New version, now with excessive comments.
I found a deadlock (where both reader and writer would go to sleep);
identified below as case 1b.
The implementation without patch is reader biased, this implementation,
as Mel pointed out, is writer biased. I should try and fix this but I'm
stepping away from the computer now as I have the feeling I'll only
wreck stuff from now on.
---
Subject: hotplug: Optimize {get,put}_online_cpus()
From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Date: Tue Sep 17 16:17:11 CEST 2013
The current implementation uses global state, change it so the reader
side uses per-cpu state in the contended fast path.
Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: Paul McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Signed-off-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
---
include/linux/cpu.h | 29 ++++++++-
kernel/cpu.c | 159 ++++++++++++++++++++++++++++++++++------------------
2 files changed, 134 insertions(+), 54 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 +176,32 @@ 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();
+
+ this_cpu_inc(__cpuhp_refcount);
+ smp_mb(); /* see comment near __get_online_cpus() */
+ if (unlikely(__cpuhp_writer))
+ __get_online_cpus();
+}
+
+extern void __put_online_cpus(void);
+
+static inline void put_online_cpus(void)
+{
+ this_cpu_dec(__cpuhp_refcount);
+ smp_mb(); /* see comment near __get_online_cpus() */
+ if (unlikely(__cpuhp_writer))
+ __put_online_cpus();
+}
+
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 +49,143 @@ 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,
-};
+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 DECLARE_WAIT_QUEUE_HEAD(cpuhp_wq);
+
+/*
+ * We must order things like:
+ *
+ * CPU0 -- read-lock CPU1 -- write-lock
+ *
+ * STORE __cpuhp_refcount STORE __cpuhp_writer
+ * MB MB
+ * LOAD __cpuhp_writer LOAD __cpuhp_refcount
+ *
+ *
+ * This gives rise to the following permutations:
+ *
+ * a) all of R happend before W
+ * b) R starts but sees the W store -- therefore W must see the R store
+ * W starts but sees the R store -- therefore R must see the W store
+ * c) all of W happens before R
+ *
+ * 1) RL vs WL:
+ *
+ * 1a) RL proceeds; WL observes refcount and goes wait for !refcount.
+ * 1b) RL drops into the slow path; WL waits for !refcount.
+ * 1c) WL proceeds; RL drops into the slow path.
+ *
+ * 2) RL vs WU:
+ *
+ * 2a) RL drops into the slow path; WU clears writer and wakes RL
+ * 2b) RL proceeds; WU continues to wake others
+ * 2d) RL proceeds.
+ *
+ * 3) RU vs WL:
+ *
+ * 3a) RU proceeds; WL proceeds.
+ * 3b) RU drops to slow path; WL proceeds
+ * 3c) WL waits for !refcount; RL drops to slow path
+ *
+ * 4) RU vs WU:
+ *
+ * Impossible since R and W state are mutually exclusive.
+ *
+ * This leaves us to consider the R slow paths:
+ *
+ * RL
+ *
+ * 1b) we must wake W
+ * 2a) nothing of importance
+ *
+ * RU
+ *
+ * 3b) nothing of importance
+ * 3c) we must wake W
+ *
+ */
-void get_online_cpus(void)
+void __get_online_cpus(void)
{
- might_sleep();
- if (cpu_hotplug.active_writer == current)
+ if (__cpuhp_writer == current)
return;
- mutex_lock(&cpu_hotplug.lock);
- cpu_hotplug.refcount++;
- mutex_unlock(&cpu_hotplug.lock);
+again:
+ /*
+ * Case 1b; we must decrement our refcount again otherwise WL will
+ * never observe !refcount and stay blocked forever. Not good since
+ * we're going to sleep too. Someone must be awake and do something.
+ *
+ * Skip recomputing the refcount, just wake the pending writer and
+ * have him check it -- writers are rare.
+ */
+ this_cpu_dec(__cpuhp_refcount);
+ wake_up_process(__cpuhp_writer); /* implies MB */
+
+ wait_event(cpuhp_wq, !__cpuhp_writer);
+
+ /* Basically re-do the fast-path. Excep we can never be the writer. */
+ this_cpu_inc(__cpuhp_refcount);
+ smp_mb();
+ if (unlikely(__cpuhp_writer))
+ goto again;
}
-EXPORT_SYMBOL_GPL(get_online_cpus);
+EXPORT_SYMBOL_GPL(__get_online_cpus);
-void put_online_cpus(void)
+void __put_online_cpus(void)
{
- if (cpu_hotplug.active_writer == current)
- return;
- mutex_lock(&cpu_hotplug.lock);
+ unsigned int refcnt = 0;
+ int cpu;
- if (WARN_ON(!cpu_hotplug.refcount))
- cpu_hotplug.refcount++; /* try to fix things up */
+ if (__cpuhp_writer == current)
+ return;
- if (!--cpu_hotplug.refcount && unlikely(cpu_hotplug.active_writer))
- wake_up_process(cpu_hotplug.active_writer);
- mutex_unlock(&cpu_hotplug.lock);
+ /* 3c */
+ for_each_possible_cpu(cpu)
+ refcnt += per_cpu(__cpuhp_refcount, cpu);
+ if (!refcnt)
+ wake_up_process(__cpuhp_writer);
}
-EXPORT_SYMBOL_GPL(put_online_cpus);
+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;
+ __cpuhp_writer = current;
for (;;) {
- mutex_lock(&cpu_hotplug.lock);
- if (likely(!cpu_hotplug.refcount))
+ unsigned int refcnt = 0;
+ int cpu;
+
+ set_current_state(TASK_UNINTERRUPTIBLE); /* implies MB */
+
+ for_each_possible_cpu(cpu)
+ refcnt += per_cpu(__cpuhp_refcount, cpu);
+
+ if (!refcnt)
break;
- __set_current_state(TASK_UNINTERRUPTIBLE);
- mutex_unlock(&cpu_hotplug.lock);
+
schedule();
}
+ __set_current_state(TASK_RUNNING);
}
void cpu_hotplug_done(void)
{
- cpu_hotplug.active_writer = NULL;
- mutex_unlock(&cpu_hotplug.lock);
+ __cpuhp_writer = NULL;
+ wake_up_all(&cpuhp_wq); /* implies MB */
}
/*
--
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