RE: [Xen-devel] cpuidle causing Dom0 soft lockups

["Jan Beulich" <JBeulich@xxxxxxxxxx>]

>>> "Yu, Ke" <ke.yu@xxxxxxxxx> 23.02.10 10:32 >>>
>>The author's assertion was that vc must be runnable and hence cannot be
>>polling and hence cannot be is_urgent. It sounded dodgy to me hence I
>>upgarded it to a BUG_ON(), but couldn't actually eyeball a way in which
>>vc->is_urgent could be true at that point in the code. We have the lock on
>>the peer cpu's schedule_lock, so is_urgent cannot change under our feet, and
>>vc is not running so it cannot be added to the domain's poll_mask under our
>>feet.

>Right. According to the code, there should be no way to this BUG_ON.
>If it happens, that reveal either bugs of code or the necessity of
>adding code to migrate urgent vcpu count. Do you have more
>information on how this BUG_ON happens?

Obviously there are vCPU-s that get inserted on a run queue with
is_urgent set (which according to my reading of Keir's description
shouldn't happen). In particular, this

--- a/xen/common/sched_credit.c
+++ b/xen/common/sched_credit.c
@@ -201,6 +201,7 @@ __runq_insert(unsigned int cpu, struct c
 
     BUG_ON( __vcpu_on_runq(svc) );
     BUG_ON( cpu != svc->vcpu->processor );
+WARN_ON(svc->vcpu->is_urgent);//temp
 
     list_for_each( iter, runq )
     {

triggers occasionally:

(XEN) Xen call trace:
(XEN)    [<ffff82c48011a1e8>] csched_vcpu_wake+0x1e8/0x200
(XEN)    [<ffff82c48011dde2>] vcpu_wake+0x152/0x3a0
(XEN)    [<ffff82c48014b6cd>] vcpu_kick+0x1d/0x80
(XEN)    [<ffff82c480108465>] evtchn_set_pending+0x145/0x1d0
(XEN)    [<ffff82c4801087e8>] evtchn_send+0xa8/0x150
(XEN)    [<ffff82c480108de2>] do_event_channel_op+0x552/0x10d0
(XEN)    [<ffff82c4801f3b61>] do_iret+0xc1/0x1a0
(XEN)    [<ffff82c4801ef169>] syscall_enter+0xa9/0xae

As I understand it, a vCPU with is_urgent set being on any run
queue is enough for the questionable BUG_ON() to eventually
trigger.

Jan


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