[Xen-changelog] [xen stable-4.2] SEDF: avoid gathering vCPU-s on pCPU0

[patchbot@xxxxxxx]

commit 69a5f4dc9357b4693c471c50f429211c8c00bc7b
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Tue Mar 12 16:15:18 2013 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Tue Mar 12 16:15:18 2013 +0100

    SEDF: avoid gathering vCPU-s on pCPU0
    
    The introduction of vcpu_force_reschedule() in 14320:215b799fa181 was
    incompatible with the SEDF scheduler: Any vCPU using
    VCPUOP_stop_periodic_timer (e.g. any vCPU of half way modern PV Linux
    guests) ends up on pCPU0 after that call. Obviously, running all PV
    guests' (and namely Dom0's) vCPU-s on pCPU0 causes problems for those
    guests rather sooner than later.
    
    So the main thing that was clearly wrong (and bogus from the beginning)
    was the use of cpumask_first() in sedf_pick_cpu(). It is being replaced
    by a construct that prefers to put back the vCPU on the pCPU that it
    got launched on.
    
    However, there's one more glitch: When reducing the affinity of a vCPU
    temporarily, and then widening it again to a set that includes the pCPU
    that the vCPU was last running on, the generic scheduler code would not
    force a migration of that vCPU, and hence it would forever stay on the
    pCPU it last ran on. Since that can again create a load imbalance, the
    SEDF scheduler wants a migration to happen regardless of it being
    apparently unnecessary.
    
    Of course, an alternative to checking for SEDF explicitly in
    vcpu_set_affinity() would be to introduce a flags field in struct
    scheduler, and have SEDF set a "always-migrate-on-affinity-change"
    flag.
    
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Acked-by: Keir Fraser <keir@xxxxxxx>
---
 xen/common/sched_sedf.c |    3 ++-
 xen/common/schedule.c   |    3 ++-
 2 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/xen/common/sched_sedf.c b/xen/common/sched_sedf.c
index 644787b..92ba36a 100644
--- a/xen/common/sched_sedf.c
+++ b/xen/common/sched_sedf.c
@@ -396,7 +396,8 @@ static int sedf_pick_cpu(const struct scheduler *ops, 
struct vcpu *v)
 
     online = cpupool_scheduler_cpumask(v->domain->cpupool);
     cpumask_and(&online_affinity, v->cpu_affinity, online);
-    return cpumask_first(&online_affinity);
+    return cpumask_cycle(v->vcpu_id % cpumask_weight(&online_affinity) - 1,
+                         &online_affinity);
 }
 
 /*
diff --git a/xen/common/schedule.c b/xen/common/schedule.c
index b82b4fe..c2cd9d5 100644
--- a/xen/common/schedule.c
+++ b/xen/common/schedule.c
@@ -611,7 +611,8 @@ int vcpu_set_affinity(struct vcpu *v, const cpumask_t 
*affinity)
     vcpu_schedule_lock_irq(v);
 
     cpumask_copy(v->cpu_affinity, affinity);
-    if ( !cpumask_test_cpu(v->processor, v->cpu_affinity) )
+    if ( VCPU2OP(v)->sched_id == XEN_SCHEDULER_SEDF ||
+         !cpumask_test_cpu(v->processor, v->cpu_affinity) )
         set_bit(_VPF_migrating, &v->pause_flags);
 
     vcpu_schedule_unlock_irq(v);
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.2

_______________________________________________
Xen-changelog mailing list
Xen-changelog@xxxxxxxxxxxxx
http://lists.xensource.com/xen-changelog