[PATCH] Handle per-subsystem mutexes for CONFIG_HOTPLUG_CPU not set.

[PATCH] Handle per-subsystem mutexes for CONFIG_HOTPLUG_CPU not set.

[Gautham R Shenoy]

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 ;-)

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 <ego@in.ibm.com>

---
 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)
+
 #define lock_cpu_hotplug()	do { } while (0)
 #define unlock_cpu_hotplug()	do { } while (0)
 #define lock_cpu_hotplug_interruptible() 0
-- 
Gautham R Shenoy
Linux Technology Center
IBM India.
"Freedom comes with a price tag of responsibility, which is still a bargain,
because Freedom is priceless!"

Re: [PATCH] Handle per-subsystem mutexes for CONFIG_HOTPLUG_CPU not set.

[Andrew Morton]

On Thu, 23 Nov 2006 18:48:52 +0530
Gautham R Shenoy <ego@in.ibm.com> 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 <ego@in.ibm.com>
> 
> ---
>  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 ;)

Re: [PATCH] Handle per-subsystem mutexes for CONFIG_HOTPLUG_CPU not set.

[Gautham R Shenoy]

Hi Andrew,

> >
> > 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 <ego@in.ibm.com>
> >
> > ---
> >  include/linux/cpu.h |   15 +++++++++++++++
[snip]
> >  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.

Why even leave it there?

Can't we do something as follows, which has already been suggested in
the patch description:

Each hotcpu aware subsystem defines the hotcpu_mutex as follows
#ifdef CONFIG_HOTPLUG_CPU
[static] DEFINE_MUTEX(my_hotcpu_mutex);
#endif

That way, we won't be having any unneeded mutexes at all.

> 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
> 

Yup, this won't work. Wish we could cook up something like this, but
alas! Most of these mutexes are defined with the static keyword,
which causes compile errors with CONFIG_HOTPLUG_CPU=n.

> or
> 
> #define cpuhotplug_mutex_lock(m)	do { (void)(m); } while (0)
> 
> 
> Given that the former won't work, I'd suggest the latter ;)
> 

regards
gautham.
-- 
Gautham R Shenoy
Linux Technology Center
IBM India.
"Freedom comes with a price tag of responsibility, which is still a bargain,
because Freedom is priceless!"

[PATCH][v2] Handle per-subsystem mutexes for CONFIG_HOTPLUG_CPU not set.

[Gautham R Shenoy]

On Fri, Nov 24, 2006 at 09:57:02AM +0530, Gautham R Shenoy wrote:
> > 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.
> 
> Why even leave it there?
> 
> Can't we do something as follows, which has already been suggested in
> the patch description:
> 
> Each hotcpu aware subsystem defines the hotcpu_mutex as follows
> #ifdef CONFIG_HOTPLUG_CPU
> [static] DEFINE_MUTEX(my_hotcpu_mutex);
> #endif
> 
> That way, we won't be having any unneeded mutexes at all.

I take back these words. In addition to the fact that this would
introduce more #ifdef noise in .c files, this approach won't work
in cases where we will be reusing existing mutexes as hotcpu_mutexes.
Eg: workqueue_mutex in kernel/workqueue.c and cache_chain_mutex in
mm/slab.c

> > or
> >
> > #define cpuhotplug_mutex_lock(m)	do { (void)(m); } while (0)
> >
> >
> > Given that the former won't work, I'd suggest the latter ;)

The patch below implements the latter :)

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.

Signed-off-by: Gautham R Shenoy <ego@in.ibm.com>

---
 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 { (void)(m); } while (0)
+#define cpuhotplug_mutex_unlock(m)	do { (void)(m); } while (0)
+
 #define lock_cpu_hotplug()	do { } while (0)
 #define unlock_cpu_hotplug()	do { } while (0)
 #define lock_cpu_hotplug_interruptible() 0
-- 
Gautham R Shenoy
Linux Technology Center
IBM India.
"Freedom comes with a price tag of responsibility, which is still a bargain,
because Freedom is priceless!"