[Xen-devel] [PATCH 23/24] xen: credit2: optimize runq_tickle() a little bit

[Dario Faggioli <dario.faggioli@xxxxxxxxxx>]

By not looking at the same cpu (to check whether
we want to preempt who's running there) twice, if
the vcpu being woken up has both soft and hard
affinity.

In fact, all the cpus that are part of both soft
affinity and hard-affinity (of the waking vcpu)
are checked during the soft-affinity balancing
step. If none turns out to be suitable, e.g.,
because they're running vcpus with higher credits,
there's no point in re-checking them, only to
re-assess the same, during the hard-affinity
step.

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 |   43 +++++++++++++++++++++++++++++++++++++++----
 1 file changed, 39 insertions(+), 4 deletions(-)

diff --git a/xen/common/sched_credit2.c b/xen/common/sched_credit2.c
index 6963872..f03ecce 100644
--- a/xen/common/sched_credit2.c
+++ b/xen/common/sched_credit2.c
@@ -997,7 +997,7 @@ runq_tickle(const struct scheduler *ops, struct 
csched2_vcpu *new, s_time_t now)
     s_time_t lowest = (1<<30);
     unsigned int bs, cpu = new->vcpu->processor;
     struct csched2_runqueue_data *rqd = RQD(ops, cpu);
-    cpumask_t mask;
+    cpumask_t mask, skip_mask;
     struct csched2_vcpu * cur;
 
     ASSERT(new->rqd == rqd);
@@ -1017,6 +1017,13 @@ runq_tickle(const struct scheduler *ops, struct 
csched2_vcpu *new, s_time_t now)
                     (unsigned char *)&d);
     }
 
+    /*
+     * Cpus that end up in this mask, have been already checked during the
+     * soft-affinity step, and need not to be checked again when doing hard
+     * affinity.
+     */
+    cpumask_clear(&skip_mask);
+
     for_each_affinity_balance_step( bs )
     {
         /*
@@ -1073,7 +1080,8 @@ runq_tickle(const struct scheduler *ops, struct 
csched2_vcpu *new, s_time_t now)
         cpumask_andnot(&mask, &rqd->active, &rqd->idle);
         cpumask_andnot(&mask, &mask, &rqd->tickled);
         cpumask_and(&mask, &mask, cpumask_scratch);
-        if ( cpumask_test_cpu(cpu, &mask) )
+        if ( cpumask_test_cpu(cpu, &mask) &&
+             !cpumask_test_cpu(cpu, &skip_mask) )
         {
             cur = CSCHED2_VCPU(curr_on_cpu(cpu));
 
@@ -1102,13 +1110,26 @@ runq_tickle(const struct scheduler *ops, struct 
csched2_vcpu *new, s_time_t now)
                     ipid = cpu;
                     goto tickle;
                 }
+
+                /*
+                 * If we're here, cpu is just not a valid candidate for being
+                 * tickled. Set its bit in skip_mask, to avoid calling
+                 * burn_credits() and check its current vcpu for preemption
+                 * twice.
+                 */
+                __cpumask_set_cpu(cpu, &skip_mask);
             }
         }
 
         for_each_cpu(i, &mask)
         {
-            /* Already looked at this one above */
-            if ( i == cpu )
+            /*
+             * Already looked at these ones above, either because it's the
+             * cpu where new was running before, or because we are at the
+             * hard-affinity step, and we checked this during the
+             * soft-affinity one
+             */
+            if ( i == cpu || cpumask_test_cpu(i, &skip_mask) )
                 continue;
 
             cur = CSCHED2_VCPU(curr_on_cpu(i));
@@ -1139,6 +1160,20 @@ runq_tickle(const struct scheduler *ops, struct 
csched2_vcpu *new, s_time_t now)
                     ipid = i;
                     lowest = cur->credit;
                 }
+
+                /*
+                 * No matter if i is the new lowest or not. We've run
+                 * burn_credits() on it, and we've checked it for preemption.
+                 *
+                 * If we are at soft-affinity balancing step, and i is indeed
+                 * the lowest, it will be tickled (and we exit the function).
+                 * If it is not the lowest among the cpus in the soft-affinity
+                 * mask, it can't be the lowest among the cpus in the hard
+                 * affinity mask (assuming we'll actually do the second
+                 * balancing step), as hard-affinity is a superset of soft
+                 * affinity, and therefore we can flag it to be skipped.
+                 */
+                __cpumask_set_cpu(i, &skip_mask);
             }
         }
 


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