[Xen-devel] [PATCH v2 3/4] xen: credit1: properly deal with pCPUs not in any cpupool

[Dario Faggioli <dario.faggioli@xxxxxxxxxx>]

Ideally, the pCPUs that are 'free', i.e., not assigned
to any cpupool, should not be considred by the scheduler
for load balancing or anything. In Credit1, we fail at
this, because of how we use cpupool_scheduler_cpumask().
In fact, for a free pCPU, cpupool_scheduler_cpumask()
returns a pointer to cpupool_free_cpus, and hence, near
the top of csched_load_balance():

 if ( unlikely(!cpumask_test_cpu(cpu, online)) )
     goto out;

is false (the pCPU _is_ free!), and we therefore do not
jump to the end right away, as we should. This, causes
the following splat when resuming from ACPI S3 with
pCPUs not assigned to any pool:

(XEN) ----[ Xen-4.6-unstable  x86_64  debug=y  Tainted:    C ]----
(XEN) ... ... ...
(XEN) Xen call trace:
(XEN)    [<ffff82d080122eaa>] csched_load_balance+0x213/0x794
(XEN)    [<ffff82d08012374c>] csched_schedule+0x321/0x452
(XEN)    [<ffff82d08012c85e>] schedule+0x12a/0x63c
(XEN)    [<ffff82d08012fa09>] __do_softirq+0x82/0x8d
(XEN)    [<ffff82d08012fa61>] do_softirq+0x13/0x15
(XEN)    [<ffff82d080164780>] idle_loop+0x5b/0x6b
(XEN)
(XEN)
(XEN) ****************************************
(XEN) Panic on CPU 8:
(XEN) GENERAL PROTECTION FAULT
(XEN) [error_code=0000]
(XEN) ****************************************

The cure is:
 * use cpupool_online_cpumask(), as a better guard to the
   case when the cpu is being offlined;
 * explicitly check whether the cpu is free.

SEDF is in a similar situation, so fix it too.

Still in Credit1, we must make sure that free (or offline)
CPUs are not considered "ticklable". Not doing so would impair
the load balancing algorithm, making the scheduler think that
it is possible to 'ask' the pCPU to pick up some work, while
in reallity, that will never happen! Evidence of such behavior
is shown in this trace:

 Name               CPU list
 Pool-0             0,1,2,3,4,5,6,7,8,9,10,11,12,13,14

    0.112998198 | ||.|| -|x||-|- d0v0 runstate_change d0v4 offline->runnable
 ]  0.112998198 | ||.|| -|x||-|- d0v0   22006(2:2:6) 1 [ f ]
 ]  0.112999612 | ||.|| -|x||-|- d0v0   28004(2:8:4) 2 [ 0 4 ]
    0.113003387 | ||.|| -||||-|x d32767v15 runstate_continue d32767v15 
running->running

where "22006(2:2:6) 1 [ f ]" means that pCPU 15, which is
free from any pool, is tickled.

The cure, in this case, is to filter out the free pCPUs,
within __runq_tickle().

Signed-off-by: Dario Faggioli <dario.faggioli@xxxxxxxxxx>
Acked-by: Juergen Gross <jgross@xxxxxxxx>
Reviewed-by: George Dunlap <george.dunlap@xxxxxxxxxxxxx>
---
 xen/common/sched_credit.c |   23 ++++++++++++++++-------
 xen/common/sched_sedf.c   |    3 ++-
 2 files changed, 18 insertions(+), 8 deletions(-)

diff --git a/xen/common/sched_credit.c b/xen/common/sched_credit.c
index 953ecb0..a1945ac 100644
--- a/xen/common/sched_credit.c
+++ b/xen/common/sched_credit.c
@@ -366,12 +366,17 @@ __runq_tickle(unsigned int cpu, struct csched_vcpu *new)
 {
     struct csched_vcpu * const cur = CSCHED_VCPU(curr_on_cpu(cpu));
     struct csched_private *prv = CSCHED_PRIV(per_cpu(scheduler, cpu));
-    cpumask_t mask, idle_mask;
+    cpumask_t mask, idle_mask, *online;
     int balance_step, idlers_empty;
 
     ASSERT(cur);
     cpumask_clear(&mask);
-    idlers_empty = cpumask_empty(prv->idlers);
+
+    /* cpu is vc->processor, so it must be in a cpupool. */
+    ASSERT(per_cpu(cpupool, cpu) != NULL);
+    online = cpupool_online_cpumask(per_cpu(cpupool, cpu));
+    cpumask_and(&idle_mask, prv->idlers, online);
+    idlers_empty = cpumask_empty(&idle_mask);
 
 
     /*
@@ -408,8 +413,8 @@ __runq_tickle(unsigned int cpu, struct csched_vcpu *new)
             /* Are there idlers suitable for new (for this balance step)? */
             csched_balance_cpumask(new->vcpu, balance_step,
                                    csched_balance_mask);
-            cpumask_and(&idle_mask, prv->idlers, csched_balance_mask);
-            new_idlers_empty = cpumask_empty(&idle_mask);
+            cpumask_and(csched_balance_mask, csched_balance_mask, &idle_mask);
+            new_idlers_empty = cpumask_empty(csched_balance_mask);
 
             /*
              * Let's not be too harsh! If there aren't idlers suitable
@@ -1510,6 +1515,7 @@ static struct csched_vcpu *
 csched_load_balance(struct csched_private *prv, int cpu,
     struct csched_vcpu *snext, bool_t *stolen)
 {
+    struct cpupool *c = per_cpu(cpupool, cpu);
     struct csched_vcpu *speer;
     cpumask_t workers;
     cpumask_t *online;
@@ -1517,10 +1523,13 @@ csched_load_balance(struct csched_private *prv, int cpu,
     int node = cpu_to_node(cpu);
 
     BUG_ON( cpu != snext->vcpu->processor );
-    online = cpupool_scheduler_cpumask(per_cpu(cpupool, cpu));
+    online = cpupool_online_cpumask(c);
 
-    /* If this CPU is going offline we shouldn't steal work. */
-    if ( unlikely(!cpumask_test_cpu(cpu, online)) )
+    /*
+     * If this CPU is going offline, or is not (yet) part of any cpupool
+     * (as it happens, e.g., during cpu bringup), we shouldn't steal work.
+     */
+    if ( unlikely(!cpumask_test_cpu(cpu, online) || c == NULL) )
         goto out;
 
     if ( snext->pri == CSCHED_PRI_IDLE )
diff --git a/xen/common/sched_sedf.c b/xen/common/sched_sedf.c
index a1a4cb7..cad5b39 100644
--- a/xen/common/sched_sedf.c
+++ b/xen/common/sched_sedf.c
@@ -790,7 +790,8 @@ static struct task_slice sedf_do_schedule(
     if ( tasklet_work_scheduled ||
          (list_empty(runq) && list_empty(waitq)) ||
          unlikely(!cpumask_test_cpu(cpu,
-                   cpupool_scheduler_cpumask(per_cpu(cpupool, cpu)))) )
+                   cpupool_online_cpumask(per_cpu(cpupool, cpu))) ||
+                  per_cpu(cpupool, cpu) == NULL) )
     {
         ret.task = IDLETASK(cpu);
         ret.time = SECONDS(1);


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