[Xen-devel] [PATCH v2 1/2] xen: credit2: fix two s_time_t handling issues in load balancing

[Dario Faggioli <dario.faggioli@xxxxxxxxxx>]

both introduced in d205f8a7f48e2ec ("xen: credit2: rework
load tracking logic").

First, in __update_runq_load(), the ASSERT() was actually
useless. Let's instead check that the computed value of
the load has not overflowed (and hence gone negative).

While there, do that in __update_svc_load() as well.

Second, in balance_load(), cpus_max needs being extended
in order to be correctly shifted, and the result compared
with an s_time_t value, without risking loosing info.

Spotted by Coverity.

Signed-off-by: Dario Faggioli <dario.faggioli@xxxxxxxxxx>
Reported-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
Cc: George Dunlap <george.dunlap@xxxxxxxxxx>
Cc: Anshul Makkar <anshul.makkar@xxxxxxxxxx>
---
Changed from v1:
 * fixed a '> 0' which wanted to be '>= 0' in the ASSERT()-s;
 * cite Coverity in the changelog.
---
 xen/common/sched_credit2.c |    8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/xen/common/sched_credit2.c b/xen/common/sched_credit2.c
index b33ba7a..a55240f 100644
--- a/xen/common/sched_credit2.c
+++ b/xen/common/sched_credit2.c
@@ -656,7 +656,8 @@ __update_runq_load(const struct scheduler *ops,
     rqd->load += change;
     rqd->load_last_update = now;
 
-    ASSERT(rqd->avgload <= STIME_MAX && rqd->b_avgload <= STIME_MAX);
+    /* Overflow, capable of making the load look negative, must not occur. */
+    ASSERT(rqd->avgload >= 0 && rqd->b_avgload >= 0);
 
     if ( unlikely(tb_init_done) )
     {
@@ -714,6 +715,9 @@ __update_svc_load(const struct scheduler *ops,
     }
     svc->load_last_update = now;
 
+    /* Overflow, capable of making the load look negative, must not occur. */
+    ASSERT(svc->avgload >= 0);
+
     if ( unlikely(tb_init_done) )
     {
         struct {
@@ -1742,7 +1746,7 @@ retry:
          * If we're under 100% capacaty, only shift if load difference
          * is > 1.  otherwise, shift if under 12.5%
          */
-        if ( load_max < (cpus_max << prv->load_precision_shift) )
+        if ( load_max < ((s_time_t)cpus_max << prv->load_precision_shift) )
         {
             if ( st.load_delta < (1ULL << (prv->load_precision_shift +
                                            opt_underload_balance_tolerance)) )


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