[Xen-devel] [PATCH 6/6] xen: credit2: avoid cpumask_any() in pick_cpu().

[Dario Faggioli <dario.faggioli@xxxxxxxxxx>]

cpumask_any() is costly (because of the randomization).
And since it does not really matter which exact CPU is
selected within a runqueue, as that will be overridden
shortly after, in runq_tickle(), spending too much time
and achieving true randomization is pretty pointless.

As the picked CPU, however, would be used as an hint,
within runq_tickle(), don't give up on it entirely,
and let's make sure we don't always return the same
CPU, or favour lower or higher ID CPUs.

To achieve that, let's record and remember, for each
runqueue, what CPU we picked for last, and start from
that the following time.

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 |   22 ++++++++++++++++++----
 1 file changed, 18 insertions(+), 4 deletions(-)

diff --git a/xen/common/sched_credit2.c b/xen/common/sched_credit2.c
index af457c1..7b9e1a1 100644
--- a/xen/common/sched_credit2.c
+++ b/xen/common/sched_credit2.c
@@ -363,6 +363,7 @@ struct csched2_runqueue_data {
     struct list_head runq; /* Ordered list of runnable vms */
     struct list_head svc;  /* List of all vcpus assigned to this runqueue */
     unsigned int max_weight;
+    unsigned int pick_bias;/* Last CPU we picked. Start from it next time */
 
     cpumask_t idle,        /* Currently idle pcpus */
         smt_idle,          /* Fully idle-and-untickled cores (see below) */
@@ -1679,7 +1680,9 @@ csched2_cpu_pick(const struct scheduler *ops, struct vcpu 
*vc)
         {
             cpumask_and(cpumask_scratch_cpu(cpu), cpumask_scratch_cpu(cpu),
                         &svc->migrate_rqd->active);
-            new_cpu = cpumask_any(cpumask_scratch_cpu(cpu));
+            new_cpu = cpumask_cycle(svc->migrate_rqd->pick_bias,
+                                    cpumask_scratch_cpu(cpu));
+            svc->migrate_rqd->pick_bias = new_cpu;
             goto out_up;
         }
         /* Fall-through to normal cpu pick */
@@ -1737,7 +1740,9 @@ csched2_cpu_pick(const struct scheduler *ops, struct vcpu 
*vc)
 
     cpumask_and(cpumask_scratch_cpu(cpu), cpumask_scratch_cpu(cpu),
                 &prv->rqd[min_rqi].active);
-    new_cpu = cpumask_any(cpumask_scratch_cpu(cpu));
+    new_cpu = cpumask_cycle(prv->rqd[min_rqi].pick_bias,
+                            cpumask_scratch_cpu(cpu));
+    prv->rqd[min_rqi].pick_bias = new_cpu;
     BUG_ON(new_cpu >= nr_cpu_ids);
 
  out_up:
@@ -1854,7 +1859,9 @@ static void migrate(const struct scheduler *ops,
                     cpupool_domain_cpumask(svc->vcpu->domain));
         cpumask_and(cpumask_scratch_cpu(cpu), cpumask_scratch_cpu(cpu),
                     &trqd->active);
-        svc->vcpu->processor = cpumask_any(cpumask_scratch_cpu(cpu));
+        svc->vcpu->processor = cpumask_cycle(trqd->pick_bias,
+                                             cpumask_scratch_cpu(cpu));
+        trqd->pick_bias = svc->vcpu->processor;
         ASSERT(svc->vcpu->processor < nr_cpu_ids);
 
         _runq_assign(svc, trqd);
@@ -2821,13 +2828,15 @@ csched2_dump(const struct scheduler *ops)
         printk("Runqueue %d:\n"
                "\tncpus              = %u\n"
                "\tcpus               = %s\n"
-               "\tmax_weight         = %d\n"
+               "\tmax_weight         = %u\n"
+               "\tpick_bias          = %u\n"
                "\tinstload           = %d\n"
                "\taveload            = %"PRI_stime" (~%"PRI_stime"%%)\n",
                i,
                cpumask_weight(&prv->rqd[i].active),
                cpustr,
                prv->rqd[i].max_weight,
+               prv->rqd[i].pick_bias,
                prv->rqd[i].load,
                prv->rqd[i].avgload,
                fraction);
@@ -2930,6 +2939,9 @@ init_pdata(struct csched2_private *prv, unsigned int cpu)
     __cpumask_set_cpu(cpu, &prv->initialized);
     __cpumask_set_cpu(cpu, &rqd->smt_idle);
 
+    if ( cpumask_weight(&rqd->active) == 1 )
+        rqd->pick_bias = cpu;
+
     return rqi;
 }
 
@@ -3042,6 +3054,8 @@ csched2_deinit_pdata(const struct scheduler *ops, void 
*pcpu, int cpu)
         printk(XENLOG_INFO " No cpus left on runqueue, disabling\n");
         deactivate_runqueue(prv, rqi);
     }
+    else if ( rqd->pick_bias == cpu )
+        rqd->pick_bias = cpumask_first(&rqd->active);
 
     spin_unlock(&rqd->lock);
 


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