[Xen-devel] [PATCH v2] xen: sched_credit: filter node-affinity mask against online cpus

[Dario Faggioli <dario.faggioli@xxxxxxxxxx>]

in _csched_cpu_pick(), as not doing so may result in the domain's
node-affinity mask (as retrieved by csched_balance_cpumask() )
and online mask (as retrieved by cpupool_scheduler_cpumask() )
having an empty intersection.

Therefore, when attempting a node-affinity load balancing step
and running this:

    ...
    /* Pick an online CPU from the proper affinity mask */
    csched_balance_cpumask(vc, balance_step, &cpus);
    cpumask_and(&cpus, &cpus, online);
    ...

we end up with an empty cpumask (in cpus). At this point, in
the following code:

    ....
    /* If present, prefer vc's current processor */
    cpu = cpumask_test_cpu(vc->processor, &cpus)
            ? vc->processor
            : cpumask_cycle(vc->processor, &cpus);
    ....

an ASSERT (from inside cpumask_cycle() ) triggers like this:

(XEN) Xen call trace:
(XEN)    [<ffff82d08011b124>] _csched_cpu_pick+0x1d2/0x652
(XEN)    [<ffff82d08011b5b2>] csched_cpu_pick+0xe/0x10
(XEN)    [<ffff82d0801232de>] vcpu_migrate+0x167/0x31e
(XEN)    [<ffff82d0801238cc>] cpu_disable_scheduler+0x1c8/0x287
(XEN)    [<ffff82d080101b3f>] cpupool_unassign_cpu_helper+0x20/0xb4
(XEN)    [<ffff82d08010544f>] continue_hypercall_tasklet_handler+0x4a/0xb1
(XEN)    [<ffff82d080127793>] do_tasklet_work+0x78/0xab
(XEN)    [<ffff82d080127a70>] do_tasklet+0x5f/0x8b
(XEN)    [<ffff82d080158985>] idle_loop+0x57/0x5e
(XEN)
(XEN)
(XEN) ****************************************
(XEN) Panic on CPU 1:
(XEN) Assertion 'cpu < nr_cpu_ids' failed at 
/home/dario/Sources/xen/xen/xen.git/xen/include/xe:16481

It is for example sufficient to have a domain with node-affinity
to NUMA node 1 running, and issueing a `xl cpupool-numa-split'
would make the above happen. That is because, by default, all
the existing domains remain assigned to the first cpupool, and
it now (after the cpupool-numa-split) only includes NUMA node 0.

This change prevents that by generalizing the function used
for figuring out whether a node-affinity load balancing step
is legit or not. This way we can, in _csched_cpu_pick(),
figure out early enough that the mask would end up empty,
skip the step all together and avoid the splat.

Signed-off-by: Dario Faggioli <dario.faggioli@xxxxxxxxxx>
Cc: George Dunlap <George.Dunlap@xxxxxxxxxxxxx>
Cc: Jan Beulich <JBeulich@xxxxxxxx>
Cc: Keir Fraser <keir@xxxxxxx>
---
Changes from v1:
 * improved code comments, as suggested during review;
---
 xen/common/sched_credit.c |   54 ++++++++++++++++++++++++++++++++++++---------
 1 file changed, 43 insertions(+), 11 deletions(-)

