[Xen-devel] [PATCH 2/2] credit2: track residual from divisions done during accounting

[George Dunlap <george.dunlap@xxxxxxxxxxxxx>]

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

Don't bother subtracting the residual from the runtime, as it can only ever
add up to one nanosecond, and will end up being debited during the next
reset interval anyway.

Original-patch-by: Jan Beulich <jbeulich@xxxxxxxx>
Signed-off-by: George Dunlap <george.dunlap@xxxxxxxxxxxxx>
---
 xen/common/sched_credit2.c |   24 ++++++++++++++++--------
 1 file changed, 16 insertions(+), 8 deletions(-)

diff --git a/xen/common/sched_credit2.c b/xen/common/sched_credit2.c
index 804049e..eb544db 100644
--- 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) */
@@ -271,14 +272,22 @@ struct csched_dom {
 
 /*
  * Time-to-credit, credit-to-time.
+ * 
+ * We keep track of the "residual" time to make sure that frequent short
+ * schedules still get accounted for in the end.
+ *
  * FIXME: Do pre-calculated division?
  */
-static s_time_t t2c(struct csched_runqueue_data *rqd, s_time_t time, struct 
csched_vcpu *svc)
+static void t2c_update(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);
+    svc->credit -= val;
 }
 
-static s_time_t c2t(struct csched_runqueue_data *rqd, s_time_t credit, struct 
csched_vcpu *svc)
+static s_time_t c2t_adjusted(struct csched_runqueue_data *rqd, s_time_t 
credit, struct csched_vcpu *svc)
 {
     return credit * svc->weight / rqd->max_weight;
 }
@@ -636,8 +645,7 @@ void burn_credits(struct csched_runqueue_data *rqd, struct 
csched_vcpu *svc, s_t
     delta = now - svc->start_time;
 
     if ( delta > 0 ) {
-        /* This will round down; should we consider rounding up...? */
-        svc->credit -= t2c(rqd, delta, svc);
+        t2c_update(rqd, delta, svc);
         svc->start_time = now;
 
         d2printk("b d%dv%d c%d\n",
-- 
1.7.9.5


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