Re: [Patch v3 2/7] sched: Add infrastructure to store and update instantaneous thermal pressure

Vincent Guittot
2019-10-18T08:05:53Z

Hi Thara,

On Thu, 17 Oct 2019 at 18:40, Thara Gopinath <thara.gopinath@linaro.org> wrote:
>
> On 10/17/2019 04:44 AM, Vincent Guittot wrote:
> > Hi Thara,
> >
> > On Wed, 16 Oct 2019 at 23:22, Thara Gopinath <thara.gopinath@linaro.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@linaro.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@linaro.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@linaro.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.

What I mean is that you don't know how the value will be provided and
if you will need a cap and a max value. It might easily provide
directly the delta, or a percent or even something else

> > 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.

Also could you rename this function. You use thermal for the pelt
function so could you align the function names ? either to use thermal
everywhere or another name but could you not mix the naming
If you want to use thermal why not using update_thermal_pressure ? at
least it's easy to understand what it does

>

>
> --
> Warm Regards
> Thara