[xen stable-4.15] xen: credit2: fix per-entity load tracking when continuing running

[patchbot@xxxxxxx]

commit 3556dc6bfb27bb8ec8bc1df1f4813319ebec4e59
Author:     Dario Faggioli <dfaggioli@xxxxxxxx>
AuthorDate: Thu Jul 15 09:18:32 2021 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Thu Jul 15 09:18:32 2021 +0200

    xen: credit2: fix per-entity load tracking when continuing running
    
    If we schedule, and the current vCPU continues to run, its statistical
    load is not properly updated, resulting in something like this, even if
    all the 8 vCPUs are 100% busy:
    
    (XEN) Runqueue 0:
    (XEN) [...]
    (XEN)   aveload            = 2097152 (~800%)
    (XEN) [...]
    (XEN)   Domain: 0 w 256 c 0 v 8
    (XEN)     1: [0.0] flags=2 cpu=4 credit=9996885 [w=256] load=35 (~0%)
    (XEN)     2: [0.1] flags=2 cpu=2 credit=9993725 [w=256] load=796 (~0%)
    (XEN)     3: [0.2] flags=2 cpu=1 credit=9995885 [w=256] load=883 (~0%)
    (XEN)     4: [0.3] flags=2 cpu=5 credit=9998833 [w=256] load=487 (~0%)
    (XEN)     5: [0.4] flags=2 cpu=6 credit=9998942 [w=256] load=1595 (~0%)
    (XEN)     6: [0.5] flags=2 cpu=0 credit=9994669 [w=256] load=22 (~0%)
    (XEN)     7: [0.6] flags=2 cpu=7 credit=9997706 [w=256] load=0 (~0%)
    (XEN)     8: [0.7] flags=2 cpu=3 credit=9992440 [w=256] load=0 (~0%)
    
    As we can see, the average load of the runqueue as a whole is, instead,
    computed properly.
    
    This issue would, in theory, potentially affect Credit2 load balancing
    logic. In practice, however, the problem only manifests (at least with
    these characteristics) when there is only 1 runqueue active in the
    cpupool, which also means there is no need to do any load-balancing.
    
    Hence its real impact is pretty much limited to wrong per-vCPU load
    percentages, when looking at the output of the 'r' debug-key.
    
    With this patch, the load is updated and displayed correctly:
    
    (XEN) Runqueue 0:
    (XEN) [...]
    (XEN)   aveload            = 2097152 (~800%)
    (XEN) [...]
    (XEN) Domain info:
    (XEN)   Domain: 0 w 256 c 0 v 8
    (XEN)     1: [0.0] flags=2 cpu=4 credit=9995584 [w=256] load=262144 (~100%)
    (XEN)     2: [0.1] flags=2 cpu=6 credit=9992992 [w=256] load=262144 (~100%)
    (XEN)     3: [0.2] flags=2 cpu=3 credit=9998918 [w=256] load=262118 (~99%)
    (XEN)     4: [0.3] flags=2 cpu=5 credit=9996867 [w=256] load=262144 (~100%)
    (XEN)     5: [0.4] flags=2 cpu=1 credit=9998912 [w=256] load=262144 (~100%)
    (XEN)     6: [0.5] flags=2 cpu=2 credit=9997842 [w=256] load=262144 (~100%)
    (XEN)     7: [0.6] flags=2 cpu=7 credit=9994623 [w=256] load=262144 (~100%)
    (XEN)     8: [0.7] flags=2 cpu=0 credit=9991815 [w=256] load=262144 (~100%)
    
    Signed-off-by: Dario Faggioli <dfaggioli@xxxxxxxx>
    Reviewed-by: George Dunlap <george.dunlap@xxxxxxxxxx>
    master commit: 89052b9fa24bf976924e40918fc9fa3b1b940e17
    master date: 2021-06-07 13:17:06 +0100
---
 xen/common/sched/credit2.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/xen/common/sched/credit2.c b/xen/common/sched/credit2.c
index f5c1e5b944..ebb09ea43a 100644
--- a/xen/common/sched/credit2.c
+++ b/xen/common/sched/credit2.c
@@ -3649,6 +3649,8 @@ static void csched2_schedule(
             runq_remove(snext);
             __set_bit(__CSFLAG_scheduled, &snext->flags);
         }
+        else
+            update_load(ops, rqd, snext, 0, now);
 
         /* Clear the idle mask if necessary */
         if ( cpumask_test_cpu(sched_cpu, &rqd->idle) )
--
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