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

[Thara Gopinath]

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@xxxxxxxxxx> 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@xxxxxxxxxx>
>> ---
>>  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@xxxxxxxxxx>
>> + */
>> +
>> +#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 ?
> 
>> +
>> +void update_periodic_maxcap(struct rq *rq)
>> +{
>> +       struct max_capacity_info *__max_cap = (&per_cpu(max_cap, cpu_of(rq)));
>> +       unsigned long delta;
>> +
>> +       if (!__max_cap)
>> +               return;
>> +
>> +       delta = __max_cap->max_capacity - __max_cap->cap_capacity;
> 
> Why don't you just save the delta in the per_cpu variable instead of
> the struct max_capacity_info ? You have to compute the delta every
> tick whereas we can expect it to not change that much compared to the
> number of tick.

Again I think thermal pressure can be applied from other entities like
firmware as well. But  I agree with your point that calculating delta
every tick is not a good idea. How about I move it to
update_maxcap_capacity so that delta is calculate every time an update
comes from cpu cooling device or anybody else ?

Warm Regards
Thara