[Xen-changelog] [xen master] xen: credit2: prevent load balancing to go mad if time goes backwards

[patchbot@xxxxxxx]

commit 11eadabc34625337f7556e9d74f638229f889fd8
Author:     Dario Faggioli <dario.faggioli@xxxxxxxxxx>
AuthorDate: Fri Jul 15 16:49:18 2016 +0200
Commit:     George Dunlap <george.dunlap@xxxxxxxxxx>
CommitDate: Mon Jul 18 17:51:13 2016 +0100

    xen: credit2: prevent load balancing to go mad if time goes backwards
    
    This really should not happen, but:
     1. it does happen! Some more info here:
        http://lists.xen.org/archives/html/xen-devel/2016-06/msg00922.html
     2. independently from 1, it makes sense and is easy enough
        to have a 'safety catch'.
    
    The reason why this is particularly bad for Credit2 is that
    negative values of delta mean out of scale high load (because
    of the conversion to unsigned). This, for instance in the
    case of runqueue load, results in a runqueue having its load
    updated to values of the order of 10000% or so, which in turns
    means that the load balancer will migrate everything off from
    the pCPUs in the runqueue, and leave them idle until the load
    gets back to something sane... which may indeed take a while!
    
    This is not a fix for the problem of time going backwards. In
    fact, if that happens a lot, load tracking accuracy is still
    compromized, but at least the effect is a lot less bad than
    before.
    
    Signed-off-by: Dario Faggioli <dario.faggioli@xxxxxxxxxx>
    Reviewed-by: George Dunlap <george.dunlap@xxxxxxxxxx>
---
 xen/common/sched_credit2.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/xen/common/sched_credit2.c b/xen/common/sched_credit2.c
index 7e572bf..6cb06e8 100644
--- a/xen/common/sched_credit2.c
+++ b/xen/common/sched_credit2.c
@@ -404,6 +404,12 @@ __update_runq_load(const struct scheduler *ops,
     else
     {
         delta = now - rqd->load_last_update;
+        if ( unlikely(delta < 0) )
+        {
+            d2printk("%s: Time went backwards? now %"PRI_stime" llu 
%"PRI_stime"\n",
+                     __func__, now, rqd->load_last_update);
+            delta = 0;
+        }
 
         rqd->avgload =
             ( ( delta * ( (unsigned long long)rqd->load << 
prv->load_window_shift ) )
@@ -455,6 +461,12 @@ __update_svc_load(const struct scheduler *ops,
     else
     {
         delta = now - svc->load_last_update;
+        if ( unlikely(delta < 0) )
+        {
+            d2printk("%s: Time went backwards? now %"PRI_stime" llu 
%"PRI_stime"\n",
+                     __func__, now, svc->load_last_update);
+            delta = 0;
+        }
 
         svc->avgload =
             ( ( delta * ( (unsigned long long)vcpu_load << 
prv->load_window_shift ) )
--
generated by git-patchbot for /home/xen/git/xen.git#master

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