[Xen-devel] [PATCH v2 1/2] xen: sched: don't call hooks of the wrong scheduler via VCPU2OP

[Dario Faggioli <dario.faggioli@xxxxxxxxxx>]

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 dealing explicitly, in VCPU2OP, with
idle vcpus, returning the scheduler of the pCPU
they (always) run on.

While there, rename VCPU2OP itself to something
that makes it easier to understand what it does.

Signed-off-by: Dario Faggioli <dario.faggioli@xxxxxxxxxx>
---
Cc: George Dunlap <george.dunlap@xxxxxxxxxx>
Cc: Juergen Gross <jgross@xxxxxxxx>
Cc: Jan Beulich <jbeulich@xxxxxxxx>
---
Changes from v1:
 - refactored according to review comments. Added code comments and an ASSERT().
---
Cc-ing Jan, as this should be backported at least to 4.8, but, IMO, as back as
possible.
---
 xen/common/schedule.c |   22 +++++++++++++++++++++-
 1 file changed, 21 insertions(+), 1 deletion(-)

diff --git a/xen/common/schedule.c b/xen/common/schedule.c
index 223a120..d344b7c 100644
--- a/xen/common/schedule.c
+++ b/xen/common/schedule.c
@@ -78,7 +78,27 @@ 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;
+
+    /*
+     * If d->cpupool is NULL, this is a vCPU of the idle domain. And this
+     * case is special because the idle domain does not really belong to
+     * a cpupool and, hence, doesn't really have a scheduler). In fact, its
+     * vCPUs (may) run on pCPUs which are in different pools, with different
+     * schedulers.
+     *
+     * What we want, in this case, is the scheduler of the pCPU where this
+     * particular idle vCPU is running. And, since v->processor never changes
+     * for idle vCPUs, it is safe to use it, with no locks, to figure that out.
+     */
+    ASSERT(is_idle_domain(d));
+    return per_cpu(scheduler, v->processor);
+}
 #define VCPU2ONLINE(_v) cpupool_domain_cpumask((_v)->domain)
 
 static inline void trace_runstate_change(struct vcpu *v, int new_state)


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