On Tue, 2005-04-19 at 11:07 +1000, Benjamin Herrenschmidt wrote:
> On Mon, 2005-04-18 at 14:38 -0500, Linas Vepstas wrote:
> >
> > Hi,
> >
> > The patch below appears to fix a problem where a number of dead processes
> > linger on the system. On a highly loaded system, dozens of processes
> > were found stuck in do_exit(), calling thier very last schedule(), and
> > then being lost forever.
> >
> > Processes that are PF_DEAD are cleaned up *after* the context switch,
> > in a routine called finish_task_switch(task_t *prev). The "prev" gets
> > the value returned by _switch() in entry.S, but this value comes from
> >
> > __switch_to (struct task_struct *prev,
> > struct task_struct *new)
> > {
> > old_thread = ¤t->thread; ///XXX shouldn't this be prev, not
> > current?
> > last = _switch(old_thread, new_thread);
> > return last;
> > }
> >
> > The way I see it, "prev" and "current" are almost always going to be
> > pointing at the same thing; however, if a "need resched" happens,
> > or there's a pre-emept or some-such, then prev and current won't be
> > the same; in which case, finish_task_switch() will end up cleaning
> > up the old current, instead of prev. This will result in dead processes
> > hanging around, which will never be scheduled again, and will never
> > get a chance to have put_task_struct() called on them.
>
> Ok, thinking moer about this ... that will need maybe some help from
> Ingo so I fully understand where schedule's are allowed ... We are
> basically in the middle of the scheduler here, so I wonder how much of
> the scheduler itself can be preempted or so ...
>
Not much. schedule() has a small preempt window at the beginning
and end of the function.
The context switch is of course run with preempt disabled. Ie.
your switch_to should never get preempted.
> Basically, under which circumstances can prev and current be different ?
>
Depends on your context switch, really. prev == current before you
switch, and when you switch to 'new' it is different. However, I think
the 'new' current has *its* old prev on the stack (which == new
current). You just have to preserve the old 'prev' somehow (ie. the
thread you switched away from).
--
SUSE Labs, Novell Inc.
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