[PATCH v6 3/3] ARM: Check if a CPU has gone offline

[Mark Rutland]

On Thu, Apr 17, 2014 at 08:15:46PM +0100, Ashwin Chaugule wrote:
> PSCIv0.2 adds a new function called AFFINITY_INFO, which
> can be used to query if a specified CPU has actually gone
> offline. Calling this function via cpu_kill ensures that
> a CPU has quiesced after a call to cpu_die.
> 
> Signed-off-by: Ashwin Chaugule <ashwin.chaugule at linaro.org>
> Reviewed-by: Rob Herring <robh at kernel.org>
> ---
>  arch/arm/kernel/psci_smp.c | 21 +++++++++++++++++++++
>  include/uapi/linux/psci.h  |  5 +++++
>  2 files changed, 26 insertions(+)
> 
> diff --git a/arch/arm/kernel/psci_smp.c b/arch/arm/kernel/psci_smp.c
> index 570a48c..c6f1420 100644
> --- a/arch/arm/kernel/psci_smp.c
> +++ b/arch/arm/kernel/psci_smp.c
> @@ -16,6 +16,7 @@
>  #include <linux/init.h>
>  #include <linux/smp.h>
>  #include <linux/of.h>
> +#include <uapi/linux/psci.h>
>  
>  #include <asm/psci.h>
>  #include <asm/smp_plat.h>
> @@ -66,6 +67,25 @@ void __ref psci_cpu_die(unsigned int cpu)
>         /* We should never return */
>         panic("psci: cpu %d failed to shutdown\n", cpu);
>  }
> +
> +int __ref psci_cpu_kill(unsigned int cpu)
> +{
> +	int err;
> +
> +	if (!psci_ops.affinity_info)
> +		return 1;
> +
> +	err = psci_ops.affinity_info(cpu_logical_map(cpu), 0);
> +
> +	if (err != PSCI_AFFINITY_INFO_RET_OFF) {
> +		pr_err("psci: Cannot kill CPU:%d, psci ret val: %d\n",
> +				cpu, err);
> +		/* Make platform_cpu_kill() fail. */
> +		return 0;
> +	}

We can race with the dying CPU here -- if we call AFFINITY_INFO before
the dying cpu is sufficiently far through its CPU_OFF call it won't
register as OFF. 

Could we poll here instead (with a reasonable limit on the number of
iterations)? That would enable us to not spuriously declare a CPU to be
dead when it happened to take slightly longer than we expect to turn
off.

Otherwise, this looks good. Thanks for implementing this :)

Cheers,
Mark.