[PATCH 06/24] xen: credit2: implement yield()

[Dario Faggioli <dario.faggioli@citrix.com>]

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).

Implement a "preempt bias", to be applied to yielding
vcpus. Basically when evaluating what vcpu to run next,
if a vcpu that has just yielded is encountered, we give
it a credit penalty, and check whether there is anyone
else that would better take over the cpu (of course,
if there isn't the yielding vcpu will continue).

The value of this bias can be configured with a boot
time parameter, and the default is set to 1 ms.

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

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
---
Cc: George Dunlap <george.dunlap@eu.citrix.com>
Cc: Anshul Makkar <anshul.makkar@citrix.com>
Cc: Jan Beulich <jbeulich@suse.com>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>
---
Note that this *only* consider the bias during the very scheduling decision
that retults from the vcpu calling yield. After that, the __CSFLAG_vcpu_yield
flag is reset, and during all furute scheduling decisions, the vcpu will
compete with the other ones with its own amount of credits.

Alternatively, we can actually _subtract_ some credits to a yielding vcpu.
That will sort of make the effect of a call to yield last in time.

I'm not sure which path is best. Personally, I like the subtract approach
(perhaps, with a smaller bias than 1ms), but I think the "one shot" behavior
implemented here is a good starting point. It is _something_, which is better
than nothing, which is what we have without this patch! :-) It's lightweight
(in its impact on the crediting algorithm, I mean), and benchmarks looks nice,
so I propose we go for this one, and explore the "permanent" --subtraction
based-- solution a bit more.
---
 docs/misc/xen-command-line.markdown |   10 ++++++
 xen/common/sched_credit2.c          |   62 +++++++++++++++++++++++++++++++----
 xen/common/schedule.c               |    2 +
 xen/include/xen/perfc_defn.h        |    1 +
 4 files changed, 68 insertions(+), 7 deletions(-)

diff --git a/docs/misc/xen-command-line.markdown b/docs/misc/xen-command-line.markdown
index 3a250cb..5f469b1 100644
--- a/docs/misc/xen-command-line.markdown
+++ b/docs/misc/xen-command-line.markdown
@@ -1389,6 +1389,16 @@ Choose the default scheduler.
 ### sched\_credit2\_migrate\_resist
 > `= <integer>`
 
+### sched\_credit2\_yield\_bias
+> `= <integer>`
+
+> Default: `1000`
+
+Set how much a yielding vcpu will be penalized, in order to actually
+give a chance to run to some other vcpu. This is basically a bias, in
+favour of the non-yielding vcpus, expressed in microseconds (default
+is 1ms).
+
 ### sched\_credit\_tslice\_ms
 > `= <integer>`
 
diff --git a/xen/common/sched_credit2.c b/xen/common/sched_credit2.c
index a3d7beb..569174b 100644
--- a/xen/common/sched_credit2.c
+++ b/xen/common/sched_credit2.c
@@ -144,6 +144,9 @@
 #define CSCHED2_MIGRATE_RESIST       ((opt_migrate_resist)*MICROSECS(1))
 /* How much to "compensate" a vcpu for L2 migration */
 #define CSCHED2_MIGRATE_COMPENSATION MICROSECS(50)
+/* How big of a bias we should have against a yielding vcpu */
+#define CSCHED2_YIELD_BIAS           ((opt_yield_bias)*MICROSECS(1))
+#define CSCHED2_YIELD_BIAS_MIN       CSCHED2_MIN_TIMER
 /* Reset: Value below which credit will be reset. */
 #define CSCHED2_CREDIT_RESET         0
 /* Max timer: Maximum time a guest can be run for. */
@@ -181,11 +184,20 @@
  */
 #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);
 
+static unsigned int __read_mostly opt_yield_bias = 1000;
+integer_param("sched_credit2_yield_bias", opt_yield_bias);
+
 /*
  * Useful macros
  */
@@ -1432,6 +1444,14 @@ 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);
@@ -2247,10 +2267,22 @@ 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));
+    int yield_bias = 0;
 
     /* Default to current if runnable, idle otherwise */
     if ( vcpu_runnable(scurr->vcpu) )
+    {
+        /*
+         * The way we actually take yields into account is like this:
+         * if scurr is yielding, when comparing its credits with other
+         * vcpus in the runqueue, act like those other vcpus had yield_bias
+         * more credits.
+         */
+        if ( unlikely(scurr->flags & CSFLAG_vcpu_yield) )
+            yield_bias = CSCHED2_YIELD_BIAS;
+
         snext = scurr;
+    }
     else
         snext = CSCHED2_VCPU(idle_vcpu[cpu]);
 
@@ -2268,6 +2300,7 @@ runq_candidate(struct csched2_runqueue_data *rqd,
     list_for_each( iter, &rqd->runq )
     {
         struct csched2_vcpu * svc = list_entry(iter, struct csched2_vcpu, runq_elem);
+        int svc_credit = svc->credit + yield_bias;
 
         /* Only consider vcpus that are allowed to run on this processor. */
         if ( !cpumask_test_cpu(cpu, svc->vcpu->cpu_hard_affinity) )
@@ -2288,19 +2321,23 @@ 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 )
+             && snext->credit + CSCHED2_MIGRATE_RESIST > svc_credit )
         {
             (*pos)++;
             SCHED_STAT_CRANK(migrate_resisted);
             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), choose it.
+         */
+        if ( svc_credit > snext->credit )
             snext = svc;
 
         /* In any case, if we got this far, break. */
@@ -2399,6 +2436,8 @@ csched2_schedule(
          && vcpu_runnable(current) )
         __set_bit(__CSFLAG_delayed_runq_add, &scurr->flags);
 
+    __clear_bit(__CSFLAG_vcpu_yield, &scurr->flags);
+
     ret.migrated = 0;
 
     /* Accounting for non-idle tasks */
@@ -2918,6 +2957,14 @@ csched2_init(struct scheduler *ops)
     printk(XENLOG_INFO "load tracking window lenght %llu ns\n",
            1ULL << opt_load_window_shift);
 
+    if ( opt_yield_bias < CSCHED2_YIELD_BIAS_MIN )
+    {
+        printk("WARNING: %s: opt_yield_bias %d too small, resetting\n",
+               __func__, opt_yield_bias);
+        opt_yield_bias = 1000; /* 1 ms */
+    }
+    printk(XENLOG_INFO "yield bias value %d us\n", opt_yield_bias);
+
     /* Basically no CPU information is available at this point; just
      * set up basic structures, and a callback when the CPU info is
      * available. */
@@ -2970,6 +3017,7 @@ 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 32a300f..abe063d 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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