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[Xen-devel] [PATCH 1/2] xen: credit2: avoid vCPUs to ever reach lower credits than idle

There have been report of stalls of guest vCPUs, when Credit2 was used.
It seemed like these vCPUs were not getting scheduled for very long
time, even under light load conditions (e.g., during dom0 boot).

Investigations led to the discovery that --although rarely-- it can
happen that a vCPU manages to run for very long timeslices. In Credit2,
this means that, when runtime accounting happens, the vCPU will lose a
large quantity of credits. This in turn may lead to the vCPU having less
credits than the idle vCPUs (-2^30). At this point, the scheduler will
pick the idle vCPU, instead of the ready to run vCPU, for a few
"epochs", which often times is enough for the guest kernel to think the
vCPU is not responding and crashing.

An example of this situation is shown here. In fact, we can see d0v1
sitting in the runqueue while all the CPUs are idle, as it has
-1254238270 credits, which is smaller than -2^30 = −1073741824:

    (XEN) Runqueue 0:
    (XEN)   ncpus              = 28
    (XEN)   cpus               = 0-27
    (XEN)   max_weight         = 256
    (XEN)   pick_bias          = 22
    (XEN)   instload           = 1
    (XEN)   aveload            = 293391 (~111%)
    (XEN)   idlers: 00,00000000,00000000,00000000,00000000,00000000,0fffffff
    (XEN)   tickled: 00,00000000,00000000,00000000,00000000,00000000,00000000
    (XEN)   fully idle cores: 
    (XEN) Runqueue 0:
    (XEN) CPU[00] runq=0, sibling=00,..., core=00,...
    (XEN) CPU[01] runq=0, sibling=00,..., core=00,...
    (XEN) CPU[26] runq=0, sibling=00,..., core=00,...
    (XEN) CPU[27] runq=0, sibling=00,..., core=00,...
    (XEN) RUNQ:
    (XEN)     0: [0.1] flags=0 cpu=5 credit=-1254238270 [w=256] load=262144 

We certainly don't want, under any circumstance, this to happen.
Therefore, let's use INT_MIN for the credits of the idle vCPU, in
Credit2, to be sure that no vCPU can get below that value.

NOTE: investigations have been done about _how_ it is possible for a
vCPU to execute for so long that its credits becomes so low. While still
not completely clear, there are evidence that:
- it only happens very rarely
- it appears to be both machine and workload specific
- it does not look to be a Credit2 (e.g., as it happens when running
  with Credit1 as well) issue, or a scheduler issue

This patch makes Credit2 more robust to events like this, whatever
the cause is, and should hence be backported (as far as possible).

Signed-off-by: Dario Faggioli <dfaggioli@xxxxxxxx>
Reported-by: Glen <glenbarney@xxxxxxxxx>
Reported-by: Tomas Mozes <hydrapolic@xxxxxxxxx>
Cc: George Dunlap <george.dunlap@xxxxxxxxxx>
Cc: Juergen Gross <jgross@xxxxxxxx>
Cc: Jan Beulich <jbeulich@xxxxxxxx>
Cc: Charles Arnold <carnold@xxxxxxxx>
Cc: Sarah Newman <srn@xxxxxxxxx>
I will provide the backports myself, at least for 4.13 and 4.12.x (and
feel free to ask for more).
For Sarah, looking back at the various threads, I am not quite sure
whether you also experienced the issue and reported it. If yes, I'm
happy to add a "Reported-by:" line about you too (or, if this is fine to
go in, for this to be done while committing, if possible).
 xen/common/sched/credit2.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/xen/common/sched/credit2.c b/xen/common/sched/credit2.c
index c7241944a8..5c0ab9cd05 100644
--- a/xen/common/sched/credit2.c
+++ b/xen/common/sched/credit2.c
@@ -234,7 +234,7 @@
  * units does not consume credits, and it must be lower than whatever
  * amount of credit 'regular' unit would end up with.
-#define CSCHED2_IDLE_CREDIT          (-(1U<<30))
+#define CSCHED2_IDLE_CREDIT          INT_MIN
  * Carryover: How much "extra" credit may be carried over after
  * a reset.

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