Fix for VCPU periodic timer.

Idle vcpu periodic timer is useless. It increased the lapic timer
interrupt number, which decreased the cpu idle residency. This patch
disables idle vcpu periodic timer via keeping v->periodic_period = 0.

The vcpu periodic timer may be expired 50us before expected time
because there is a 50us TIMER_SLOP used for soft timer (xen/common/timer.c,
timer_softirq_action()). This will cause vcpu_periodic_timer_work() be
continuously called tens of times in a single call of
timer_softirq_action() until (now > periodic_next_event). It brings
unnecessary overhead. This patch adds a similar time slop in vcpu
periodic timer to eliminate this overhead.

Signed-off-by: Wei Gang <gang.wei@intel.com>

diff --git a/xen/arch/x86/domain.c b/xen/arch/x86/domain.c
index 1dc24924e6f8db028de819a521bf4a5bdbcd71dc..10d0cef7c652bef24899a6984a6c60790e273e56 100644 (file)
--- a/xen/arch/x86/domain.c
+++ b/xen/arch/x86/domain.c
@@ -302,7 +302,8 @@ int vcpu_initialise(struct vcpu *v)
     else
     {
         /* PV guests by default have a 100Hz ticker. */
-        v->periodic_period = MILLISECS(10);
+        if ( !is_idle_domain(d) )
+            v->periodic_period = MILLISECS(10);
 
         /* PV guests get an emulated PIT too for video BIOSes to use. */
         if ( !is_idle_domain(d) && (v->vcpu_id == 0) )

diff --git a/xen/common/schedule.c b/xen/common/schedule.c
index 2934d07029d43b373709c8a0584a6b429d5ad765..7d32c99485572e09c2e58a2054f29df0eab04a23 100644 (file)
--- a/xen/common/schedule.c
+++ b/xen/common/schedule.c
@@ -628,7 +628,9 @@ static void vcpu_periodic_timer_work(struct vcpu *v)
         return;
 
     periodic_next_event = v->periodic_last_event + v->periodic_period;
-    if ( now > periodic_next_event )
+
+    /* The timer subsystem may call us up to TIME_SLOP ahead of deadline. */
+    if ( (now + TIME_SLOP) > periodic_next_event )
     {
         send_timer_event(v);
         v->periodic_last_event = now;