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
> >>>> @@ -4230,7 +4230,7 @@ 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)
> >> @@ -1790,7 +1790,6 @@ 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)
> >> @@ -406,6 +406,8 @@ 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)
> >> @@ -940,7 +940,6 @@ 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.
--
John Baldwin
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