[Xen-devel] [PATCH 2/2] credit2: Reset until the front of the runqueue is positive

[George Dunlap <george.dunlap@xxxxxxxxxxxxx>]

Under normal circumstances, snext->credit should never be less than
-CSCHED_MIN_TIMER.  However, under some circumstances, a vcpu with low
credits may be allowed to run long enough that its credits are
actually less than -CSCHED_CREDIT_INIT.

(Instances have been observed, for example, where a vcpu with 200us of
credit was allowed to run for 11ms, giving it -10.8ms of credit.  Thus
it was still negative even after the reset.)

If this is the case for snext, we simply want to keep moving everyone
up until it is in the black again.  This fair because none of the
other vcpus want to run at the moment.

Signed-off-by: George Dunlap <george.dunlap@xxxxxxxxxxxxx>
---
 xen/common/sched_credit2.c |   81 +++++++++++++++++++++++++++-----------------
 1 file changed, 49 insertions(+), 32 deletions(-)

diff --git a/xen/common/sched_credit2.c b/xen/common/sched_credit2.c
index 5bf5ebc..7265d5b 100644
--- a/xen/common/sched_credit2.c
+++ b/xen/common/sched_credit2.c
@@ -588,41 +588,58 @@ no_tickle:
 /*
  * Credit-related code
  */
-static void reset_credit(const struct scheduler *ops, int cpu, s_time_t now)
+static void reset_credit(const struct scheduler *ops, int cpu, s_time_t now,
+                         struct csched_vcpu *snext)
 {
     struct csched_runqueue_data *rqd = RQD(ops, cpu);
     struct list_head *iter;
 
-    list_for_each( iter, &rqd->svc )
-    {
-        struct csched_vcpu * svc = list_entry(iter, struct csched_vcpu, 
rqd_elem);
-
-        int start_credit;
-
-        BUG_ON( is_idle_vcpu(svc->vcpu) );
-        BUG_ON( svc->rqd != rqd );
-
-        start_credit = svc->credit;
-
-        /* "Clip" credits to max carryover */
-        if ( svc->credit > CSCHED_CARRYOVER_MAX )
-            svc->credit = CSCHED_CARRYOVER_MAX;
-        /* And add INIT */
-        svc->credit += CSCHED_CREDIT_INIT;
-        svc->start_time = now;
-
-        /* TRACE */ {
-            struct {
-                unsigned dom:16,vcpu:16;
-                unsigned credit_start, credit_end;
-            } d;
-            d.dom = svc->vcpu->domain->domain_id;
-            d.vcpu = svc->vcpu->vcpu_id;
-            d.credit_start = start_credit;
-            d.credit_end = svc->credit;
-            trace_var(TRC_CSCHED2_CREDIT_RESET, 1,
-                      sizeof(d),
-                      (unsigned char *)&d);
+    /*
+     * Under normal circumstances, snext->credit should never be less
+     * than -CSCHED_MIN_TIMER.  However, under some circumstances, a
+     * vcpu with low credits may be allowed to run long enough that
+     * its credits are actually less than -CSCHED_CREDIT_INIT.
+     * (Instances have been observed, for example, where a vcpu with
+     * 200us of credit was allowed to run for 11ms, giving it -10.8ms
+     * of credit.  Thus it was still negative even after the reset.)
+     *
+     * If this is the case for snext, we simply want to keep moving
+     * everyone up until it is in the black again.  This fair because
+     * none of the other vcpus want to run at the moment.
+     */
+    while (snext->credit <= CSCHED_CREDIT_RESET ) {
+        list_for_each( iter, &rqd->svc )
+        {
+            struct csched_vcpu * svc;
+            int start_credit;
+
+            svc = list_entry(iter, struct csched_vcpu, rqd_elem);
+
+            BUG_ON( is_idle_vcpu(svc->vcpu) );
+            BUG_ON( svc->rqd != rqd );
+
+            start_credit = svc->credit;
+
+            /* "Clip" credits to max carryover */
+            if ( svc->credit > CSCHED_CARRYOVER_MAX )
+                svc->credit = CSCHED_CARRYOVER_MAX;
+            /* And add INIT */
+            svc->credit += CSCHED_CREDIT_INIT;
+            svc->start_time = now;
+
+            /* TRACE */ {
+                struct {
+                    unsigned dom:16,vcpu:16;
+                    unsigned credit_start, credit_end;
+                } d;
+                d.dom = svc->vcpu->domain->domain_id;
+                d.vcpu = svc->vcpu->vcpu_id;
+                d.credit_start = start_credit;
+                d.credit_end = svc->credit;
+                trace_var(TRC_CSCHED2_CREDIT_RESET, 1,
+                          sizeof(d),
+                          (unsigned char *)&d);
+            }
         }
     }
 
@@ -1731,7 +1748,7 @@ csched_schedule(
         /* Check for the reset condition */
         if ( snext->credit <= CSCHED_CREDIT_RESET )
         {
-            reset_credit(ops, cpu, now);
+            reset_credit(ops, cpu, now, snext);
             balance_load(ops, cpu, now);
         }
 
-- 
1.7.9.5


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