[Xen-devel] [PATCH v6 02/20] xen/sched: introduce unit_runnable_state()

[Juergen Gross <jgross@xxxxxxxx>]

Today the vcpu runstate of a new scheduled vcpu is always set to
"running" even if at that time vcpu_runnable() is already returning
false due to a race (e.g. with pausing the vcpu).

With core scheduling this can no longer work as not all vcpus of a
schedule unit have to be "running" when being scheduled. So the vcpu's
new runstate has to be selected at the same time as the runnability of
the related schedule unit is probed.

For this purpose introduce a new helper unit_runnable_state() which
will save the new runstate of all tested vcpus in a new field of the
vcpu struct.

Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
Reviewed-by: Dario Faggioli <dfaggioli@xxxxxxxx>
---
RFC V2:
- new patch
V3:
- add vcpu loop to unit_runnable_state() right now instead of doing
  so in next patch (Jan Beulich, Dario Faggioli)
- make new_state unsigned int (Jan Beulich)
V4:
- add comment explaining unit_runnable_state() (Jan Beulich)
---
 xen/common/domain.c         |  1 +
 xen/common/sched_arinc653.c |  2 +-
 xen/common/sched_credit.c   | 49 ++++++++++++++++++++++++---------------------
 xen/common/sched_credit2.c  |  7 ++++---
 xen/common/sched_null.c     |  3 ++-
 xen/common/sched_rt.c       |  8 +++++++-
 xen/common/schedule.c       |  2 +-
 xen/include/xen/sched-if.h  | 30 +++++++++++++++++++++++++++
 xen/include/xen/sched.h     |  1 +
 9 files changed, 73 insertions(+), 30 deletions(-)

diff --git a/xen/common/domain.c b/xen/common/domain.c
index 601da28c9c..a9882509ed 100644
--- a/xen/common/domain.c
+++ b/xen/common/domain.c
@@ -157,6 +157,7 @@ struct vcpu *vcpu_create(struct domain *d, unsigned int 
vcpu_id)
     if ( is_idle_domain(d) )
     {
         v->runstate.state = RUNSTATE_running;
+        v->new_state = RUNSTATE_running;
     }
     else
     {
diff --git a/xen/common/sched_arinc653.c b/xen/common/sched_arinc653.c
index fcf81db19a..dd5876eacd 100644
--- a/xen/common/sched_arinc653.c
+++ b/xen/common/sched_arinc653.c
@@ -563,7 +563,7 @@ a653sched_do_schedule(
     if ( !((new_task != NULL)
            && (AUNIT(new_task) != NULL)
            && AUNIT(new_task)->awake
-           && unit_runnable(new_task)) )
+           && unit_runnable_state(new_task)) )
         new_task = IDLETASK(cpu);
     BUG_ON(new_task == NULL);
 
diff --git a/xen/common/sched_credit.c b/xen/common/sched_credit.c
index 299eff21ac..00beac3ea4 100644
--- a/xen/common/sched_credit.c
+++ b/xen/common/sched_credit.c
@@ -1894,7 +1894,7 @@ static void csched_schedule(
     if ( !test_bit(CSCHED_FLAG_UNIT_YIELD, &scurr->flags)
          && !tasklet_work_scheduled
          && prv->ratelimit
-         && unit_runnable(unit)
+         && unit_runnable_state(unit)
          && !is_idle_unit(unit)
          && runtime < prv->ratelimit )
     {
@@ -1939,33 +1939,36 @@ static void csched_schedule(
         dec_nr_runnable(sched_cpu);
     }
 
-    snext = __runq_elem(runq->next);
-
-    /* Tasklet work (which runs in idle UNIT context) overrides all else. */
-    if ( tasklet_work_scheduled )
-    {
-        TRACE_0D(TRC_CSCHED_SCHED_TASKLET);
-        snext = CSCHED_UNIT(sched_idle_unit(sched_cpu));
-        snext->pri = CSCHED_PRI_TS_BOOST;
-    }
-
     /*
      * Clear YIELD flag before scheduling out
      */
     clear_bit(CSCHED_FLAG_UNIT_YIELD, &scurr->flags);
 
