[xen stable-4.20] xen/credit2: factor in previous active unit's credit in csched2_runtime()

[patchbot@xxxxxxx]

commit ec5752e154d768a3c12442f732f07dfe79a36385
Author:     Koichiro Den <den@xxxxxxxxxxxxx>
AuthorDate: Thu Jul 31 09:51:31 2025 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Thu Jul 31 09:51:31 2025 +0200

    xen/credit2: factor in previous active unit's credit in csched2_runtime()
    
    When a running unit is about to be scheduled out due to a competing unit
    with the highest remaining credit, the residual credit of the previous
    unit is currently ignored in csched2_runtime() because it hasn't yet
    been reinserted into the runqueue.
    
    As a result, two equally weighted, busy units can often each be granted
    almost the maximum possible runtime (i.e. consuming CSCHED2_CREDIT_INIT
    in one shot) when only those two are active. In broad strokes two units
    switch back and forth every 10ms (CSCHED2_MAX_TIMER). In contrast, when
    more than two busy units are competing, such coarse runtime allocations
    are rarely seen, since at least one active unit remains in the runqueue.
    
    To ensure consistent behavior, have csched2_runtime() take into account
    the previous unit's latest credit when it still can/wants to run.
    
    Signed-off-by: Koichiro Den <den@xxxxxxxxxxxxx>
    Reviewed-by: Juergen Gross <jgross@xxxxxxxx>
    master commit: ae648e9f8013a6c6382e54ddc4230ea05792c802
    master date: 2025-07-28 13:04:40 +0200
---
 xen/common/sched/credit2.c | 28 +++++++++++++++++++++-------
 1 file changed, 21 insertions(+), 7 deletions(-)

diff --git a/xen/common/sched/credit2.c b/xen/common/sched/credit2.c
index 0a83f23725..0b74fa3c5d 100644
--- a/xen/common/sched/credit2.c
+++ b/xen/common/sched/credit2.c
@@ -3278,13 +3278,14 @@ csched2_unit_remove(const struct scheduler *ops, struct 
sched_unit *unit)
 /* How long should we let this unit run for? */
 static s_time_t
 csched2_runtime(const struct scheduler *ops, int cpu,
-                struct csched2_unit *snext, s_time_t now)
+                struct csched2_unit *snext, s_time_t now, int inflight_credit)
 {
     s_time_t time, min_time;
     int rt_credit; /* Proposed runtime measured in credits */
     struct csched2_runqueue_data *rqd = c2rqd(cpu);
     struct list_head *runq = &rqd->runq;
     const struct csched2_private *prv = csched2_priv(ops);
+    int swait_credit = 0;
 
     /*
      * If we're idle, just stay so. Others (or external events)
@@ -3320,17 +3321,21 @@ csched2_runtime(const struct scheduler *ops, int cpu,
     /*
      * 2) If there's someone waiting whose credit is positive,
      *    run until your credit ~= his.
+     *    Note that this someone might be the one who was just
+     *    running and is about to be placed back on the runqueue.
      */
     if ( ! list_empty(runq) )
     {
         struct csched2_unit *swait = runq_elem(runq->next);
 
-        if ( ! is_idle_unit(swait->unit)
-             && swait->credit > 0 )
-        {
-            rt_credit = snext->credit - swait->credit;
-        }
+        if ( !is_idle_unit(swait->unit) && swait->credit > 0 )
+            swait_credit = swait->credit;
     }
+    if ( swait_credit < inflight_credit )
+        swait_credit = inflight_credit;
+
+    if ( swait_credit > 0 )
+        rt_credit = snext->credit - swait_credit;
 
     /*
      * The next guy on the runqueue may actually have a higher credit,
@@ -3582,6 +3587,7 @@ static void cf_check csched2_schedule(
     struct csched2_runqueue_data *rqd;
     struct csched2_unit * const scurr = csched2_unit(currunit);
     struct csched2_unit *snext = NULL;
+    int inflight_credit = 0;
     bool tickled;
     bool migrated = false;
 
@@ -3716,6 +3722,13 @@ static void cf_check csched2_schedule(
             balance_load(ops, sched_cpu, now);
         }
 
+        /*
+         * This must occur after the potential credit reset above,
+         * otherwise it would capture a negative credit.
+         */
+        if ( test_bit(__CSFLAG_delayed_runq_add, &scurr->flags) )
+            inflight_credit = scurr->credit;
+
         snext->start_time = now;
         snext->tickled_cpu = -1;
 
@@ -3756,7 +3769,8 @@ static void cf_check csched2_schedule(
     /*
      * Return task to run next...
      */
-    currunit->next_time = csched2_runtime(ops, sched_cpu, snext, now);
+    currunit->next_time = csched2_runtime(ops, sched_cpu, snext, now,
+                                          inflight_credit);
     currunit->next_task = snext->unit;
     snext->unit->migrated = migrated;
 
--
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