[Xen-changelog] [xen stable-4.6] xen: sched: don't call hooks of the wrong scheduler via VCPU2OP

[patchbot@xxxxxxx]

commit 7ff6d9fc19ef88d2ca3304a312c0e8a46b61f546
Author:     Dario Faggioli <dario.faggioli@xxxxxxxxxx>
AuthorDate: Fri Mar 31 09:03:32 2017 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Fri Mar 31 09:03:32 2017 +0200

    xen: sched: don't call hooks of the wrong scheduler via VCPU2OP
    
    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.
    
    Signed-off-by: Dario Faggioli <dario.faggioli@xxxxxxxxxx>
    Reviewed-by: Juergen Gross <jgross@xxxxxxxx>
    Reviewed-by: George Dunlap <george.dunlap@xxxxxxxxxx>
    master commit: a3653e6a279213ba4e883b2252415dc98633106a
    master date: 2017-03-27 14:28:05 +0100
---
 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 fddcd52..ca29a1c 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_online_cpumask((_v)->domain->cpupool)
 
 static inline void trace_runstate_change(struct vcpu *v, int new_state)
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.6

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