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@suse.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>
master commit: 89052b9fa2
master date: 2021-06-07 13:17:06 +0100

xen/common/sched_credit2.c

@@ -3554,6 +3554,8 @@ 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) )