[RFC PATCH 4/4] timers: Migrating non pinned regular and hrtimers.

[Arun R Bharadwaj <arun@linux.vnet.ibm.com>]

This patch migrates all the non cpu-pinned regular and hrtimers
from a particular cpu to the target cpu specified.

The migration is triggered by setting the target cpu number at
/sys/devices/system/cpu/cpuX/timer_migration.

In the situation where the cpuX goes offline *after* the user sets
X as the target_cpu, then timers are not migrated and stay on
the source cpu. But this is just a short term fix and any discussion
in this regard will help greatly.

I have tested this patch by stressing the system using a script
which continuously hotplug-add and removes the cpus.


Signed-off-by: Arun R Bharadwaj <arun@linux.vnet.ibm.com>
---
 kernel/hrtimer.c |    6 ++++++
 kernel/timer.c   |    7 ++++++-
 2 files changed, 12 insertions(+), 1 deletion(-)

Index: linux-2.6.27/kernel/timer.c
===================================================================
--- linux-2.6.27.orig/kernel/timer.c
+++ linux-2.6.27/kernel/timer.c
@@ -547,7 +547,7 @@ int __mod_timer(struct timer_list *timer
 {
 	struct tvec_base *base, *new_base;
 	unsigned long flags;
-	int ret = 0;
+	int ret = 0, target_cpu;
 
 	timer_stats_timer_set_start_info(timer);
 	BUG_ON(!timer->function);
@@ -562,6 +562,11 @@ int __mod_timer(struct timer_list *timer
 	debug_timer_activate(timer);
 
 	new_base = __get_cpu_var(tvec_bases);
+	target_cpu = __get_cpu_var(enable_timer_migration);
+	if (target_cpu != smp_processor_id() && !tbase_get_pinned(timer->base)
+			&& cpu_online(target_cpu))
+		new_base = per_cpu(tvec_bases, target_cpu);
+
 
 	if (base != new_base) {
 		/*
Index: linux-2.6.27/kernel/hrtimer.c
===================================================================
--- linux-2.6.27.orig/kernel/hrtimer.c
+++ linux-2.6.27/kernel/hrtimer.c
@@ -206,8 +206,14 @@ switch_hrtimer_base(struct hrtimer *time
 {
 	struct hrtimer_clock_base *new_base;
 	struct hrtimer_cpu_base *new_cpu_base;
+	int target_cpu;
 
 	new_cpu_base = &__get_cpu_var(hrtimer_bases);
+	target_cpu = __get_cpu_var(enable_timer_migration);
+	if (target_cpu != smp_processor_id() && !is_hrtimer_pinned(timer)
+			&& cpu_online(target_cpu))
+		new_cpu_base = &per_cpu(hrtimer_bases, target_cpu);
+
 	new_base = &new_cpu_base->clock_base[base->index];
 
 	if (base != new_base) {