[PATCH] xen: sched: don't call hooks of the wrong scheduler via VCPU2OP

[Dario Faggioli <dario.faggioli@citrix.com>]

Within context_saved(), we call the context_saved hook,
and we use VCPU2OP() to determine from what scheduler.
VCPU2OP uses DOM2OP, which uses d->cpupool, which is
NULL when d is the idle domain. And in that case,
DOM2OP just returns ops, the scheduler of cpupool0.

Therefore, if:
- cpupool0's scheduler defines context_saved (like
  Credit2 and RTDS do),
- we are not in cpupool0 (i.e., our scheduler is
  not ops),
- we are context switching from idle,

we call VCPU2OP(idle_vcpu), which means
DOM2OP(idle->cpupool), which is ops.

Therefore, we both:
- check if context_saved is defined in the wrong
  scheduler;
- if yes, call the wrong one.

When using Credit2 at boot, and also Credit2 in
the other cpupool, this is wrong but innocuous,
because it only involves the idle vcpus.

When using Credit2 at boot, and Credit1 in the
other cpupool, this is *totally* wrong, and
it's by chance it does not explode!

When using Credit2 and other schedulers I'm
developping, I hit the following assert (in
sched_credit2.c, on a CPU inside a cpupool that
does not use Credit2):

csched2_context_saved()
{
 ...
 ASSERT(!vcpu_on_runq(svc));
 ...
}

Fix this by taking care, in VCPU2OP, of the case
when the vcpu is an idle one.

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
---
Cc: George Dunlap <george.dunlap@citrix.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: Jan Beulich <jbeulich@suse.com>
---
Cc-ing Jan, as this should be backported at least to 4.8, but, IMO, as back as
possible.
---
 xen/common/schedule.c |   14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/xen/common/schedule.c b/xen/common/schedule.c
index 223a120..d12f346 100644
--- a/xen/common/schedule.c
+++ b/xen/common/schedule.c
@@ -78,7 +78,19 @@ static struct scheduler __read_mostly ops;
           : (typeof((opsptr)->fn(opsptr, ##__VA_ARGS__)))0 )
 
 #define DOM2OP(_d)    (((_d)->cpupool == NULL) ? &ops : ((_d)->cpupool->sched))
-#define VCPU2OP(_v)   (DOM2OP((_v)->domain))
+static inline struct scheduler* VCPU2OP(const struct vcpu *v)
+{
+    struct domain *d = v->domain;
+
+    if ( likely(d->cpupool != NULL) )
+        return d->cpupool->sched;
+
+    /* v->processor never changes for idle vcpus, so using it here is safe */
+    if ( likely(is_idle_domain(d)) )
+        return per_cpu(scheduler, v->processor);
+    else
+        return &ops;
+}
 #define VCPU2ONLINE(_v) cpupool_domain_cpumask((_v)->domain)
 
 static inline void trace_runstate_change(struct vcpu *v, int new_state)


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