[PATCH] sched/rtds: refill cur_budget when extratime is toggled on a depleted vCPU

[Oleksii Moisieiev <Oleksii_Moisieiev@xxxxxxxx>]

XEN_DOMCTL_SCHEDOP_putvcpuinfo can flip the RTDS_extratime bit on a vCPU
that is currently depleted (cur_budget == 0, possibly sitting on the
depleted queue). rt_dom_cntl() touches only svc->flags; cur_budget is
left unchanged. As a result the next code path that calls runq_insert()
on this vCPU - rt_unit_wake() after a domain_unpause(),
rt_context_saved() following a delayed runq add, or repl_timer_handler()
after a replenishment - places the vCPU on the run queue, because
has_extratime(svc) is now true and runq_insert() admits extratime units
regardless of cur_budget:

    /* add svc to runq if svc still has budget or its extratime is set */
    if ( svc->cur_budget > 0 ||
         has_extratime(svc) )
        deadline_runq_insert(svc, &svc->q_elem, runq);
    else
        list_add(&svc->q_elem, &prv->depletedq);

The very next rt_schedule() iterates the run queue from runq_pick()
and trips the ASSERT(iter_svc->cur_budget > 0) at the bottom of the
loop, panicking the host. Observed trace:

    Assertion 'iter_svc->cur_budget > 0' failed at common/sched/rt.c:1035
    ----[ Xen-4.22-unstable  arm64  debug=y ubsan=y  Not tainted ]----
    [<...>] rt.c#rt_schedule+0x1558/0x33e0 (PC)
    [<...>] core.c#do_schedule+0x2e4/0x15b4
    [<...>] core.c#schedule+0xb14/0xe50
    [<...>] softirq.c#__do_softirq+0x20c/0x3d4
    [<...>] do_softirq+0x14/0x1c
    [<...>] domain.c#idle_loop+0x194/0x558

Minimal reproducer: pin a single-vCPU domU to a pCPU, program RTDS with
extratime off and a low utilisation (e.g. budget = 10ms / period = 100ms)
so the vCPU spends most of its time in the depleted queue, pause the
domain, issue a putvcpuinfo that sets XEN_DOMCTL_SCHEDRT_extra, then
unpause. As soon as the schedule softirq fires on the pCPU, the BUG
hits. The same sequence is reachable without an explicit pause: any
window in which rt_dom_cntl() runs between burn_budget()'s budget
exhaustion and rt_context_saved()'s runq_insert() also closes onto the
same broken state, because the per-scheduler lock is dropped between
those two points.

The semantics for "extratime gets exhausted budget refilled" already
live in burn_budget():

    if ( has_extratime(svc) )
    {
        svc->priority_level++;
        svc->cur_budget = svc->budget;
    }

Apply the same priority-demotion-and-refill in rt_dom_cntl() when the
flag transitions from off to on while the vCPU is depleted, clear
RTDS_depleted to match, and - if the vCPU is currently on the depleted
queue - move it to the run queue using the same q_remove() +
runq_insert() pattern already used by repl_timer_handler(). The vCPU
remains on the replenishment queue throughout, so its normal
replenishment cadence is preserved.

The complementary transition (on -> off) is already safe: clearing the
flag only narrows the runq_insert() admission condition, so subsequent
depleted insertions correctly route to the depleted queue.

No other call sites need changes: with cur_budget restored before the
flag is observable to runq_insert(), runq_pick()'s long-standing
invariant (every run-queue entry has cur_budget > 0) is preserved.

Signed-off-by: Oleksii Moisieiev <oleksii_moisieiev@xxxxxxxx>
---

 xen/common/sched/rt.c | 34 ++++++++++++++++++++++++++++++++++
 1 file changed, 34 insertions(+)

diff --git a/xen/common/sched/rt.c b/xen/common/sched/rt.c
index b156f61afa..18e39977be 100644
--- a/xen/common/sched/rt.c
+++ b/xen/common/sched/rt.c
@@ -1465,7 +1465,41 @@ rt_dom_cntl(
                 svc->period = period;
                 svc->budget = budget;
                 if ( local_sched.u.rtds.flags & XEN_DOMCTL_SCHEDRT_extra )
+                {
+                    /*
+                     * Turning extratime on while the vCPU is depleted
+                     * (cur_budget <= 0) leaves cur_budget unchanged. The
+                     * next runq_insert() on this vCPU - from
+                     * rt_unit_wake() after a domain unpause,
+                     * rt_context_saved() following a delayed runq add, or
+                     * repl_timer_handler() - then places it on the run
+                     * queue because has_extratime() is now true, even
+                     * though cur_budget is 0. The very next rt_schedule()
+                     * iterates the run queue from runq_pick() and trips
+                     * the ASSERT(iter_svc->cur_budget > 0).
+                     *
+                     * Apply the same priority-demotion-and-refill that
+                     * burn_budget() would have performed if the flag had
+                     * been set when the budget ran out, clear the
+                     * depleted state, and - if the vCPU is currently on
+                     * the depleted queue - move it to the run queue so
+                     * the new extratime allocation is picked up
+                     * immediately instead of waiting for the next
+                     * replenishment.
+                     */
+                    if ( !has_extratime(svc) && svc->cur_budget <= 0 )
+                    {
+                        svc->priority_level++;
+                        svc->cur_budget = svc->budget;
+                        __clear_bit(__RTDS_depleted, &svc->flags);
+                        if ( unit_on_q(svc) )
+                        {
+                            q_remove(svc);
+                            runq_insert(ops, svc);
+                        }
+                    }
                     __set_bit(__RTDS_extratime, &svc->flags);
+                }
                 else
                     __clear_bit(__RTDS_extratime, &svc->flags);
                 spin_unlock_irqrestore(&prv->lock, flags);
-- 
2.43.0

base-commit: d6fb54fa71926e2ab44ccd84f968ec9228319d65
branch: amoi_rtds_extra_budget_par