Meh, I should stop poking at this..
This one lost all the comments again :/
It uses preempt_disable/preempt_enable vs synchronize_sched() to remove
the barriers from the fast path.
After that it waits for !refcount before setting state, which stops new
readers.
I used a per-cpu spinlock to keep the state check and refcount inc
atomic vs the setting of state.
So the slow path is still per-cpu and mostly uncontended even in the
pending writer case.
After setting state it again waits for !refcount -- someone could have
sneaked in between the last !refcount and setting state. But this time
we know refcount will stay 0.
The only thing I don't really like is the unconditional writer wake in
the read-unlock slowpath, but I couldn't come up with anything better.
Here at least we guarantee that there is a wakeup after the last dec --
although there might be far too many wakes.
---
Subject: hotplug: Optimize {get,put}_online_cpus()
From: Peter Zijlstra <[email protected]>
Date: Tue Sep 17 16:17:11 CEST 2013
Cc: Oleg Nesterov <[email protected]>
Cc: Paul McKenney <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Steven Rostedt <[email protected]>
Signed-off-by: Peter Zijlstra <[email protected]>
---
include/linux/cpu.h | 32 ++++++++++-
kernel/cpu.c | 151 +++++++++++++++++++++++++++++-----------------------
2 files changed, 116 insertions(+), 67 deletions(-)
--- a/include/linux/cpu.h
+++ b/include/linux/cpu.h
@@ -16,6 +16,7 @@
#include <linux/node.h>
#include <linux/compiler.h>
#include <linux/cpumask.h>
+#include <linux/percpu.h>
struct device;
@@ -175,8 +176,35 @@ extern struct bus_type cpu_subsys;
extern void cpu_hotplug_begin(void);
extern void cpu_hotplug_done(void);
-extern void get_online_cpus(void);
-extern void put_online_cpus(void);
+
+extern struct task_struct *__cpuhp_writer;
+DECLARE_PER_CPU(unsigned int, __cpuhp_refcount);
+
+extern void __get_online_cpus(void);
+
+static inline void get_online_cpus(void)
+{
+ might_sleep();
+
+ preempt_disable();
+ if (likely(!__cpuhp_writer || __cpuhp_writer == current))
+ this_cpu_inc(__cpuhp_refcount);
+ else
+ __get_online_cpus();
+ preempt_enable();
+}
+
+extern void __put_online_cpus(void);
+
+static inline void put_online_cpus(void)
+{
+ preempt_disable();
+ this_cpu_dec(__cpuhp_refcount);
+ if (unlikely(__cpuhp_writer && __cpuhp_writer != current))
+ __put_online_cpus();
+ preempt_enable();
+}
+
extern void cpu_hotplug_disable(void);
extern void cpu_hotplug_enable(void);
#define hotcpu_notifier(fn, pri) cpu_notifier(fn, pri)
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -49,88 +49,109 @@ static int cpu_hotplug_disabled;
#ifdef CONFIG_HOTPLUG_CPU
-static struct {
- struct task_struct *active_writer;
- struct mutex lock; /* Synchronizes accesses to refcount, */
- /*
- * Also blocks the new readers during
- * an ongoing cpu hotplug operation.
