Re: [Xen-devel] dom0 hang

[Mukesh Rathor <mukesh.rathor@xxxxxxxxxx>]

George Dunlap wrote:
> On Thu, Jul 2, 2009 at 4:19 AM, Mukesh Rathor<mukesh.rathor@xxxxxxxxxx> wrote:
> > dom0 hang:
> >    vcpu0 is trying to wakeup a task and in try_to_wake_up() calls
> >    task_rq_lock(). since the task has cpu set to 1, it gets runq lock
> >    for vcpu1. next it calls resched_task() which results in sending IPI
> >    to vcpu1. for that, vcpu0 gets into the HYPERVISOR_event_channel_op
> >    HCALL and is waiting to return. Meanwhile, vcpu1 got running, and is
> >    spinning on it's runq lock in "schedule():spin_lock_irq(&rq->lock);",
> >    that vcpu0 is holding (and is waiting to return from the HCALL).
> >
> >    As I had noticed before, vcpu0 never gets scheduled in xen. So
> >    looking further into xen:
> >
> > xen:
> >    Both vcpu's are on the same runq, in this case cpu1. But the
> >    priority of vcpu1 has been set to CSCHED_PRI_TS_BOOST. As a result,
> >    the scheduler always picks vcpu1, and vcpu0 is starved. Also, I see in
> >    kdb that the scheduler timer is not set on cpu 0. That would've
> >    allowed csched_load_balance() to kick in on cpu0. [Also, on
> >    cpu1, the accounting timer, csched_tick, is not set.  Altho,
> >    csched_tick() is running on cpu0, it only checks runq for cpu0.]
> >
> >    Looks like c/s 19500 changed csched_schedule():
> >
> > -    ret.time = MILLISECS(CSCHED_MSECS_PER_TSLICE);
> > +    ret.time = (is_idle_vcpu(snext->vcpu) ?
> > +                -1 : MILLISECS(CSCHED_MSECS_PER_TSLICE));
> >
> >  The quickest fix for us would be to just back that out.
> >
> >
> >  BTW, just a comment on following (all in sched_credit.c):
> >
> >      if ( svc->pri == CSCHED_PRI_TS_UNDER &&
> >         !(svc->flags & CSCHED_FLAG_VCPU_PARKED) )
> >      {
> >         svc->pri = CSCHED_PRI_TS_BOOST;
> >      }
> >  comibined with
> >    if ( snext->pri > CSCHED_PRI_TS_OVER )
> >            __runq_remove(snext);
> >
> >      Setting CSCHED_PRI_TS_BOOST as pri of vcpu seems dangerous. To me,
> >      since csched_schedule() never checks for time accumulated by a
> >      vcpu at pri CSCHED_PRI_TS_BOOST, that is same as pinning a vcpu to a
> >      pcpu. if that vcpu never makes progress, essentially, the system
> >      has lost a physical cpu.  Optionally, csched_schedule() should always
> >      check for cpu time accumulated and reduce the priority over time.
> >      I can't tell right off if it already does that. or something like
> >      that :)...  my 2 cents.
> Hmm... what's supposed to happen is that eventually a timer tick will
> interrupt vcpu1.  If cpu1 is set to be "active", then it will be
> debited 10ms worth of credit.  Eventually, it will go into OVER, and
> lose BOOST.  If it's "inactive", then when the tick happens, it will
> be set to "active" and be debited 10ms again, setting it directly into
> OVER (and thus also losing boost).
>
> Can you see if the timer ticks are still happening, and perhaps put
> some tracing it to verify that what I described above is happening?
>
>  -George
George,

Is that in csched_acct()? Looks like that's somehow gotten removed. If
true, then may be that's the fundamental problem to chase.

Here's what the trq looks like when hung, not in any schedule function:

[0]xkdb> dtrq
CPU[00]: NOW:0x00003f2db9af369e
 1: exp=0x00003ee31cb32200 fn:csched_tick data:0000000000000000
 2: exp=0x00003ee347ece164 fn:time_calibration data:0000000000000000
 3: exp=0x00003ee69a28f04b fn:mce_work_fn data:0000000000000000
 4: exp=0x00003f055895e25f fn:plt_overflow data:0000000000000000
 5: exp=0x00003ee353810216 fn:rtc_update_second data:ffff83007f0226d8

CPU[01]: NOW:0x00003f2db9af369e
 1: exp=0x00003ee30b847988 fn:s_timer_fn data:0000000000000000
 2: exp=0x00003f1b309ebd45 fn:pmt_timer_callback data:ffff83007f022a68


thanks
Mukesh

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