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SubjectRe: [PATCH v2 1/2] cpumask: introduce cpumask for hotpluggable CPUs
See comments inline.

On 11 Aug 10, Mike Turquette wrote:
> On some platforms it is possible to have some CPUs which support CPU
> hotplug and some which do not. Currently the prescence of an 'online'
> sysfs entry in userspace is adequate for applications to know that a CPU
> supports hotplug, but there is no convenient way to make the same
> determination in the kernel.
>
> To better model this relationship this patch introduces a new cpumask to
> track CPUs that support CPU hotplug operations.
>
> This new cpumask is populated at boot-time and remains static for the
> life of the machine. Bits set in the mask indicate a CPU which supports
> hotplug, but make no guarantees about whether that CPU is currently
> online or not. Likewise a cleared bit in the mask indicates either a
> CPU which cannot hotplug or a lack of a populated CPU.
>
> The purpose of this new cpumask is to aid kernel code which uses CPU to
> take CPUs online and offline. Possible uses are as a thermal event
> mitigation technique or as a power capping mechanism.
>
> Signed-off-by: Mike Turquette <mturquette@ti.com>
> ---
> Change log:
> v2: fixed missing parentheses in cpumask_test_cpu and improved grammar
> in comments
>
> include/linux/cpumask.h | 27 ++++++++++++++++++++++-----
> kernel/cpu.c | 18 ++++++++++++++++++
> 2 files changed, 40 insertions(+), 5 deletions(-)
>
> diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h
> index b24ac56..52e64a7 100644
> --- a/include/linux/cpumask.h
> +++ b/include/linux/cpumask.h
> @@ -39,10 +39,11 @@ extern int nr_cpu_ids;
> * The following particular system cpumasks and operations manage
> * possible, present, active and online cpus.
> *
> - * cpu_possible_mask- has bit 'cpu' set iff cpu is populatable
> - * cpu_present_mask - has bit 'cpu' set iff cpu is populated
> - * cpu_online_mask - has bit 'cpu' set iff cpu available to scheduler
> - * cpu_active_mask - has bit 'cpu' set iff cpu available to migration
> + * cpu_possible_mask - has bit 'cpu' set iff cpu is populatable
> + * cpu_hotpluggable_mask - has bit 'cpu' set iff cpu is hotpluggable
> + * cpu_present_mask - has bit 'cpu' set iff cpu is populated
> + * cpu_online_mask - has bit 'cpu' set iff cpu available to scheduler
> + * cpu_active_mask - has bit 'cpu' set iff cpu available to migration
> *
> * If !CONFIG_HOTPLUG_CPU, present == possible, and active == online.
> *
> @@ -51,7 +52,11 @@ extern int nr_cpu_ids;
> * life of that system boot. The cpu_present_mask is dynamic(*),
> * representing which CPUs are currently plugged in. And
> * cpu_online_mask is the dynamic subset of cpu_present_mask,
> - * indicating those CPUs available for scheduling.
> + * indicating those CPUs available for scheduling. The
> + * cpu_hotpluggable_mask is also fixed at boot time as the set of CPU
> + * id's which are possible AND can hotplug. Cleared bits in this mask
> + * mean that either the CPU is not possible, or it is possible but does
> + * not support CPU hotplug operations.
> *
> * If HOTPLUG is enabled, then cpu_possible_mask is forced to have
> * all NR_CPUS bits set, otherwise it is just the set of CPUs that
> @@ -61,6 +66,9 @@ extern int nr_cpu_ids;
> * depending on what ACPI reports as currently plugged in, otherwise
> * cpu_present_mask is just a copy of cpu_possible_mask.
> *
> + * If HOTPLUG is not enabled then cpu_hotpluggable_mask is the empty
> + * set.
> + *
> * (*) Well, cpu_present_mask is dynamic in the hotplug case. If not
> * hotplug, it's a copy of cpu_possible_mask, hence fixed at boot.
> *
> @@ -76,6 +84,7 @@ extern int nr_cpu_ids;
> */
>
> extern const struct cpumask *const cpu_possible_mask;
> +extern const struct cpumask *const cpu_hotpluggable_mask;
> extern const struct cpumask *const cpu_online_mask;
> extern const struct cpumask *const cpu_present_mask;
> extern const struct cpumask *const cpu_active_mask;
> @@ -85,19 +94,23 @@ extern const struct cpumask *const cpu_active_mask;
> #define num_possible_cpus() cpumask_weight(cpu_possible_mask)
> #define num_present_cpus() cpumask_weight(cpu_present_mask)
> #define num_active_cpus() cpumask_weight(cpu_active_mask)
> +#define num_hotpluggable_cpus() cpumask_weight(cpu_hotpluggable_mask)
> #define cpu_online(cpu) cpumask_test_cpu((cpu), cpu_online_mask)
> #define cpu_possible(cpu) cpumask_test_cpu((cpu), cpu_possible_mask)
> #define cpu_present(cpu) cpumask_test_cpu((cpu), cpu_present_mask)
> #define cpu_active(cpu) cpumask_test_cpu((cpu), cpu_active_mask)
> +#define cpu_hotpluggable(cpu) cpumask_test_cpu((cpu, cpu_hotpluggable_mask))

The bracket should be around cpu, like this (cpu) so that there are no
side-effects when passing anything to the macro. See the other #defines above.

