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Subject[RFC PATCH 26/33] subarch suport for idle loop (NO_IDLE_HZ for Xen)
Paravirtualize the idle loop to explicitly trap to the hypervisor when
blocking, and to use the NO_IDLE_HZ functionality introduced by s390
to inform the rcu subsystem that the CPU is quiescent.

Signed-off-by: Ian Pratt <ian.pratt@xensource.com>
Signed-off-by: Christian Limpach <Christian.Limpach@cl.cam.ac.uk>
Signed-off-by: Chris Wright <chrisw@sous-sol.org>
Signed-off-by: Jeremy Fitzhardinge <jeremy@xensource.com>
---
arch/i386/mach-xen/setup-xen.c | 59 ++++++++++++++++++++++++++++++++++
drivers/xen/Kconfig | 8 ++++
drivers/xen/core/time.c | 21 ++++++++++++
include/asm-i386/mach-xen/mach_time.h | 3 +
4 files changed, 91 insertions(+)
diff -r 793d8e45fb1e arch/i386/mach-xen/setup-xen.c
--- a/arch/i386/mach-xen/setup-xen.c Tue Jul 18 03:41:42 2006 -0400
+++ b/arch/i386/mach-xen/setup-xen.c Tue Jul 18 03:43:53 2006 -0400
@@ -7,6 +7,7 @@
#include <linux/mm.h>
#include <linux/bootmem.h>
#include <linux/initrd.h>
+#include <linux/pm.h>

#include <asm/e820.h>
#include <asm/setup.h>
@@ -15,6 +16,9 @@

#include <xen/interface/physdev.h>

+#include "mach_time.h"
+
+static void xen_idle(void);

struct start_info *xen_start_info;
EXPORT_SYMBOL(xen_start_info);
@@ -82,4 +86,59 @@ void __init machine_specific_arch_setup(
initrd_reserve_bootmem = 0;
initrd_below_start_ok = 1;
}
+
+
+ pm_idle = xen_idle;
}
+
+/*
+ * stop_hz_timer / start_hz_timer - enter/exit 'tickless mode' on an idle cpu
+ * These functions are based on implementations from arch/s390/kernel/time.c
+ */
+static void stop_hz_timer(void)
+{
+ unsigned int cpu = smp_processor_id();
+ unsigned long j;
+
+ cpu_set(cpu, nohz_cpu_mask);
+
+ /* See matching smp_mb in rcu_start_batch in rcupdate.c. These mbs */
+ /* ensure that if __rcu_pending (nested in rcu_needs_cpu) fetches a */
+ /* value of rcp->cur that matches rdp->quiescbatch and allows us to */
+ /* stop the hz timer then the cpumasks created for subsequent values */
+ /* of cur in rcu_start_batch are guaranteed to pick up the updated */
+ /* nohz_cpu_mask and so will not depend on this cpu. */
+
+ smp_mb();
+
+ /* Leave ourselves in 'tick mode' if rcu or softirq or timer pending. */
+ if (rcu_needs_cpu(cpu) || local_softirq_pending() ||
+ (j = next_timer_interrupt(), time_before_eq(j, jiffies))) {
+ cpu_clear(cpu, nohz_cpu_mask);
+ j = jiffies + 1;
+ }
+
+ BUG_ON(HYPERVISOR_set_timer_op(jiffies_to_st(j)) != 0);
+}
+
+static void start_hz_timer(void)
+{
+ cpu_clear(smp_processor_id(), nohz_cpu_mask);
+}
+
+static void xen_idle(void)
+{
+ local_irq_disable();
+
+ if (need_resched())
+ local_irq_enable();
+ else {
+ current_thread_info()->status &= ~TS_POLLING;
+ smp_mb__after_clear_bit();
+ stop_hz_timer();
+ /* Blocking includes an implicit local_irq_enable(). */
+ HYPERVISOR_sched_op(SCHEDOP_block, 0);
+ start_hz_timer();
+ current_thread_info()->status |= TS_POLLING;
+ }
+}
diff -r 793d8e45fb1e drivers/xen/Kconfig
--- a/drivers/xen/Kconfig Tue Jul 18 03:41:42 2006 -0400
+++ b/drivers/xen/Kconfig Tue Jul 18 03:43:53 2006 -0400
@@ -12,6 +12,14 @@ config XEN

if XEN

+config NO_IDLE_HZ
+ bool
+ default y
+ help
+ Switches the regular HZ timer off when the system is going idle.
+ This helps Xen to detect that the Linux system is idle, reducing
+ the overhead of idle systems.
+
config XEN_SHADOW_MODE
bool
default y
diff -r 793d8e45fb1e drivers/xen/core/time.c
--- a/drivers/xen/core/time.c Tue Jul 18 03:41:42 2006 -0400
+++ b/drivers/xen/core/time.c Tue Jul 18 03:43:53 2006 -0400
@@ -227,6 +227,27 @@ void do_timer_interrupt_hook(struct pt_r
update_process_times(user_mode_vm(regs));
}

+
+/* Convert jiffies to Xen system time. */
+u64 jiffies_to_st(unsigned long j)
+{
+ unsigned long seq;
+ long delta;
+ u64 st;
+
+ do {
+ seq = read_seqbegin(&xtime_lock);
+ delta = j - jiffies;
+ /* NB. The next check can trigger in some wrap-around cases,
+ * but that's ok: we'll just end up with a shorter timeout. */
+ if (delta < 1)
+ delta = 1;
+ st = processed_system_time + (delta * (u64)NS_PER_TICK);
+ } while (read_seqretry(&xtime_lock, seq));
+
+ return st;
+}
+
static cycle_t xen_clocksource_read(void)
{
struct shadow_time_info *shadow = &per_cpu(shadow_time, smp_processor_id());
diff -r 793d8e45fb1e include/asm-i386/mach-xen/mach_time.h
--- a/include/asm-i386/mach-xen/mach_time.h Tue Jul 18 03:41:42 2006 -0400
+++ b/include/asm-i386/mach-xen/mach_time.h Tue Jul 18 03:43:53 2006 -0400
@@ -13,4 +13,7 @@ int mach_set_rtc_mmss(unsigned long nowt
int mach_set_rtc_mmss(unsigned long nowtime);
unsigned long mach_get_cmos_time(void);

+/* Convert jiffies to Xen system time. */
+u64 jiffies_to_st(unsigned long j);
+
#endif /* !_MACH_TIME_H */
--
-
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