[RFC PATCH 22/35] subarch suport for idle loop (NO_IDLE_HZ for Xen)

[Chris Wright <chrisw@sous-sol.org>]

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>
---
 drivers/xen/Kconfig                         |    8 ++++++++
 include/asm-i386/mach-xen/setup_arch_post.h |   24 ++++++++++++++++++++++++
 2 files changed, 32 insertions(+)

--- xen-subarch-2.6.orig/drivers/xen/Kconfig
+++ xen-subarch-2.6/drivers/xen/Kconfig
@@ -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
--- xen-subarch-2.6.orig/include/asm-i386/mach-xen/setup_arch_post.h
+++ xen-subarch-2.6/include/asm-i386/mach-xen/setup_arch_post.h
@@ -8,6 +8,11 @@
 
 #include <xen/interface/physdev.h>
 
+extern void stop_hz_timer(void);
+extern void start_hz_timer(void);
+
+void xen_idle(void);
+
 static char * __init machine_specific_memory_setup(void)
 {
 	unsigned long max_pfn = xen_start_info->nr_pages;
@@ -74,4 +79,23 @@ static void __init machine_specific_arch
 		console_use_vt = 0;
 		conswitchp = NULL;
 	}
+
+	pm_idle = xen_idle;
+}
+
+void xen_idle(void)
+{
+	local_irq_disable();
+
+	if (need_resched())
+		local_irq_enable();
+	else {
+		clear_thread_flag(TIF_POLLING_NRFLAG);
+		smp_mb__after_clear_bit();
+		stop_hz_timer();
+		/* Blocking includes an implicit local_irq_enable(). */
+		HYPERVISOR_sched_op(SCHEDOP_block, 0);
+		start_hz_timer();
+		set_thread_flag(TIF_POLLING_NRFLAG);
+	}
 }

--
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