[Xen-devel] [PATCH] xen: migrate vcpus only when needed

[Ryan Harper <ryanh@xxxxxxxxxx>]

Currently, when pinning vcpus, the target vcpu is migrated to the first
bit in the new cpumask even if the vcpu's current process is included in
the new mask.  This produces a non-obvious side-effect when pinning
vcpus.


[x460-3 ~]# xm vcpu-list 5
Name                              ID  VCPU  CPU  State  Time(s)  CPU Affinity
x460-3-guest                       5     0   31   -b-       4.2  any cpu
[x460-3 ~]# xm vcpu-list 5
[x460-3 ~]# xm vcpu-pin 5 0 28-31
[x460-3 ~]# xm vcpu-list 5
Name                              ID  VCPU  CPU  State  Time(s)  CPU Affinity
x460-3-guest                       5     0   28   -b-       4.2  28-31

This patch adds a check to determine if a vcpu needs to switch
processors during a pin operation, removing the above side-effect.

[x460-3 ~]# xm vcpu-list 2
Name                              ID  VCPU  CPU  State  Time(s)  CPU Affinity
x460-3-guest                       2     0   31   -b-       4.2  any cpu
[x460-3 ~]# xm vcpu-pin 2 0 28-31
Name                              ID  VCPU  CPU  State  Time(s)  CPU Affinity
x460-3-guest                       2     0   31   -b-       4.2  28-31
[x460-3 ~]# xm vcpu-pin 2 0 28-30
[x460-3 ~]# xm vcpu-list 2
Name                              ID  VCPU  CPU  State  Time(s)  CPU Affinity
x460-3-guest                       2     0   28   -b-       4.2  28-30

Tested on sedf scheduler.

-- 
Ryan Harper
Software Engineer; Linux Technology Center
IBM Corp., Austin, Tx
(512) 838-9253   T/L: 678-9253
ryanh@xxxxxxxxxx


diffstat output:
 sched_bvt.c  |   14 ++++++++++----
 sched_sedf.c |   12 +++++++++---
 2 files changed, 19 insertions(+), 7 deletions(-)

---
# HG changeset patch
# User rharper@x460-3
# Node ID ffa67b655a8f5855549880a0733c7fae6b86f3ae
# Parent  ad686e96536fe8c9318efa80e777d5c2555f2682
When setting a vcpu's affinity via pin operations, only 
migrate the vcpu if its current processor is not in the 
new affinity mask.

Signed-off-by: Ryan Harper <ryanh@xxxxxxxxxx>

diff -r ad686e96536f -r ffa67b655a8f xen/common/sched_bvt.c
--- a/xen/common/sched_bvt.c    Sat Apr 15 04:17:27 2006
+++ b/xen/common/sched_bvt.c    Sat Apr 15 05:12:00 2006
@@ -287,11 +287,17 @@
     if ( v == current )
         return cpu_isset(v->processor, *affinity) ? 0 : -EBUSY;
 
-    vcpu_pause(v);
+    /* 
+     * only migrate the vcpu if the vcpu's processor is not in 
+     * the new cpumask_t
+     */
+    if ( !cpu_isset(v->processor, *affinity) ) {
+        vcpu_pause(v);
+        v->processor = first_cpu(*affinity);
+        EBVT_INFO(v)->migrated = 1;
+        vcpu_unpause(v);
+    }
     v->cpu_affinity = *affinity;
-    v->processor = first_cpu(v->cpu_affinity);
-    EBVT_INFO(v)->migrated = 1;
-    vcpu_unpause(v);
 
     return 0;
 }
diff -r ad686e96536f -r ffa67b655a8f xen/common/sched_sedf.c
--- a/xen/common/sched_sedf.c   Sat Apr 15 04:17:27 2006
+++ b/xen/common/sched_sedf.c   Sat Apr 15 05:12:00 2006
@@ -1186,10 +1186,16 @@
     if ( v == current )
         return cpu_isset(v->processor, *affinity) ? 0 : -EBUSY;
 
-    vcpu_pause(v);
+    /* 
+     * only migrate the vcpu if the vcpu's processor is not in 
+     * the new cpumask_t
+     */
+    if ( !cpu_isset(v->processor,*affinity) ) {
+        vcpu_pause(v);
+        v->processor = first_cpu(*affinity);
+        vcpu_unpause(v);
+    }
     v->cpu_affinity = *affinity;
-    v->processor = first_cpu(v->cpu_affinity);
-    vcpu_unpause(v);
 
     return 0;
 }

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