diff --git a/xen/common/sched_credit.c b/xen/common/sched_credit.c
index dbe6de6..d7afa50 100644
--- a/xen/common/sched_credit.c
+++ b/xen/common/sched_credit.c
@@ -296,15 +296,25 @@ static void csched_set_node_affinity(
  * vcpu-affinity balancing is always necessary and must never be skipped.
  * OTOH, if a domain's node-affinity is said to be automatically computed
  * (or if it just spans all the nodes), we can safely avoid dealing with
- * node-affinity entirely. Ah, node-affinity is also deemed meaningless
- * in case it has empty intersection with the vcpu's vcpu-affinity, as it
- * would mean trying to schedule it on _no_ pcpu!
+ * node-affinity entirely.
+ *
+ * Node-affinity is also deemed meaningless in case it has empty
+ * intersection with mask, to cover the cases where using the node-affinity
+ * mask seems legit, but would instead led to trying to schedule the vcpu
+ * on _no_ pcpu! Typical use cases are for mask to be equal to the vcpu's
+ * vcpu-affinity, or to the && of vcpu-affinity and the set of online cpus
+ * in the domain's cpupool.
  */
-#define __vcpu_has_node_affinity(vc)                                          \
-    ( !(cpumask_full(CSCHED_DOM(vc->domain)->node_affinity_cpumask)           \
-        || !cpumask_intersects(vc->cpu_affinity,                              \
-                               CSCHED_DOM(vc->domain)->node_affinity_cpumask) \
-        || vc->domain->auto_node_affinity == 1) )
+static inline int __vcpu_has_node_affinity(struct vcpu *vc, cpumask_t *mask)
+{
+    if ( vc->domain->auto_node_affinity == 1
+         || cpumask_full(CSCHED_DOM(vc->domain)->node_affinity_cpumask)
+         || !cpumask_intersects(CSCHED_DOM(vc->domain)->node_affinity_cpumask,
+                                mask) )
+        return 0;
+
+    return 1;
+}
 
 /*
  * Each csched-balance step uses its own cpumask. This function determines
@@ -393,7 +403,8 @@ __runq_tickle(unsigned int cpu, struct csched_vcpu *new)
             int new_idlers_empty;
 
             if ( balance_step == CSCHED_BALANCE_NODE_AFFINITY
-                 && !__vcpu_has_node_affinity(new->vcpu) )
+                 && !__vcpu_has_node_affinity(new->vcpu,
+                                              new->vcpu->cpu_affinity) )
                 continue;
 
             /* Are there idlers suitable for new (for this balance step)? */
@@ -626,11 +637,32 @@ _csched_cpu_pick(const struct scheduler *ops, struct vcpu 
*vc, bool_t commit)
     int cpu = vc->processor;
     int balance_step;
 
+    /* Store in cpus the mask of online cpus on which the domain can run */
     online = cpupool_scheduler_cpumask(vc->domain->cpupool);
+    cpumask_and(&cpus, vc->cpu_affinity, online);
+
     for_each_csched_balance_step( balance_step )
     {
+        /*
+         * We want to pick up a pcpu among the ones that are online and
+         * can accommodate vc, which is basically what we computed above
+         * and stored in cpus. As far as vcpu-affinity is concerned,
+         * there always will be at least one of these pcpus, hence cpus
+         * is never empty and the calls to cpumask_cycle() and
+         * cpumask_test_cpu() below are ok.
+         *
+         * On the other hand, when considering node-affinity too, it
+         * is possible for the mask to become empty (for instance, if the
+         * domain has been put in a cpupool that does not contain any of the
+         * nodes in its node-affinity), which would result in the ASSERT()-s
+         * inside cpumask_*() operations triggering (in debug builds).
+         *
+         * Therefore, in this case, we filter the node-affinity mask against
+         * cpus and, if the result is empty, we just skip the node-affinity
+         * balancing step all together.
+         */
         if ( balance_step == CSCHED_BALANCE_NODE_AFFINITY
-             && !__vcpu_has_node_affinity(vc) )
+             && !__vcpu_has_node_affinity(vc, &cpus) )
             continue;
 
         /* Pick an online CPU from the proper affinity mask */
@@ -1445,7 +1477,7 @@ csched_runq_steal(int peer_cpu, int cpu, int pri, int 
balance_step)
              * or counter.
              */
             if ( balance_step == CSCHED_BALANCE_NODE_AFFINITY
-                 && !__vcpu_has_node_affinity(vc) )
+                 && !__vcpu_has_node_affinity(vc, vc->cpu_affinity) )
                 continue;
 
             csched_balance_cpumask(vc, balance_step, csched_balance_mask);


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