[PATCH] xen/sched: rtds: re-arm repl_timer after timer re-initialization

[Oleksii Moisieiev <Oleksii_Moisieiev@xxxxxxxx>]

In RTDS, removing the last eligible pCPU can kill repl_timer.
When a pCPU is later re-added, rt_switch_sched() reinitializes the
timer object, but pending entries may already exist in replq.

Without re-arming from replq head, replenishment can remain inactive
until some unrelated event programs the timer again. This may stall
budget replenishment for non-extratime units.

Fix this by re-arming repl_timer in rt_switch_sched() immediately after
init_timer() when replq is non-empty, using the earliest pending
deadline.

This keeps behavior unchanged when replq is empty.

Add a lightweight regression script,
test_rtds_repl_timer_rearm.sh, to verify source-level guards and model
the expected re-arm decision logic.
Signed-off-by: Oleksii Moisieiev <oleksii_moisieiev@xxxxxxxx>
---

 xen/common/sched/rt.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/xen/common/sched/rt.c b/xen/common/sched/rt.c
index 7b1f64a779..59021e1110 100644
--- a/xen/common/sched/rt.c
+++ b/xen/common/sched/rt.c
@@ -741,8 +741,17 @@ rt_switch_sched(struct scheduler *new_ops, unsigned int 
cpu,
     if ( prv->repl_timer.status == TIMER_STATUS_invalid ||
          prv->repl_timer.status == TIMER_STATUS_killed )
     {
+        struct list_head *replq = rt_replq(new_ops);
+
         init_timer(&prv->repl_timer, repl_timer_handler, (void *)new_ops, cpu);
         dprintk(XENLOG_DEBUG, "RTDS: timer initialized on cpu %u\n", cpu);
+
+        /*
+         * When re-adding CPUs after all RTDS CPUs were removed, replq may
+         * already contain pending replenishment events. Re-arm immediately.
+         */
+        if ( !list_empty(replq) )
+            set_timer(&prv->repl_timer, replq_elem(replq->next)->cur_deadline);
     }
 
     sched_idle_unit(cpu)->priv = vdata;
-- 
2.43.0

base-commit: a7bf8ff218ca05eb3674fdfd2817f6cff471e96a