> #else
> #define num_online_cpus() 1U
> #define num_possible_cpus() 1U
> #define num_present_cpus() 1U
> #define num_active_cpus() 1U
> +#define num_hotpluggable_cpus() 0
> #define cpu_online(cpu) ((cpu) == 0)
> #define cpu_possible(cpu) ((cpu) == 0)
> #define cpu_present(cpu) ((cpu) == 0)
> #define cpu_active(cpu) ((cpu) == 0)
> +#define cpu_hotpluggable(cpu) 0
> #endif
>
> /* verify cpu argument to cpumask_* operators */
> @@ -678,16 +691,20 @@ extern const DECLARE_BITMAP(cpu_all_bits, NR_CPUS);
> #define cpu_none_mask to_cpumask(cpu_bit_bitmap[0])
>
> #define for_each_possible_cpu(cpu) for_each_cpu((cpu), cpu_possible_mask)
> +#define for_each_hotpluggable_cpu(cpu) \
> + for_each_cpu((cpu), cpu_hotpluggable_mask)
> #define for_each_online_cpu(cpu) for_each_cpu((cpu), cpu_online_mask)
> #define for_each_present_cpu(cpu) for_each_cpu((cpu), cpu_present_mask)
>
> /* Wrappers for arch boot code to manipulate normally-constant masks */
> void set_cpu_possible(unsigned int cpu, bool possible);
> +void set_cpu_hotpluggable(unsigned int cpu, bool hotpluggable);
> void set_cpu_present(unsigned int cpu, bool present);
> void set_cpu_online(unsigned int cpu, bool online);
> void set_cpu_active(unsigned int cpu, bool active);
> void init_cpu_present(const struct cpumask *src);
> void init_cpu_possible(const struct cpumask *src);
> +void init_cpu_hotpluggable(const struct cpumask *src);
> void init_cpu_online(const struct cpumask *src);
>
> /**
> diff --git a/kernel/cpu.c b/kernel/cpu.c
> index 12b7458..8c397c9 100644
> --- a/kernel/cpu.c
> +++ b/kernel/cpu.c
> @@ -536,6 +536,11 @@ static DECLARE_BITMAP(cpu_possible_bits, CONFIG_NR_CPUS) __read_mostly;
> const struct cpumask *const cpu_possible_mask = to_cpumask(cpu_possible_bits);
> EXPORT_SYMBOL(cpu_possible_mask);
>
> +static DECLARE_BITMAP(cpu_hotpluggable_bits, CONFIG_NR_CPUS) __read_mostly;
> +const struct cpumask *const cpu_hotpluggable_mask =
> + to_cpumask(cpu_hotpluggable_bits);
> +EXPORT_SYMBOL(cpu_hotpluggable_mask);
> +
> static DECLARE_BITMAP(cpu_online_bits, CONFIG_NR_CPUS) __read_mostly;
> const struct cpumask *const cpu_online_mask = to_cpumask(cpu_online_bits);
> EXPORT_SYMBOL(cpu_online_mask);
> @@ -556,6 +561,14 @@ void set_cpu_possible(unsigned int cpu, bool possible)
> cpumask_clear_cpu(cpu, to_cpumask(cpu_possible_bits));
> }
>
> +void set_cpu_hotpluggable(unsigned int cpu, bool hotpluggable)
> +{
> + if (hotpluggable)
> + cpumask_set_cpu(cpu, to_cpumask(cpu_hotpluggable_bits));
> + else
> + cpumask_clear_cpu(cpu, to_cpumask(cpu_hotpluggable_bits));
> +}
> +
> void set_cpu_present(unsigned int cpu, bool present)
> {
> if (present)
> @@ -590,6 +603,11 @@ void init_cpu_possible(const struct cpumask *src)
> cpumask_copy(to_cpumask(cpu_possible_bits), src);
> }
>
> +void init_cpu_hotpluggable(const struct cpumask *src)
> +{
> + cpumask_copy(to_cpumask(cpu_hotpluggable_bits), src);
> +}
> +

Just to be clear I understand this code, init_cpu_hotpluggable will be called
in arch code, right? e.g. arch/arm/kernel/smp.c or do you intend for it to be
called in platform-specific code?

> void init_cpu_online(const struct cpumask *src)
> {
> cpumask_copy(to_cpumask(cpu_online_bits), src);
> --
> 1.7.4.1


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