Re: [Xen-devel] dom0less + sched=null => broken in staging

[Dario Faggioli <dfaggioli@suse.com>]

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On Wed, 2019-08-07 at 11:22 -0700, Stefano Stabellini wrote:
> Hi Dario, George,
> 
> Dom0less with sched=null is broken on staging, it simply hangs soon
> after Xen is finished loading things. My impression is that vcpus are
> not actually started. I did a git bisection and it pointed to:
> 
> commit d545f1d6c2519a183ed631cfca7aff0baf29fde5 (refs/bisect/bad)
> Author: Dario Faggioli <dfaggioli@suse.com>
> Date:   Mon Aug 5 11:50:55 2019 +0100
> 
>     xen: sched: deal with vCPUs being or becoming online or offline
>     
> Any ideas?
> 
Ok, I've done some basic testing, and inspected the code again, and
honestly I am not finding anything really suspicious.

Of course, I'm not really testing dom0less, and I'm not sure I can
easily do that.

Can you help me with this, e.g., by providing some more info and, if
possible, logs?

E.g., you say boot stops after Xen loading. Is there a bootlog that we
can see (ideally from a debug build, and with "loglvl=all
guest_loglvl=all")?

Does the system respond to debug-keys? If yes, the log after triggering
the 'r' debug-key would be useful.

These patches are about vcpus going offline and online... does dom0less
play with vcpu onffline/online in any way?

I've put together a debug patch (attached), focusing on what the
mentioned commit does, but it's nothing more than a shot in the dark,
for now...

Thanks and Regards
-- 
Dario Faggioli, Ph.D
http://about.me/dario.faggioli
Virtualization Software Engineer
SUSE Labs, SUSE https://www.suse.com/
-------------------------------------------------------------------
<<This happens because _I_ choose it to happen!>> (Raistlin Majere)


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diff --git a/xen/common/sched_null.c b/xen/common/sched_null.c
index 26c6f0f129..afd42e552f 100644
--- a/xen/common/sched_null.c
+++ b/xen/common/sched_null.c
@@ -455,6 +455,7 @@ static void null_vcpu_insert(const struct scheduler *ops, struct vcpu *v)
 
     if ( unlikely(!is_vcpu_online(v)) )
     {
+        dprintk(XENLOG_G_INFO, "Not inserting %pv (not online!)\n", v);
         vcpu_schedule_unlock_irq(lock, v);
         return;
     }
@@ -516,6 +517,7 @@ static void null_vcpu_remove(const struct scheduler *ops, struct vcpu *v)
     /* If offline, the vcpu shouldn't be assigned, nor in the waitqueue */
     if ( unlikely(!is_vcpu_online(v)) )
     {
+        dprintk(XENLOG_G_INFO, "Not removing %pv (wasn't online!)\n", v);
         ASSERT(per_cpu(npc, v->processor).vcpu != v);
         ASSERT(list_empty(&nvc->waitq_elem));
         goto out;
@@ -635,6 +637,8 @@ static void null_vcpu_sleep(const struct scheduler *ops, struct vcpu *v)
         }
         else if ( per_cpu(npc, cpu).vcpu == v )
             tickled = vcpu_deassign(prv, v);
+
+        dprintk(XENLOG_G_INFO, "%pv is, apparently, going offline (tickled=%d)\n", v, tickled);
     }
 
     /* If v is not assigned to a pCPU, or is not running, no need to bother */
@@ -697,6 +701,8 @@ static void null_vcpu_migrate(const struct scheduler *ops, struct vcpu *v,
      */
     if ( unlikely(!is_vcpu_online(v)) )
     {
+        dprintk(XENLOG_G_INFO, "%pv is, apparently, going offline\n", v);
+
         spin_lock(&prv->waitq_lock);
         list_del_init(&nvc->waitq_elem);
         spin_unlock(&prv->waitq_lock);

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