Implement tasklets as running in VCPU context (sepcifically, idle-VCPU context)

...rather than in softirq context. This is expected to avoid a lot of
subtle deadlocks relating to the fact that softirqs can interrupt a
scheduled vcpu.

Signed-off-by: Keir Fraser <keir.fraser@citrix.com>

1 files changed, 13 insertions, 1 deletions

diff --git a/xen/common/sched_credit.c b/xen/common/sched_credit.c
index ce42165402..cc191e26dc 100644
--- a/xen/common/sched_credit.c
+++ b/xen/common/sched_credit.c
@@ -1167,12 +1167,17 @@ csched_schedule(s_time_t now)
     CSCHED_STAT_CRANK(schedule);
     CSCHED_VCPU_CHECK(current);
 
-    /* Update credits */
     if ( !is_idle_vcpu(scurr->vcpu) )
     {
+        /* Update credits of a non-idle VCPU. */
         burn_credits(scurr, now);
         scurr->start_time -= now;
     }
+    else
+    {
+        /* Re-instate a boosted idle VCPU as normal-idle. */
+        scurr->pri = CSCHED_PRI_IDLE;
+    }
 
     /*
      * Select next runnable local VCPU (ie top of local runq)
@@ -1184,6 +1189,13 @@ csched_schedule(s_time_t now)
 
     snext = __runq_elem(runq->next);
 
+    /* Tasklet work (which runs in idle VCPU context) overrides all else. */
+    if ( !tasklet_queue_empty(cpu) )
+    {
+        snext = CSCHED_VCPU(idle_vcpu[cpu]);
+        snext->pri = CSCHED_PRI_TS_BOOST;
+    }
+
     /*
      * SMP Load balance:
      *