credit: track residual from divisions done during accounting

This should help with under-accounting of vCPU-s running for extremly
short periods of time, but becoming runnable again at a high frequency.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/common/sched_credit.c
+++ b/xen/common/sched_credit.c
@@ -19,6 +19,7 @@
 #include <xen/sched-if.h>
 #include <xen/softirq.h>
 #include <asm/atomic.h>
+#include <asm/div64.h>
 #include <xen/errno.h>
 #include <xen/keyhandler.h>
 #include <xen/trace.h>
@@ -136,6 +137,7 @@ struct csched_vcpu {
     struct csched_dom *sdom;
     struct vcpu *vcpu;
     atomic_t credit;
+    unsigned int residual;
     s_time_t start_time;   /* When we were scheduled (used for credit) */
     uint16_t flags;
     int16_t pri;
@@ -242,6 +244,7 @@ __runq_remove(struct csched_vcpu *svc)
 static void burn_credits(struct csched_vcpu *svc, s_time_t now)
 {
     s_time_t delta;
+    uint64_t val;
     unsigned int credits;
 
     /* Assert svc is current */
@@ -250,7 +253,10 @@ static void burn_credits(struct csched_v
     if ( (delta = now - svc->start_time) <= 0 )
         return;
 
-    credits = (delta*CSCHED_CREDITS_PER_MSEC + MILLISECS(1)/2) / MILLISECS(1);
+    val = delta * CSCHED_CREDITS_PER_MSEC + svc->residual;
+    svc->residual = do_div(val, MILLISECS(1));
+    credits = val;
+    ASSERT(credits == val);
     atomic_sub(credits, &svc->credit);
     svc->start_time += (credits * MILLISECS(1)) / CSCHED_CREDITS_PER_MSEC;
 }
--- a/xen/common/sched_credit2.c
+++ b/xen/common/sched_credit2.c
@@ -21,7 +21,7 @@
 #include <xen/perfc.h>
 #include <xen/sched-if.h>
 #include <xen/softirq.h>
-#include <asm/atomic.h>
+#include <asm/div64.h>
 #include <xen/errno.h>
 #include <xen/trace.h>
 #include <xen/cpu.h>
@@ -205,7 +205,7 @@ struct csched_runqueue_data {
 
     struct list_head runq; /* Ordered list of runnable vms */
     struct list_head svc;  /* List of all vcpus assigned to this runqueue */
-    int max_weight;
+    unsigned int max_weight;
 
     cpumask_t idle,        /* Currently idle */
         tickled;           /* Another cpu in the queue is already targeted for this one */
@@ -244,7 +244,8 @@ struct csched_vcpu {
     struct csched_dom *sdom;
     struct vcpu *vcpu;
 
-    int weight;
+    unsigned int weight;
+    unsigned int residual;
 
     int credit;
     s_time_t start_time; /* When we were scheduled (used for credit) */
@@ -275,12 +276,15 @@ struct csched_dom {
  */
 static s_time_t t2c(struct csched_runqueue_data *rqd, s_time_t time, struct csched_vcpu *svc)
 {
-    return time * rqd->max_weight / svc->weight;
+    uint64_t val = time * rqd->max_weight + svc->residual;
+
+    svc->residual = do_div(val, svc->weight);
+    return val;
 }
 
 static s_time_t c2t(struct csched_runqueue_data *rqd, s_time_t credit, struct csched_vcpu *svc)
 {
-    return credit * svc->weight / rqd->max_weight;
+    return (credit * svc->weight - svc->residual) / rqd->max_weight;
 }
 
 /*