- */
- int refcount;
-} cpu_hotplug = {
- .active_writer = NULL,
- .lock = __MUTEX_INITIALIZER(cpu_hotplug.lock),
- .refcount = 0,
-};
-
-void get_online_cpus(void)
-{
- might_sleep();
- if (cpu_hotplug.active_writer == current)
- return;
- mutex_lock(&cpu_hotplug.lock);
- cpu_hotplug.refcount++;
- mutex_unlock(&cpu_hotplug.lock);
-
-}
-EXPORT_SYMBOL_GPL(get_online_cpus);
-
-void put_online_cpus(void)
-{
- if (cpu_hotplug.active_writer == current)
- return;
- mutex_lock(&cpu_hotplug.lock);
-
- if (WARN_ON(!cpu_hotplug.refcount))
- cpu_hotplug.refcount++; /* try to fix things up */
-
- if (!--cpu_hotplug.refcount && unlikely(cpu_hotplug.active_writer))
- wake_up_process(cpu_hotplug.active_writer);
- mutex_unlock(&cpu_hotplug.lock);
+struct task_struct *__cpuhp_writer = NULL;
+EXPORT_SYMBOL_GPL(__cpuhp_writer);
+DEFINE_PER_CPU(unsigned int, __cpuhp_refcount);
+EXPORT_PER_CPU_SYMBOL_GPL(__cpuhp_refcount);
+
+static DEFINE_PER_CPU(int, cpuhp_state);
+static DEFINE_PER_CPU(spinlock_t, cpuhp_lock);
+static DECLARE_WAIT_QUEUE_HEAD(cpuhp_wq);
+
+void __get_online_cpus(void)
+{
+ spin_lock(__this_cpu_ptr(&cpuhp_lock));
+ for (;;) {
+ if (!__this_cpu_read(cpuhp_state)) {
+ __this_cpu_inc(__cpuhp_refcount);
+ break;
+ }
+
+ spin_unlock(__this_cpu_ptr(&cpuhp_lock));
+ preempt_enable();
+
+ wait_event(cpuhp_wq, !__cpuhp_writer);
+
+ preempt_disable();
+ spin_lock(__this_cpu_ptr(&cpuhp_lock));
+ }
+ spin_unlock(__this_cpu_ptr(&cpuhp_lock));
+}
+EXPORT_SYMBOL_GPL(__get_online_cpus);
+
+void __put_online_cpus(void)
+{
+ wake_up_process(__cpuhp_writer);
+}
+EXPORT_SYMBOL_GPL(__put_online_cpus);
+
+static void cpuph_wait_refcount(void)
+{
+ for (;;) {
+ unsigned int refcnt = 0;
+ int cpu;
+
+ set_current_state(TASK_UNINTERRUPTIBLE);
+
+ for_each_possible_cpu(cpu)
+ refcnt += per_cpu(__cpuhp_refcount, cpu);
+
+ if (!refcnt)
+ break;
+
+ schedule();
+ }
+ __set_current_state(TASK_RUNNING);
+}
+
+static void cpuhp_set_state(int state)
+{
+ int cpu;
+
+ for_each_possible_cpu(cpu) {
+ spinlock_t *lock = &per_cpu(cpuhp_lock, cpu);
+
+ spin_lock(lock);
+ per_cpu(cpuhp_state, cpu) = state;
+ spin_unlock(lock);
+ }
}
-EXPORT_SYMBOL_GPL(put_online_cpus);
/*
* This ensures that the hotplug operation can begin only when the
* refcount goes to zero.
*
- * Note that during a cpu-hotplug operation, the new readers, if any,
- * will be blocked by the cpu_hotplug.lock
- *
* Since cpu_hotplug_begin() is always called after invoking
* cpu_maps_update_begin(), we can be sure that only one writer is active.
- *
- * Note that theoretically, there is a possibility of a livelock:
- * - Refcount goes to zero, last reader wakes up the sleeping
- * writer.
- * - Last reader unlocks the cpu_hotplug.lock.
- * - A new reader arrives at this moment, bumps up the refcount.
- * - The writer acquires the cpu_hotplug.lock finds the refcount
- * non zero and goes to sleep again.
- *
- * However, this is very difficult to achieve in practice since
- * get_online_cpus() not an api which is called all that often.
- *
*/
void cpu_hotplug_begin(void)
{
- cpu_hotplug.active_writer = current;
+ lockdep_assert_held(&cpu_add_remove_lock);
- for (;;) {
- mutex_lock(&cpu_hotplug.lock);
- if (likely(!cpu_hotplug.refcount))
- break;
- __set_current_state(TASK_UNINTERRUPTIBLE);
- mutex_unlock(&cpu_hotplug.lock);
- schedule();
- }
+ __cpuhp_writer = current;
+
+ /* After this everybody will observe _writer and take the slow path. */
+ synchronize_sched();
+
+ /* Wait for no readers -- reader preference */
+ cpuhp_wait_refcount();
+
+ /* Stop new readers. */
+ cpuhp_set_state(1);
+
+ /* Wait for no readers */
+ cpuhp_wait_refcount();
}
void cpu_hotplug_done(void)
{
- cpu_hotplug.active_writer = NULL;
- mutex_unlock(&cpu_hotplug.lock);
+ __cpuhp_writer = NULL;
+
+ /* Allow new readers */
+ cpuhp_set_state(0);
+
+ wake_up_all(&cpuhp_wq);
}
/*
--
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