[Xen-devel] [PATCH 1 of 2] credit2: Don't migrate cpus unnecessarily

[George Dunlap <george.dunlap@xxxxxxxxxxxxx>]

Modern processors have one or two L3's per socket, and generally only
one core per L2.  Credit2's design relies on having credit shared across
several.  So as a first step to sharing a queue across L3 while avoiding
excessive cross-L2 migration

Step one: If the vcpu's current cpu is acceptable, just run it there.

Signed-off-by: George Dunlap <george.dunlap@xxxxxxxxxxxxx>

diff -r 93bf801bd87a -r c48e8e6b9a69 xen/common/sched_credit2.c
--- a/xen/common/sched_credit2.c        Thu Dec 09 17:50:54 2010 +0000
+++ b/xen/common/sched_credit2.c        Thu Dec 09 17:50:54 2010 +0000
@@ -334,6 +334,7 @@
     s_time_t lowest=(1<<30);
     struct csched_runqueue_data *rqd = RQD(ops, cpu);
     cpumask_t *online, mask;
+    struct csched_vcpu * cur;
 
     d2printk("rqt d%dv%d cd%dv%d\n",
              new->vcpu->domain->domain_id,
@@ -341,8 +342,18 @@
              current->domain->domain_id,
              current->vcpu_id);
 
-    online = CSCHED_CPUONLINE(per_cpu(cpupool, cpu));
+    BUG_ON(new->vcpu->processor != cpu);
 
+    /* Look at the cpu it's running on first */
+    cur = CSCHED_VCPU(per_cpu(schedule_data, cpu).curr);
+    burn_credits(rqd, cur, now);
+
+    if ( cur->credit < new->credit )
+    {
+        ipid = cpu;
+        goto tickle;
+    }
+    
     /* Get a mask of idle, but not tickled */
     cpus_andnot(mask, rqd->idle, rqd->tickled);
     
@@ -355,6 +366,8 @@
 
     /* Otherwise, look for the non-idle cpu with the lowest credit,
      * skipping cpus which have been tickled but not scheduled yet */
+    online = CSCHED_CPUONLINE(per_cpu(cpupool, cpu));
+
     cpus_andnot(mask, *online, rqd->idle);
     cpus_andnot(mask, mask, rqd->tickled);
 
@@ -362,6 +375,10 @@
     {
         struct csched_vcpu * cur;
 
+        /* Already looked at this one above */
+        if ( i == cpu )
+            continue;
+
         cur = CSCHED_VCPU(per_cpu(schedule_data, i).curr);
 
         BUG_ON(is_idle_vcpu(cur->vcpu));

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