Re: [PATCHv4 4/4] cpuidle: coupled: add parallel barrier function

[Colin Cross <ccross@xxxxxxxxxxx>]

On Wed, May 9, 2012 at 2:31 PM, Rafael J. Wysocki <rjw@xxxxxxx> wrote:
> On Tuesday, May 08, 2012, Colin Cross wrote:
>> Adds cpuidle_coupled_parallel_barrier, which can be used by coupled
>> cpuidle state enter functions to handle resynchronization after
>> determining if any cpu needs to abort.  The normal use case will
>> be:
>>
>> static bool abort_flag;
>> static atomic_t abort_barrier;
>>
>> int arch_cpuidle_enter(struct cpuidle_device *dev, ...)
>> {
>>       if (arch_turn_off_irq_controller()) {
>>               /* returns an error if an irq is pending and would be lost
>>                  if idle continued and turned off power */
>>               abort_flag = true;
>>       }
>>
>>       cpuidle_coupled_parallel_barrier(dev, &abort_barrier);
>>
>>       if (abort_flag) {
>>               /* One of the cpus didn't turn off it's irq controller */
>>               arch_turn_on_irq_controller();
>>               return -EINTR;
>>       }
>>
>>       /* continue with idle */
>>       ...
>> }
>>
>> This will cause all cpus to abort idle together if one of them needs
>> to abort.
>>
>> Reviewed-by: Santosh Shilimkar <santosh.shilimkar@xxxxxx>
>> Tested-by: Santosh Shilimkar <santosh.shilimkar@xxxxxx>
>> Reviewed-by: Kevin Hilman <khilman@xxxxxx>
>> Tested-by: Kevin Hilman <khilman@xxxxxx>
>> Signed-off-by: Colin Cross <ccross@xxxxxxxxxxx>
>> ---
>>  drivers/cpuidle/coupled.c |   37 +++++++++++++++++++++++++++++++++++++
>>  include/linux/cpuidle.h   |    4 ++++
>>  2 files changed, 41 insertions(+), 0 deletions(-)
>>
>> diff --git a/drivers/cpuidle/coupled.c b/drivers/cpuidle/coupled.c
>> index 93101fb..3e65de1 100644
>> --- a/drivers/cpuidle/coupled.c
>> +++ b/drivers/cpuidle/coupled.c
>> @@ -130,6 +130,43 @@ struct cpuidle_coupled {
>>  static cpumask_t cpuidle_coupled_poked_mask;
>>
>>  /**
>> + * cpuidle_coupled_parallel_barrier - synchronize all online coupled cpus
>> + * @dev: cpuidle_device of the calling cpu
>> + * @a:   atomic variable to hold the barrier
>> + *
>> + * No caller to this function will return from this function until all online
>> + * cpus in the same coupled group have called this function.  Once any caller
>> + * has returned from this function, the barrier is immediately available for
>> + * reuse.
>> + *
>> + * The atomic variable a must be initialized to 0 before any cpu calls
>> + * this function, will be reset to 0 before any cpu returns from this function.
>> + *
>> + * Must only be called from within a coupled idle state handler
>> + * (state.enter when state.flags has CPUIDLE_FLAG_COUPLED set).
>> + *
>> + * Provides full smp barrier semantics before and after calling.
>> + */
>> +void cpuidle_coupled_parallel_barrier(struct cpuidle_device *dev, atomic_t *a)
>> +{
>> +     int n = dev->coupled->online_count;
>> +
>> +     smp_mb__before_atomic_inc();
>> +     atomic_inc(a);
>> +
>> +     while (atomic_read(a) < n)
>> +             cpu_relax();
>> +
>> +     if (atomic_inc_return(a) == n * 2) {
>> +             atomic_set(a, 0);
>> +             return;
>> +     }
>> +
>> +     while (atomic_read(a) > n)
>> +             cpu_relax();
>> +}
>
> Well, this looks like "wait until all CPUs execute this code".  Don't we have
> anything like this already somewhere?
>
>> +
>> +/**
>>   * cpuidle_state_is_coupled - check if a state is part of a coupled set
>>   * @dev: struct cpuidle_device for the current cpu
>>   * @drv: struct cpuidle_driver for the platform
>> diff --git a/include/linux/cpuidle.h b/include/linux/cpuidle.h
>> index 6038448..5ab7183 100644
>> --- a/include/linux/cpuidle.h
>> +++ b/include/linux/cpuidle.h
>> @@ -183,6 +183,10 @@ static inline int cpuidle_wrap_enter(struct cpuidle_device *dev,
>>
>>  #endif
>>
>> +#ifdef CONFIG_ARCH_NEEDS_CPU_IDLE_COUPLED
>> +void cpuidle_coupled_parallel_barrier(struct cpuidle_device *dev, atomic_t *a);
>> +#endif
>
> Why exactly is the extra Kconfig option necessary?

It prevents compiling in coupled.o (2k text section) on the majority
of kernels that will never use it.
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