[xen stable-4.15] credit2: make sure we pick a runnable unit from the runq if there is one

[patchbot@xxxxxxx]

commit 13ea8afded421c60dd4a0a1cd7afc8600819a7dd
Author:     Dario Faggioli <dfaggioli@xxxxxxxx>
AuthorDate: Thu Jul 15 09:17:26 2021 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Thu Jul 15 09:17:26 2021 +0200

    credit2: make sure we pick a runnable unit from the runq if there is one
    
    A !runnable unit (temporarily) present in the runq may cause us to
    stop scanning the runq itself too early. Of course, we don't run any
    non-runnable vCPUs, but we end the scan and we fallback to picking
    the idle unit. In other word, this prevent us to find there and pick
    the actual unit that we're meant to start running (which might be
    further ahead in the runq).
    
    Depending on the vCPU pinning configuration, this may lead to such
    unit to be stuck in the runq for long time, causing malfunctioning
    inside the guest.
    
    Fix this by checking runnable/non-runnable status up-front, in the runq
    scanning function.
    
    Reported-by: MichaÅ? LeszczyÅ?ski <michal.leszczynski@xxxxxxx>
    Reported-by: Dion Kant <g.w.kant@xxxxxxxxxx>
    Signed-off-by: Dario Faggioli <dfaggioli@xxxxxxxx>
    Reviewed-by: George Dunlap <george.dunlap@xxxxxxxxxx>
    master commit: 07b0eb5d0ef0be154606aa46b5b4c5c59b158505
    master date: 2021-06-07 13:16:36 +0100
---
 xen/common/sched/credit2.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/xen/common/sched/credit2.c b/xen/common/sched/credit2.c
index eb5e5a78c5..f5c1e5b944 100644
--- a/xen/common/sched/credit2.c
+++ b/xen/common/sched/credit2.c
@@ -3463,6 +3463,10 @@ runq_candidate(struct csched2_runqueue_data *rqd,
                         (unsigned char *)&d);
         }
 
+        /* Skip non runnable units that we (temporarily) have in the runq */
+        if ( unlikely(!unit_runnable_state(svc->unit)) )
+            continue;
+
         /* Only consider vcpus that are allowed to run on this processor. */
         if ( !cpumask_test_cpu(cpu, svc->unit->cpu_hard_affinity) )
             continue;
@@ -3496,8 +3500,7 @@ runq_candidate(struct csched2_runqueue_data *rqd,
          * some budget, then choose it.
          */
         if ( (yield || svc->credit > snext->credit) &&
-             (!has_cap(svc) || unit_grab_budget(svc)) &&
-             unit_runnable_state(svc->unit) )
+             (!has_cap(svc) || unit_grab_budget(svc)) )
             snext = svc;
 
         /* In any case, if we got this far, break. */
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.15