[Xen-devel] [PATCH 21/24] xen: credit2: optimize runq_candidate() a little bit

[Dario Faggioli <dario.faggioli@xxxxxxxxxx>]

By factoring into one (at the top) all the checks
to see whether current is the idle vcpu, and mark
it as unlikely().

In fact, if current is idle, all the logic for
dealing with yielding, context switching rate
limiting and soft-affinity, is just pure overhead,
and we better rush checking the runq and pick some
vcpu up.

Signed-off-by: Dario Faggioli <dario.faggioli@xxxxxxxxxx>
---
Cc: George Dunlap <george.dunlap@xxxxxxxxxx>
Cc: Anshul Makkar <anshul.makkar@xxxxxxxxxx>
---
 xen/common/sched_credit2.c |   18 +++++++++++++-----
 1 file changed, 13 insertions(+), 5 deletions(-)

diff --git a/xen/common/sched_credit2.c b/xen/common/sched_credit2.c
index ab0122b..21b1f91 100644
--- a/xen/common/sched_credit2.c
+++ b/xen/common/sched_credit2.c
@@ -2730,10 +2730,18 @@ runq_candidate(struct csched2_runqueue_data *rqd,
      * if scurr is yielding, when comparing its credits with other vcpus in
      * the runqueue, act like those other vcpus had yield_bias more credits.
      */
-    int yield_bias = __test_and_clear_bit(__CSFLAG_vcpu_yield, &scurr->flags) ?
-                     CSCHED2_YIELD_BIAS : 0;
+    int yield_bias = 0;
     bool_t cpu_in_soft_aff = 1;
 
+    if ( unlikely(is_idle_vcpu(scurr->vcpu)) )
+    {
+        snext = scurr;
+        goto check_runq;
+    }
+
+    if ( __test_and_clear_bit(__CSFLAG_vcpu_yield, &scurr->flags) )
+        yield_bias = CSCHED2_YIELD_BIAS;
+
     /*
      * Return the current vcpu if it has executed for less than ratelimit.
      * Adjuststment for the selected vcpu's credit and decision
@@ -2748,7 +2756,7 @@ runq_candidate(struct csched2_runqueue_data *rqd,
      * has been cleared already above.
      */
     if ( !yield_bias &&
-         prv->ratelimit_us && !is_idle_vcpu(scurr->vcpu) &&
+         prv->ratelimit_us &&
          vcpu_runnable(scurr->vcpu) &&
          (now - scurr->vcpu->runstate.state_entry_time) <
           MICROSECS(prv->ratelimit_us) )
@@ -2769,8 +2777,7 @@ runq_candidate(struct csched2_runqueue_data *rqd,
         return scurr;
     }
 
-    if ( !is_idle_vcpu(scurr->vcpu) &&
-         has_soft_affinity(scurr->vcpu, scurr->vcpu->cpu_hard_affinity) )
+    if ( has_soft_affinity(scurr->vcpu, scurr->vcpu->cpu_hard_affinity) )
     {
         affinity_balance_cpumask(scurr->vcpu, BALANCE_SOFT_AFFINITY,
                                  cpumask_scratch);
@@ -2804,6 +2811,7 @@ runq_candidate(struct csched2_runqueue_data *rqd,
     else
         snext = CSCHED2_VCPU(idle_vcpu[cpu]);
 
+ check_runq:
     list_for_each( iter, &rqd->runq )
     {
         struct csched2_vcpu * svc = list_entry(iter, struct csched2_vcpu, 
runq_elem);


_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
https://lists.xen.org/xen-devel