[Xen-devel] [PATCH v3 1/2] xen: credit2: implement yield()

[Dario Faggioli <dario.faggioli@xxxxxxxxxx>]

When a vcpu explicitly yields it is usually giving
us an advice of "let someone else run and come back
to me in a bit."

Credit2 isn't, so far, doing anything when a vcpu
yields, which means an yield is basically a NOP (well,
actually, it's pure overhead, as it causes the scheduler
kick in, but the result is --at least 99% of the time--
that the very same vcpu that yielded continues to run).

With this patch, when a vcpu yields, we go and try
picking the next vcpu on the runqueue that can run on
the pcpu where the yielding vcpu is running. Of course,
if we don't find any other vcpu that wants and can run
there, the yielding vcpu will continue.

Also, add an yield performance counter, and fix the
style of a couple of comments.

Signed-off-by: Dario Faggioli <dario.faggioli@xxxxxxxxxx>
---
Cc: George Dunlap <george.dunlap@xxxxxxxxxxxxx>
Cc: Anshul Makkar <anshul.makkar@xxxxxxxxxx>
Cc: Jan Beulich <jbeulich@xxxxxxxx>
Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
Changes from v2:
 * implement yield as a "switch", rather than as a "knob", as suggested
   during review.

Changes from v1:
 * add _us to the parameter name, as suggested during review;
 * get rid of the minimum value for the yield bias;
 * apply the idle bias via subtraction of credits to the yielding vcpu, rather
   than via addition to all the others;
 * merge the Credit2 bits of what was patch 7 here, as suggested during review.
---
 xen/common/sched_credit2.c   |   54 +++++++++++++++++++++++++++++++-----------
 xen/common/schedule.c        |    2 ++
 xen/include/xen/perfc_defn.h |    1 +
 3 files changed, 43 insertions(+), 14 deletions(-)

diff --git a/xen/common/sched_credit2.c b/xen/common/sched_credit2.c
index 72e31b5..c0646e9 100644
--- a/xen/common/sched_credit2.c
+++ b/xen/common/sched_credit2.c
@@ -181,7 +181,13 @@
  */
 #define __CSFLAG_runq_migrate_request 3
 #define CSFLAG_runq_migrate_request (1<<__CSFLAG_runq_migrate_request)
-
+/*
+ * CSFLAG_vcpu_yield: this vcpu was running, and has called vcpu_yield(). The
+ * scheduler is invoked to see if we can give the cpu to someone else, and
+ * get back to the yielding vcpu in a while.
+ */
+#define __CSFLAG_vcpu_yield 4
+#define CSFLAG_vcpu_yield (1<<__CSFLAG_vcpu_yield)
 
 static unsigned int __read_mostly opt_migrate_resist = 500;
 integer_param("sched_credit2_migrate_resist", opt_migrate_resist);
@@ -1431,6 +1437,14 @@ out:
 }
 
 static void
+csched2_vcpu_yield(const struct scheduler *ops, struct vcpu *v)
+{
+    struct csched2_vcpu * const svc = CSCHED2_VCPU(v);
+
+    __set_bit(__CSFLAG_vcpu_yield, &svc->flags);
+}
+
+static void
 csched2_context_saved(const struct scheduler *ops, struct vcpu *vc)
 {
     struct csched2_vcpu * const svc = CSCHED2_VCPU(vc);
@@ -2250,26 +2264,31 @@ runq_candidate(struct csched2_runqueue_data *rqd,
     struct list_head *iter;
     struct csched2_vcpu *snext = NULL;
     struct csched2_private *prv = CSCHED2_PRIV(per_cpu(scheduler, cpu));
+    bool yield = __test_and_clear_bit(__CSFLAG_vcpu_yield, &scurr->flags);
 
     *skipped = 0;
 
-    /* Default to current if runnable, idle otherwise */
-    if ( vcpu_runnable(scurr->vcpu) )
-        snext = scurr;
-    else
-        snext = CSCHED2_VCPU(idle_vcpu[cpu]);
-
     /*
      * Return the current vcpu if it has executed for less than ratelimit.
      * Adjuststment for the selected vcpu's credit and decision
      * for how long it will run will be taken in csched2_runtime.
+     *
+     * Note that, if scurr is yielding, we don't let rate limiting kick in.
+     * In fact, it may be the case that scurr is about to spin, and there's
+     * no point forcing it to do so until rate limiting expires.
      */
-    if ( prv->ratelimit_us && !is_idle_vcpu(scurr->vcpu) &&
+    if ( !yield && prv->ratelimit_us && !is_idle_vcpu(scurr->vcpu) &&
          vcpu_runnable(scurr->vcpu) &&
          (now - scurr->vcpu->runstate.state_entry_time) <
           MICROSECS(prv->ratelimit_us) )
         return scurr;
 
