[Xen-devel] [PATCH 6/6] xen: sched: optimize exclusive pinning case (Credit1 & 2)

[Dario Faggioli <dario.faggioli@xxxxxxxxxx>]

Exclusive pinning of vCPUs is used, sometimes, for
achieving the highest level of determinism, and the
least possible overhead, for the vCPUs in question.

Although static 1:1 pinning is not recommended, for
general use cases, optimizing the tickling code (of
Credit1 and Credit2) is easy and cheap enough, so go
for it.

Signed-off-by: Dario Faggioli <dario.faggioli@xxxxxxxxxx>
---
Cc: George Dunlap <george.dunlap@xxxxxxxxxx>
Cc: Anshul Makkar <anshulmakkar@xxxxxxxxx>
---
 xen/common/sched_credit.c    |   19 +++++++++++++++++++
 xen/common/sched_credit2.c   |   21 ++++++++++++++++++++-
 xen/include/xen/perfc_defn.h |    1 +
 3 files changed, 40 insertions(+), 1 deletion(-)

diff --git a/xen/common/sched_credit.c b/xen/common/sched_credit.c
index 4f6330e..85e014d 100644
--- a/xen/common/sched_credit.c
+++ b/xen/common/sched_credit.c
@@ -429,6 +429,24 @@ static inline void __runq_tickle(struct csched_vcpu *new)
     idlers_empty = cpumask_empty(&idle_mask);
 
     /*
+     * Exclusive pinning is when a vcpu has hard-affinity with only one
+     * cpu, and there is no other vcpu that has hard-affinity with that
+     * same cpu. This is infrequent, but if it happens, is for achieving
+     * the most possible determinism, and least possible overhead for
+     * the vcpus in question.
+     *
+     * Try to identify the vast majority of these situations, and deal
+     * with them quickly.
+     */
+    if ( unlikely(cpumask_cycle(cpu, new->vcpu->cpu_hard_affinity) == cpu &&
+                  cpumask_test_cpu(cpu, &idle_mask)) )
+    {
+        SCHED_STAT_CRANK(tickled_idle_cpu_excl);
+        __cpumask_set_cpu(cpu, &mask);
+        goto tickle;
+    }
+
+    /*
      * If the pcpu is idle, or there are no idlers and the new
      * vcpu is a higher priority than the old vcpu, run it here.
      *
@@ -524,6 +542,7 @@ static inline void __runq_tickle(struct csched_vcpu *new)
         }
     }
 
+ tickle:
     if ( !cpumask_empty(&mask) )
     {
         if ( unlikely(tb_init_done) )
diff --git a/xen/common/sched_credit2.c b/xen/common/sched_credit2.c
index 9814072..3a1ecbb 100644
--- a/xen/common/sched_credit2.c
+++ b/xen/common/sched_credit2.c
@@ -1186,7 +1186,26 @@ runq_tickle(const struct scheduler *ops, struct 
csched2_vcpu *new, s_time_t now)
                 cpupool_domain_cpumask(new->vcpu->domain));
 
     /*
-     * First of all, consider idle cpus, checking if we can just
+     * Exclusive pinning is when a vcpu has hard-affinity with only one
+     * cpu, and there is no other vcpu that has hard-affinity with that
+     * same cpu. This is infrequent, but if it happens, is for achieving
+     * the most possible determinism, and least possible overhead for
+     * the vcpus in question.
+     *
+     * Try to identify the vast majority of these situations, and deal
+     * with them quickly.
+     */
+    if ( unlikely(cpumask_cycle(cpu, cpumask_scratch_cpu(cpu)) == cpu &&
+                  cpumask_test_cpu(cpu, &rqd->idle) &&
+                  !cpumask_test_cpu(cpu, &rqd->tickled)) )
+    {
+        SCHED_STAT_CRANK(tickled_idle_cpu_excl);
+        ipid = cpu;
+        goto tickle;
+    }
+
+    /*
+     * Afterwards, let's consider idle cpus, checking if we can just
      * re-use the pcpu where we were running before.
      *
      * If there are cores where all the siblings are idle, consider
diff --git a/xen/include/xen/perfc_defn.h b/xen/include/xen/perfc_defn.h
index 53849af..ad914dc 100644
--- a/xen/include/xen/perfc_defn.h
+++ b/xen/include/xen/perfc_defn.h
@@ -30,6 +30,7 @@ PERFCOUNTER(vcpu_wake_runnable,     "sched: 
vcpu_wake_runnable")
 PERFCOUNTER(vcpu_wake_not_runnable, "sched: vcpu_wake_not_runnable")
 PERFCOUNTER(tickled_no_cpu,         "sched: tickled_no_cpu")
 PERFCOUNTER(tickled_idle_cpu,       "sched: tickled_idle_cpu")
+PERFCOUNTER(tickled_idle_cpu_excl,  "sched: tickled_idle_cpu_exclusive")
 PERFCOUNTER(tickled_busy_cpu,       "sched: tickled_busy_cpu")
 PERFCOUNTER(vcpu_check,             "sched: vcpu_check")
 


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