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SubjectRe: [PATCHv4 4/4] cpuidle: coupled: add parallel barrier function
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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@ti.com>
> Tested-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
> Reviewed-by: Kevin Hilman <khilman@ti.com>
> Tested-by: Kevin Hilman <khilman@ti.com>
> Signed-off-by: Colin Cross <ccross@android.com>
> ---
> 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?

> +
> /******************************
> * CPUIDLE GOVERNOR INTERFACE *
> ******************************/

Rafael


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