Re: Stop scheduler on panic

On Thursday, November 17, 2011 2:42:51 pm Andriy Gapon wrote:
> on 17/11/2011 21:09 John Baldwin said the following:
> > On Thursday, November 17, 2011 11:58:03 am Andriy Gapon wrote:
> >> on 17/11/2011 18:37 John Baldwin said the following:
> >>> On Thursday, November 17, 2011 4:47:42 am Andriy Gapon wrote:
> >>>> on 17/11/2011 10:34 Andriy Gapon said the following:
> >>>>> on 17/11/2011 10:15 Kostik Belousov said the following:
> >>>>>> I have the following change for eons on my test boxes. Without it,
> >>>>>> I simply cannot get _any_ dump.
> >>>>>>
> >>>>>> diff --git a/sys/cam/cam_xpt.c b/sys/cam/cam_xpt.c
> >>>>>> index 10b89c7..a38e42f 100644
> >>>>>> --- a/sys/cam/cam_xpt.c
> >>>>>> +++ b/sys/cam/cam_xpt.c
> >>>>>> _at__at_ -4230,7 +4230,7 _at__at_ xpt_done(union ccb *done_ccb)
> >>>>>>  			TAILQ_INSERT_TAIL(&cam_simq, sim, links);
> >>>>>>  			mtx_unlock(&cam_simq_lock);
> >>>>>>  			sim->flags |= CAM_SIM_ON_DONEQ;
> >>>>>> -			if (first)
> >>>>>> +			if (first && panicstr == NULL)
> >>>>>>  				swi_sched(cambio_ih, 0);
> >>>>>>  		}
> >>>>>>  	}
> >>>>>
> >>>>> I think that this (or similar) change should go into the patch and the tree.
> >>>>>
> >>>>
> >>>> And, BTW, I still would like to do something like the following (perhaps with
> >>>> td_oncpu = NOCPU and td_flags &= ~TDF_NEEDRESCHED also moved to the common code):
> >>>>
> >>>> Index: sys/kern/sched_ule.c
> >>>> ===================================================================
> >>>> --- sys/kern/sched_ule.c	(revision 227608)
> >>>> +++ sys/kern/sched_ule.c	(working copy)
> >>>> _at__at_ -1790,7 +1790,6 _at__at_ sched_switch(struct thread *td, struct thread *new
> >>>>  	td->td_oncpu = NOCPU;
> >>>>  	if (!(flags & SW_PREEMPT))
> >>>>  		td->td_flags &= ~TDF_NEEDRESCHED;
> >>>> -	td->td_owepreempt = 0;
> >>>>  	tdq->tdq_switchcnt++;
> >>>>  	/*
> >>>>  	 * The lock pointer in an idle thread should never change.  Reset it
> >>>> Index: sys/kern/kern_synch.c
> >>>> ===================================================================
> >>>> --- sys/kern/kern_synch.c	(revision 227608)
> >>>> +++ sys/kern/kern_synch.c	(working copy)
> >>>> _at__at_ -406,6 +406,8 _at__at_ mi_switch(int flags, struct thread *newtd)
> >>>>  	    ("mi_switch: switch must be voluntary or involuntary"));
> >>>>  	KASSERT(newtd != curthread, ("mi_switch: preempting back to ourself"));
> >>>>
> >>>> +	td->td_owepreempt = 0;
> >>>> +
> >>>>  	/*
> >>>>  	 * Don't perform context switches from the debugger.
> >>>>  	 */
> >>>> Index: sys/kern/sched_4bsd.c
> >>>> ===================================================================
> >>>> --- sys/kern/sched_4bsd.c	(revision 227608)
> >>>> +++ sys/kern/sched_4bsd.c	(working copy)
> >>>> _at__at_ -940,7 +940,6 _at__at_ sched_switch(struct thread *td, struct thread *new
> >>>>  	td->td_lastcpu = td->td_oncpu;
> >>>>  	if (!(flags & SW_PREEMPT))
> >>>>  		td->td_flags &= ~TDF_NEEDRESCHED;
> >>>> -	td->td_owepreempt = 0;
> >>>>  	td->td_oncpu = NOCPU;
> >>>>
> >>>>  	/*
> >>>>
> >>>> Does anybody see any potential problems with such a change?
> >>>
> >>> Hmm, does this mean the preemption will be lost if you break into the
> >>> debugger and continue in the non-panic case?
> >>
> >> Not sure which exact scenario you have in mind.
> >> Please note that the above diff just moves resetting of td_owepreempt to an
> >> earlier place.  As far as I can see there are no checks of td_owepreempt value
> >> between the new place and the old places.
> > 
> > I'm worried that you are clearing td_owepreempt even in cases where a context
> > switch is not performed.  So say you enter DDB with td_owepreempt set and that
> > DDB bails on a context switch.  With your change it will now clear td_owepreempt
> > and "lose" the preemption.
> > 
> 
> And without the change we get the recursion and double-fault because of
> kdb_switch -> thread_unlock ->  spinlock_exit ->  critical_exit ->
> mi_switch in this case ?
> 
> BTW, it is my opinion that we really should not let the debugger code call
> mi_switch for any reason.

Hmmm, you could also make critical_exit() not perform deferred preemptions
if SCHEDULER_STOPPED?  That would fix the recursion and still let the
preemption "work" when resuming from the debugger?

-- 
John Baldwin