[xen staging] xen/credit2: let the tickled pCPU pick up the unit it was tickled for

[patchbot@xxxxxxx]

commit e4da1829737fb575133d1f79d9ad987000891649
Author:     Jiaqing Zhao <Zhao.Jiaqing@xxxxxxx>
AuthorDate: Thu Oct 1 01:57:47 2026 +0800
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Thu Oct 1 19:14:42 2026 +0100

    xen/credit2: let the tickled pCPU pick up the unit it was tickled for
    
    runq_tickle() picks an idle pCPU without taking CSCHED2_MIGRATE_RESIST
    into account, but runq_candidate() on that pCPU still applies it. With
    the unit's credit within CSCHED2_MIGRATE_RESIST of CSCHED2_CREDIT_MIN,
    the tickled pCPU declines the unit because it is not its master, while
    the master has already skipped it as deferred to the tickled pCPU. Both
    go idle and the unit stays runnable on the runqueue with nobody to run
    it.
    
    Seen as a permanent PVH dom0 stall under nested virtualization, where
    dom0 overruns its slice far enough to be clamped at CSCHED2_CREDIT_MIN.
    
    Don't resist migration to the pCPU that was tickled for the unit.
    
    Fixes: 069cf39fb171 ("xen: credit2: make tickling more deterministic")
    Signed-off-by: Jiaqing Zhao <Zhao.Jiaqing@xxxxxxx>
    Reviewed-by: George Dunlap <gwd@xxxxxxxxxxxxxx>
    Reviewed-by: Juergen Gross <jgross@xxxxxxxx>
    Reviewed-by: Jason Andryuk <jason.andryuk@xxxxxxx>
---
 xen/common/sched/credit2.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/xen/common/sched/credit2.c b/xen/common/sched/credit2.c
index 4949606881..c32e753bb4 100644
--- a/xen/common/sched/credit2.c
+++ b/xen/common/sched/credit2.c
@@ -3514,9 +3514,11 @@ runq_candidate(struct csched2_runqueue_data *rqd,
 
         /*
          * If this is on a different processor, don't pull it unless
-         * its credit is at least CSCHED2_MIGRATE_RESIST higher.
+         * its credit is at least CSCHED2_MIGRATE_RESIST higher, or the
+         * processor was tickled for it.
          */
         if ( sched_unit_master(svc->unit) != cpu
+             && svc->tickled_cpu != cpu
              && snext->credit + CSCHED2_MIGRATE_RESIST > svc->credit )
         {
             SCHED_STAT_CRANK(migrate_resisted);
--
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