[Xen-devel] [PATCH v3] schedule: move last_run_time to the credit scheduler privates

[Andrii Anisov <andrii.anisov@xxxxxxxxx>]

From: Andrii Anisov <andrii_anisov@xxxxxxxx>

The structure member last_run_time is used by credit scheduler only.
So move it from a generic vcpu structure to the credit scheduler private
vcpu definition.
With this move we have slight changes in functionality:
 - last_run_time is not updated for an idle vcpu. But the idle vcpu is,
   in fact, a per-pcpu stub and never migrates so last_run_time is
   meaningless for it.
 - The value of last_run_time is updated on every schedule, even if the
   vcpu is not being changed. It is still ok, because last_run_time is
   only used for runnable vcpu migration decision, and we have it correct
   at that moment. Scheduling parameters and statistics are tracked by
   other entities.

While here, also:
  - turn last_run_time into s_time_t, which is more appropriate.
  - properly const-ify related argument of __csched_vcpu_is_cache_hot().

Signed-off-by: Andrii Anisov <andrii_anisov@xxxxxxxx>
---

Changes in
 v3:
    - commit message updated accordingly to [1]
 v2:
    - last_run_time type changed to s_time_t
    - scurr changed to svc
    - dropped stray blanks
    - pointers to const are used appropriately

[1] https://lists.xenproject.org/archives/html/xen-devel/2018-09/msg01201.html

---
 xen/common/sched_credit.c | 12 +++++++++---
 xen/common/schedule.c     |  1 -
 xen/include/xen/sched.h   |  3 ---
 3 files changed, 9 insertions(+), 7 deletions(-)

diff --git a/xen/common/sched_credit.c b/xen/common/sched_credit.c
index 7b7facb..b097048 100644
--- a/xen/common/sched_credit.c
+++ b/xen/common/sched_credit.c
@@ -175,6 +175,9 @@ struct csched_vcpu {
     atomic_t credit;
     unsigned int residual;
 
+    /* last time when vCPU is scheduled out */
+    s_time_t last_run_time;
+
 #ifdef CSCHED_STATS
     struct {
         int credit_last;
@@ -701,10 +704,11 @@ static unsigned int vcpu_migration_delay_us;
 integer_param("vcpu_migration_delay", vcpu_migration_delay_us);
 
 static inline bool
-__csched_vcpu_is_cache_hot(const struct csched_private *prv, struct vcpu *v)
+__csched_vcpu_is_cache_hot(const struct csched_private *prv,
+                           const struct csched_vcpu *svc)
 {
     bool hot = prv->vcpu_migr_delay &&
-               (NOW() - v->last_run_time) < prv->vcpu_migr_delay;
+               (NOW() - svc->last_run_time) < prv->vcpu_migr_delay;
 
     if ( hot )
         SCHED_STAT_CRANK(vcpu_hot);
@@ -716,6 +720,7 @@ static inline int
 __csched_vcpu_is_migrateable(const struct csched_private *prv, struct vcpu *vc,
                              int dest_cpu, cpumask_t *mask)
 {
+    const struct csched_vcpu *svc = CSCHED_VCPU(vc);
     /*
      * Don't pick up work that's hot on peer PCPU, or that can't (or
      * would prefer not to) run on cpu.
@@ -725,7 +730,7 @@ __csched_vcpu_is_migrateable(const struct csched_private 
*prv, struct vcpu *vc,
      */
     ASSERT(!vc->is_running);
 
-    return !__csched_vcpu_is_cache_hot(prv, vc) &&
+    return !__csched_vcpu_is_cache_hot(prv, svc) &&
            cpumask_test_cpu(dest_cpu, mask);
 }
 
@@ -1870,6 +1875,7 @@ csched_schedule(
         /* Update credits of a non-idle VCPU. */
         burn_credits(scurr, now);
         scurr->start_time -= now;
+        scurr->last_run_time = now;
     }
     else
     {
diff --git a/xen/common/schedule.c b/xen/common/schedule.c
index 66f1e26..4deacf6 100644
--- a/xen/common/schedule.c
+++ b/xen/common/schedule.c
@@ -1493,7 +1493,6 @@ static void schedule(void)
         ((prev->pause_flags & VPF_blocked) ? RUNSTATE_blocked :
          (vcpu_runnable(prev) ? RUNSTATE_runnable : RUNSTATE_offline)),
         now);
-    prev->last_run_time = now;
 
     ASSERT(next->runstate.state != RUNSTATE_running);
     vcpu_runstate_change(next, RUNSTATE_running, now);
diff --git a/xen/include/xen/sched.h b/xen/include/xen/sched.h
index 2201fac..7aa0be6 100644
--- a/xen/include/xen/sched.h
+++ b/xen/include/xen/sched.h
@@ -174,9 +174,6 @@ struct vcpu
     } runstate_guest; /* guest address */
 #endif
 
-    /* last time when vCPU is scheduled out */
-    uint64_t last_run_time;
-
     /* Has the FPU been initialised? */
     bool             fpu_initialised;
     /* Has the FPU been used since it was last saved? */
-- 
2.7.4


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