[RFC][PATCH 1/4] sched: more accurate credit scheduling

[NISHIGUCHI Naoki <nisiguti@jp.fujitsu.com>]

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By applying this patch, the credit scheduler subtracts accurately credit 
consumed and sets correctly priority.
CSCHED_CREDITS_PER_TICK is changed from 100 to 10000, because vcpu's 
credit is subtracted in csched_schedule().

The difference between this patch and last patch is that start_time 
variable was moved from csched_vcpu structure to csched_pcpu structure.

Best regards,
Naoki Nishiguchi

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diff -r 6595393a3d28 xen/common/sched_credit.c
--- a/xen/common/sched_credit.c	Tue Dec 09 16:28:02 2008 +0000
+++ b/xen/common/sched_credit.c	Tue Dec 16 19:15:18 2008 +0900
@@ -42,7 +42,7 @@
 #define CSCHED_MSECS_PER_TICK       10
 #define CSCHED_MSECS_PER_TSLICE     \
     (CSCHED_MSECS_PER_TICK * CSCHED_TICKS_PER_TSLICE)
-#define CSCHED_CREDITS_PER_TICK     100
+#define CSCHED_CREDITS_PER_TICK     10000
 #define CSCHED_CREDITS_PER_TSLICE   \
     (CSCHED_CREDITS_PER_TICK * CSCHED_TICKS_PER_TSLICE)
 #define CSCHED_CREDITS_PER_ACCT     \
@@ -188,6 +188,7 @@ struct csched_pcpu {
     uint32_t runq_sort_last;
     struct timer ticker;
     unsigned int tick;
+    s_time_t start_time;
 };
 
 /*
@@ -545,11 +546,6 @@ csched_vcpu_acct(unsigned int cpu)
         svc->pri = CSCHED_PRI_TS_UNDER;
 
     /*
-     * Update credits
-     */
-    atomic_sub(CSCHED_CREDITS_PER_TICK, &svc->credit);
-
-    /*
      * Put this VCPU and domain back on the active list if it was
      * idling.
      *
@@ -1168,12 +1164,27 @@ csched_schedule(s_time_t now)
 {
     const int cpu = smp_processor_id();
     struct list_head * const runq = RUNQ(cpu);
+    struct csched_pcpu * const spc = CSCHED_PCPU(cpu);
     struct csched_vcpu * const scurr = CSCHED_VCPU(current);
     struct csched_vcpu *snext;
     struct task_slice ret;
+    s_time_t passed = now - spc->start_time;
+    int consumed;
 
     CSCHED_STAT_CRANK(schedule);
     CSCHED_VCPU_CHECK(current);
+
+    /*
+     * Update credit
+     */
+    consumed = ( passed +
+                 (MILLISECS(CSCHED_MSECS_PER_TSLICE) /
+                  CSCHED_CREDITS_PER_TSLICE - 1)
+               ) /
+               ( MILLISECS(CSCHED_MSECS_PER_TSLICE) /
+                 CSCHED_CREDITS_PER_TSLICE );
+    if ( consumed > 0 && !is_idle_vcpu(current) )
+        atomic_sub(consumed, &scurr->credit);
 
     /*
      * Select next runnable local VCPU (ie top of local runq)
@@ -1217,6 +1228,8 @@ csched_schedule(s_time_t now)
      */
     ret.time = MILLISECS(CSCHED_MSECS_PER_TSLICE);
     ret.task = snext->vcpu;
+
+    spc->start_time = now;
 
     CSCHED_VCPU_CHECK(ret.task);
     return ret;

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