[v3,1/2] cpumask: introduce cpumask for hotpluggable CPUs

Message ID 1313092487-15858-1-git-send-email-mturquette@ti.com
State New
Headers show

Commit Message

Mike Turquette Aug. 11, 2011, 7:54 p.m.
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
v3: really fixed the parentheses issue from v2

 include/linux/cpumask.h |   27 ++++++++++++++++++++++-----
 kernel/cpu.c            |   18 ++++++++++++++++++
 2 files changed, 40 insertions(+), 5 deletions(-)

Comments

Peter Zijlstra Aug. 11, 2011, 8:02 p.m. | #1
On Thu, 2011-08-11 at 12:54 -0700, 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. 

Nacked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>

the kernel really shouldn't be using hotplug for this (nor should
userspace really). hot-unplugging random cpus wrecks things like
cpusets. Furthermore hotplug does way too much work to use as a simple
means to idle a cpu.

Even the availability of this mask is wrong, since that implies the
information is useful, which per the above it is not, the kernel
shouldn't care about this full-stop.

The only reason for the OS to unplug a CPU is imminent and unavoidable
hardware failure. Thermal capping is not that (and yes ACPI-4.0 is a
broken piece of shit).

Patch

diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h
index b24ac56..9584807 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)
 #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);
+}
+
 void init_cpu_online(const struct cpumask *src)
 {
 	cpumask_copy(to_cpumask(cpu_online_bits), src);