-    /*
-     * SMP Load balance:
-     *
-     * If the next highest priority local runnable UNIT has already eaten
-     * through its credits, look on other PCPUs to see if we have more
-     * urgent work... If not, csched_load_balance() will return snext, but
-     * already removed from the runq.
-     */
-    if ( snext->pri > CSCHED_PRI_TS_OVER )
-        __runq_remove(snext);
-    else
-        snext = csched_load_balance(prv, sched_cpu, snext, &migrated);
+    do {
+        snext = __runq_elem(runq->next);
+
+        /* Tasklet work (which runs in idle UNIT context) overrides all else. 
*/
+        if ( tasklet_work_scheduled )
+        {
+            TRACE_0D(TRC_CSCHED_SCHED_TASKLET);
+            snext = CSCHED_UNIT(sched_idle_unit(sched_cpu));
+            snext->pri = CSCHED_PRI_TS_BOOST;
+        }
+
+        /*
+         * SMP Load balance:
+         *
+         * If the next highest priority local runnable UNIT has already eaten
+         * through its credits, look on other PCPUs to see if we have more
+         * urgent work... If not, csched_load_balance() will return snext, but
+         * already removed from the runq.
+         */
+        if ( snext->pri > CSCHED_PRI_TS_OVER )
+            __runq_remove(snext);
+        else
+            snext = csched_load_balance(prv, sched_cpu, snext, &migrated);
+
+    } while ( !unit_runnable_state(snext->unit) );
 
     /*
      * Update idlers mask if necessary. When we're idling, other CPUs
diff --git a/xen/common/sched_credit2.c b/xen/common/sched_credit2.c
index 87d142bbe4..0e29e56d5a 100644
--- a/xen/common/sched_credit2.c
+++ b/xen/common/sched_credit2.c
@@ -3291,7 +3291,7 @@ runq_candidate(struct csched2_runqueue_data *rqd,
      * 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 ( !yield && prv->ratelimit_us && unit_runnable(scurr->unit) &&
+    if ( !yield && prv->ratelimit_us && unit_runnable_state(scurr->unit) &&
          (now - scurr->unit->state_entry_time) < MICROSECS(prv->ratelimit_us) )
     {
         if ( unlikely(tb_init_done) )
@@ -3345,7 +3345,7 @@ runq_candidate(struct csched2_runqueue_data *rqd,
      *
      * Of course, we also default to idle also if scurr is not runnable.
      */
-    if ( unit_runnable(scurr->unit) && !soft_aff_preempt )
+    if ( unit_runnable_state(scurr->unit) && !soft_aff_preempt )
         snext = scurr;
     else
         snext = csched2_unit(sched_idle_unit(cpu));
@@ -3405,7 +3405,8 @@ runq_candidate(struct csched2_runqueue_data *rqd,
          * some budget, then choose it.
          */
         if ( (yield || svc->credit > snext->credit) &&
-             (!has_cap(svc) || unit_grab_budget(svc)) )
+             (!has_cap(svc) || unit_grab_budget(svc)) &&
+             unit_runnable_state(svc->unit) )
             snext = svc;
 
         /* In any case, if we got this far, break. */
diff --git a/xen/common/sched_null.c b/xen/common/sched_null.c
index 80a7d45935..3dde1dcd00 100644
--- a/xen/common/sched_null.c
+++ b/xen/common/sched_null.c
@@ -864,7 +864,8 @@ static void null_schedule(const struct scheduler *ops, 
struct sched_unit *prev,
             cpumask_set_cpu(sched_cpu, &prv->cpus_free);
     }
 
-    if ( unlikely(prev->next_task == NULL || !unit_runnable(prev->next_task)) )
+    if ( unlikely(prev->next_task == NULL ||
+                  !unit_runnable_state(prev->next_task)) )
         prev->next_task = sched_idle_unit(sched_cpu);
 