+    /* Default to current if runnable, idle otherwise */
+    if ( vcpu_runnable(scurr->vcpu) )
+        snext = scurr;
+    else
+        snext = CSCHED2_VCPU(idle_vcpu[cpu]);
+
     list_for_each( iter, &rqd->runq )
     {
         struct csched2_vcpu * svc = list_entry(iter, struct csched2_vcpu, 
runq_elem);
@@ -2293,8 +2312,10 @@ runq_candidate(struct csched2_runqueue_data *rqd,
             continue;
         }
 
-        /* If this is on a different processor, don't pull it unless
-         * its credit is at least CSCHED2_MIGRATE_RESIST higher. */
+        /*
+         * If this is on a different processor, don't pull it unless
+         * its credit is at least CSCHED2_MIGRATE_RESIST higher.
+         */
         if ( svc->vcpu->processor != cpu
              && snext->credit + CSCHED2_MIGRATE_RESIST > svc->credit )
         {
@@ -2303,9 +2324,12 @@ runq_candidate(struct csched2_runqueue_data *rqd,
             continue;
         }
 
-        /* If the next one on the list has more credit than current
-         * (or idle, if current is not runnable), choose it. */
-        if ( svc->credit > snext->credit )
+        /*
+         * If the next one on the list has more credit than current
+         * (or idle, if current is not runnable), or if current is
+         * yielding, choose it.
+         */
+        if ( yield || svc->credit > snext->credit )
             snext = svc;
 
         /* In any case, if we got this far, break. */
@@ -2391,7 +2415,8 @@ csched2_schedule(
      */
     if ( tasklet_work_scheduled )
     {
-        trace_var(TRC_CSCHED2_SCHED_TASKLET, 1, 0,  NULL);
+        __clear_bit(__CSFLAG_vcpu_yield, &scurr->flags);
+        trace_var(TRC_CSCHED2_SCHED_TASKLET, 1, 0, NULL);
         snext = CSCHED2_VCPU(idle_vcpu[cpu]);
     }
     else
@@ -2975,6 +3000,7 @@ static const struct scheduler sched_credit2_def = {
 
     .sleep          = csched2_vcpu_sleep,
     .wake           = csched2_vcpu_wake,
+    .yield          = csched2_vcpu_yield,
 
     .adjust         = csched2_dom_cntl,
     .adjust_global  = csched2_sys_cntl,
diff --git a/xen/common/schedule.c b/xen/common/schedule.c
index 104d203..5b444c4 100644
--- a/xen/common/schedule.c
+++ b/xen/common/schedule.c
@@ -947,6 +947,8 @@ long vcpu_yield(void)
     SCHED_OP(VCPU2OP(v), yield, v);
     vcpu_schedule_unlock_irq(lock, v);
 
+    SCHED_STAT_CRANK(vcpu_yield);
+
     TRACE_2D(TRC_SCHED_YIELD, current->domain->domain_id, current->vcpu_id);
     raise_softirq(SCHEDULE_SOFTIRQ);
     return 0;
diff --git a/xen/include/xen/perfc_defn.h b/xen/include/xen/perfc_defn.h
index 4a835b8..900fddd 100644
--- a/xen/include/xen/perfc_defn.h
+++ b/xen/include/xen/perfc_defn.h
@@ -23,6 +23,7 @@ PERFCOUNTER(vcpu_alloc,             "sched: vcpu_alloc")
 PERFCOUNTER(vcpu_insert,            "sched: vcpu_insert")
 PERFCOUNTER(vcpu_remove,            "sched: vcpu_remove")
 PERFCOUNTER(vcpu_sleep,             "sched: vcpu_sleep")
+PERFCOUNTER(vcpu_yield,             "sched: vcpu_yield")
 PERFCOUNTER(vcpu_wake_running,      "sched: vcpu_wake_running")
 PERFCOUNTER(vcpu_wake_onrunq,       "sched: vcpu_wake_onrunq")
 PERFCOUNTER(vcpu_wake_runnable,     "sched: vcpu_wake_runnable")


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