On Thu, 23 Nov 2006 18:48:52 +0530
Gautham R Shenoy <[email protected]> wrote:
> When CONFIG_HOTPLUG_CPU is not set, lock_cpu_hotplug() and
> unlock_cpu_hotplug() default to no-ops.
>
> However, in case of the proposed per-subsystem hotcpu mutex scheme,
> the hotcpu mutexes will be taken/released, even when
> CONFIG_HOTPLUG_CPU is not set. This is an unnecessary overhead
> both w.r.t space and time ;-)
hm.
> This patch
>
> * Provides a common interface for all the subsystems to lock and
> unlock their per-subsystem hotcpu mutexes.
> When CONFIG_HOTPLUG_CPU is not set, these operations would be no-ops.
>
> Usage:
> a) Each hotcpu aware subsystem defines the hotcpu_mutex as follows
> #ifdef CONFIG_HOTPLUG_CPU
> DEFINE_MUTEX(my_hotcpu_mutex);
> #endif
>
> b) The hotcpu aware subsystem uses
> cpuhotplug_mutex_lock(&my_hotcpu_mutex)
> and
> cpuhotplug_mutex_unlock(&my_hotcpu_mutex)
> instead of the usual mutex_lock/mutex_unlock.
>
> Signed-off-by: Gautham R Shenoy <[email protected]>
>
> ---
> include/linux/cpu.h | 15 +++++++++++++++
> 1 files changed, 15 insertions(+)
>
> Index: hotplug/include/linux/cpu.h
> ===================================================================
> --- hotplug.orig/include/linux/cpu.h
> +++ hotplug/include/linux/cpu.h
> @@ -24,6 +24,7 @@
> #include <linux/compiler.h>
> #include <linux/cpumask.h>
> #include <asm/semaphore.h>
> +#include <linux/mutex.h>
>
> struct cpu {
> int node_id; /* The node which contains the CPU */
> @@ -74,6 +75,17 @@ extern struct sysdev_class cpu_sysdev_cl
>
> #ifdef CONFIG_HOTPLUG_CPU
> /* Stop CPUs going up and down. */
> +
> +static inline void cpuhotplug_mutex_lock(struct mutex *cpu_hp_mutex)
> +{
> + mutex_lock(cpu_hp_mutex);
> +}
> +
> +static inline void cpuhotplug_mutex_unlock(struct mutex *cpu_hp_mutex)
> +{
> + mutex_unlock(cpu_hp_mutex);
> +}
> +
> extern void lock_cpu_hotplug(void);
> extern void unlock_cpu_hotplug(void);
> #define hotcpu_notifier(fn, pri) { \
> @@ -86,6 +98,9 @@ extern void unlock_cpu_hotplug(void);
> int cpu_down(unsigned int cpu);
> #define cpu_is_offline(cpu) unlikely(!cpu_online(cpu))
> #else
> +#define cpuhotplug_mutex_lock(m) do { } while (0)
> +#define cpuhotplug_mutex_unlock(m) do { } while (0)
> +
But what to do about the now-unneeded mutex?
We can just leave it there if CONFIG_HOTPLUG_CPU=n but then we'll get
unused variable warnings for statically-defined mutexes.
To fix that would require either
#ifdef CONFIG_HOTPLUG_CPU
#define DEFINE_MUTEX_HOTPLUG_CPU(m) DEFINE_MUTEX(m)
#else
#define DEFINE_MUTEX_HOTPLUG_CPU(m)
#endif
or
#define cpuhotplug_mutex_lock(m) do { (void)(m); } while (0)
Given that the former won't work, I'd suggest the latter ;)
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