Re: [Patch v5 2/6] sched/fair: Add infrastructure to store and update instantaneous thermal pressure

[Thara Gopinath]

Hi Vincent,
Thanks for the review
On 11/06/2019 03:27 AM, Vincent Guittot wrote:
> Hi Thara,
> 
> 
> On Tue, 5 Nov 2019 at 19:49, Thara Gopinath <thara.gopinath@xxxxxxxxxx> wrote:
>>
>> Add interface APIs to initialize, update/average, track, accumulate
>> and decay thermal pressure per cpu basis. A per cpu variable
>> thermal_pressure is introduced to keep track of instantaneous per
>> cpu thermal pressure. Thermal pressure is the delta between maximum
>> capacity and capped capacity due to a thermal event.
>> API trigger_thermal_pressure_average is called for periodic accumulate
>> and decay of the thermal pressure.This API passes on the instantaneous
>> thermal pressure of a cpu to update_thermal_load_avg to do the necessary
>> accumulate, decay and average.
>> API update_thermal_pressure is for the system to update the thermal
>> pressure by providing a capped maximum capacity.
>> Considering, trigger_thermal_pressure_average reads thermal_pressure and
>> update_thermal_pressure writes into thermal_pressure, one can argue for
>> some sort of locking mechanism to avoid a stale value.
>> But considering trigger_thermal_pressure_average 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 thermal_pressure 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>
>> ---
>>
>> v3->v4:
>>         - Dropped per cpu max_capacity_info struct and instead added a per
>>           delta_capacity variable to store the delta between maximum
>>           capacity and capped capacity. The delta is now calculated when
>>           thermal pressure is updated and not every tick.
>>         - Dropped populate_max_capacity_info api as only per cpu delta
>>           capacity is stored.
>>         - Renamed update_periodic_maxcap to
>>           trigger_thermal_pressure_average and update_maxcap_capacity to
>>           update_thermal_pressure.
>> v4->v5:
>>         - As per Peter's review comments folded thermal.c into fair.c.
>>         - As per Ionela's review comments revamped update_thermal_pressure
>>           to take maximum available capacity as input instead of maximum
>>           capped frequency ration.
>>
>> ---
>>  include/linux/sched.h |  9 +++++++++
>>  kernel/sched/fair.c   | 37 +++++++++++++++++++++++++++++++++++++
>>  2 files changed, 46 insertions(+)
>>
>> diff --git a/include/linux/sched.h b/include/linux/sched.h
>> index 263cf08..3c31084 100644
>> --- a/include/linux/sched.h
>> +++ b/include/linux/sched.h
>> @@ -1993,6 +1993,15 @@ static inline void rseq_syscall(struct pt_regs *regs)
>>
>>  #endif
>>
>> +#ifdef CONFIG_SMP
>> +void update_thermal_pressure(int cpu, unsigned long capped_capacity);
>> +#else
>> +static inline void
>> +update_thermal_pressure(int cpu, unsigned long capped_capacity)
>> +{
>> +}
>> +#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/fair.c b/kernel/sched/fair.c
>> index 682a754..2e907cc 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -86,6 +86,12 @@ static unsigned int normalized_sysctl_sched_wakeup_granularity       = 1000000UL;
>>
>>  const_debug unsigned int sysctl_sched_migration_cost   = 500000UL;
>>
>> +/*
>> + * Per-cpu instantaneous delta between maximum capacity
>> + * and maximum available capacity due to thermal events.
>> + */
>> +static DEFINE_PER_CPU(unsigned long, thermal_pressure);
>> +
>>  #ifdef CONFIG_SMP
>>  /*
>>   * For asym packing, by default the lower numbered CPU has higher priority.
>> @@ -10401,6 +10407,37 @@ static unsigned int get_rr_interval_fair(struct rq *rq, struct task_struct *task
>>         return rr_interval;
>>  }
>>
>> +#ifdef CONFIG_SMP
>> +/**
>> + * update_thermal_pressure: Update thermal pressure
>> + * @cpu: the cpu for which thermal pressure is to be updated for
>> + * @capped_capacity: maximum capacity of the cpu after the capping
>> + *                  due to thermal event.
>> + *
>> + * Delta between the arch_scale_cpu_capacity and capped max capacity is
>> + * stored in per cpu thermal_pressure variable.
>> + */
>> +void update_thermal_pressure(int cpu, unsigned long capped_capacity)
>> +{
>> +       unsigned long delta;
>> +
>> +       delta = arch_scale_cpu_capacity(cpu) - capped_capacity;
>> +       per_cpu(thermal_pressure, cpu) = delta;
> 
> use WRITE_ONCE
Will do.
> 
>> +}
>> +#endif
>> +
>> +/**
>> + * trigger_thermal_pressure_average: Trigger the thermal pressure accumulate
>> + *                                  and average algorithm
>> + */
>> +static void trigger_thermal_pressure_average(struct rq *rq)
>> +{
>> +#ifdef CONFIG_SMP
>> +       update_thermal_load_avg(rq_clock_task(rq), rq,
>> +                               per_cpu(thermal_pressure, cpu_of(rq)));
>> +#endif
>> +}
>> +
>>  /*
>>   * All the scheduling class methods:
>>   */
>> --
>> 2.1.4
>>


-- 
Warm Regards
Thara