> +struct max_capacity_info {
>>>> +       unsigned long max_capacity;
>>>> +       unsigned long cap_capacity;
>>>> +};
>>>> +
>>>> +static DEFINE_PER_CPU(struct max_capacity_info, max_cap);
>>>> +
>>>> +void update_maxcap_capacity(int cpu, u64 capacity)
>>>> +{
>>>> +       struct max_capacity_info *__max_cap;
>>>> +       unsigned long __capacity;
>>>> +
>>>> +       __max_cap = (&per_cpu(max_cap, cpu));
>>>> +       if (!__max_cap) {
>>>> +               pr_err("no max_capacity_info structure for cpu %d\n", cpu);
>>>> +               return;
>>>> +       }
>>>> +
>>>> +       /* Normalize the capacity */
>>>> +       __capacity = (capacity * arch_scale_cpu_capacity(cpu)) >>
>>>> +                                                       SCHED_CAPACITY_SHIFT;
>>>> +       pr_debug("updating cpu%d capped capacity from %lu to %lu\n", cpu, __max_cap->cap_capacity, __capacity);
>>>> +
>>>> +       __max_cap->cap_capacity = __capacity;
>>>> +}
>>>> +
>>>> +void populate_max_capacity_info(void)
>>>> +{
>>>> +       struct max_capacity_info *__max_cap;
>>>> +       u64 capacity;
>>>> +       int cpu;
>>>> +
>>>> +       for_each_possible_cpu(cpu) {
>>>> +               __max_cap = (&per_cpu(max_cap, cpu));
>>>> +               if (!__max_cap)
>>>> +                       continue;
>>>> +               capacity = arch_scale_cpu_capacity(cpu);
>>>> +               __max_cap->max_capacity = capacity;
>>>> +               __max_cap->cap_capacity = capacity;
>>>> +               pr_debug("cpu %d max capacity set to %ld\n", cpu, __max_cap->max_capacity);
>>>> +       }
>>>> +}
>>>
>>> everything above seems to be there for the cpu cooling device and
>>> should be included in it instead. The scheduler only need the capacity
>>> capping
>>> The cpu cooling device should just set the delta capacity in the
>>> per-cpu variable (see my comment below)
>> It can be a firmware  updating the thermal pressure instead of cpu
>> cooling device. Or may be some other entity .So instead of replicating
>> this code, isnt't it better to reside in one place ?
> 
> But you have now idea which kind of metrics will be provided by
> firmware. It might not be a capped frequency and a max frequency but
> other metrics.
hmm. Why ? It is thermal pressure. It is finally applied to the
scheduler metric cpu_capacity. It should not be anything other than
capacity reduction due to thermal events that should be provided.
> Also, __max_cap->max_capacity just save in a new per cpu struct the
> return of arch_scale_cpu_capacity but this doesn't give us real
> benefit
I agree. I will change the struct into a single variable delta and
initialize it to 0. populate_max_capacity_info need not be there. I will
have update_maxcap_capacity calculate the delta with
arch_scale_cpu_capacity and store it into a per cpu variable.


-- 
Warm Regards
Thara

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