     NULL_UNIT_CHECK(prev->next_task);
diff --git a/xen/common/sched_rt.c b/xen/common/sched_rt.c
index cfd7d334fa..fd882f2ca4 100644
--- a/xen/common/sched_rt.c
+++ b/xen/common/sched_rt.c
@@ -1092,12 +1092,18 @@ rt_schedule(const struct scheduler *ops, struct 
sched_unit *currunit,
     else
     {
         snext = runq_pick(ops, cpumask_of(sched_cpu));
+
         if ( snext == NULL )
             snext = rt_unit(sched_idle_unit(sched_cpu));
+        else if ( !unit_runnable_state(snext->unit) )
+        {
+            q_remove(snext);
+            snext = rt_unit(sched_idle_unit(sched_cpu));
+        }
 
         /* if scurr has higher priority and budget, still pick scurr */
         if ( !is_idle_unit(currunit) &&
-             unit_runnable(currunit) &&
+             unit_runnable_state(currunit) &&
              scurr->cur_budget > 0 &&
              ( is_idle_unit(snext->unit) ||
                compare_unit_priority(scurr, snext) > 0 ) )
diff --git a/xen/common/schedule.c b/xen/common/schedule.c
index ff67fb3633..9c1b044b49 100644
--- a/xen/common/schedule.c
+++ b/xen/common/schedule.c
@@ -280,7 +280,7 @@ static inline void sched_unit_runstate_change(struct 
sched_unit *unit,
     for_each_sched_unit_vcpu ( unit, v )
     {
         if ( running )
-            vcpu_runstate_change(v, RUNSTATE_running, new_entry_time);
+            vcpu_runstate_change(v, v->new_state, new_entry_time);
         else
             vcpu_runstate_change(v,
                 ((v->pause_flags & VPF_blocked) ? RUNSTATE_blocked :
diff --git a/xen/include/xen/sched-if.h b/xen/include/xen/sched-if.h
index c65dfa943b..7e568a9d9f 100644
--- a/xen/include/xen/sched-if.h
+++ b/xen/include/xen/sched-if.h
@@ -93,6 +93,36 @@ static inline bool unit_runnable(const struct sched_unit 
*unit)
     return false;
 }
 
+/*
+ * Returns whether a sched_unit is runnable and sets new_state for each of its
+ * vcpus. It is mandatory to determine the new runstate for all vcpus of a unit
+ * without dropping the schedule lock (which happens when synchronizing the
+ * context switch of the vcpus of a unit) in order to avoid races with e.g.
+ * vcpu_sleep().
+ */
+static inline bool unit_runnable_state(const struct sched_unit *unit)
+{
+    struct vcpu *v;
+    bool runnable, ret = false;
+
+    if ( is_idle_unit(unit) )
+        return true;
+
+    for_each_sched_unit_vcpu ( unit, v )
+    {
+        runnable = vcpu_runnable(v);
+
+        v->new_state = runnable ? RUNSTATE_running
+                                : (v->pause_flags & VPF_blocked)
+                                  ? RUNSTATE_blocked : RUNSTATE_offline;
+
+        if ( runnable )
+            ret = true;
+    }
+
+    return ret;
+}
+
 static inline void sched_set_res(struct sched_unit *unit,
                                  struct sched_resource *res)
 {
diff --git a/xen/include/xen/sched.h b/xen/include/xen/sched.h
index c770ab4aa0..12f00cd78d 100644
--- a/xen/include/xen/sched.h
+++ b/xen/include/xen/sched.h
@@ -174,6 +174,7 @@ struct vcpu
         XEN_GUEST_HANDLE(vcpu_runstate_info_compat_t) compat;
     } runstate_guest; /* guest address */
 #endif
+    unsigned int     new_state;
 
     /* Has the FPU been initialised? */
     bool             fpu_initialised;
-- 
2.16.4


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