Re: PATCH [PPC64]: dead processes never reaped

From: Linas Vepstas
Date: Tue Apr 19 2005 - 12:41:02 EST


On Tue, Apr 19, 2005 at 11:01:25AM +1000, Benjamin Herrenschmidt was heard to remark:
> 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 = &current->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.
>
> I wonder why we bother doing all that at all... we could just return
> "prev" from __switch_to() no ? Like x86 does...

Probably. I assume this funny two-step is left-over from a 2.4 kernel
design point. Naively, we could rturn "prev", this would save a few
cycles. Cut the "addi r3,r3,-THREAD" from entry.S as well. I was being
conservative with the patch, making the smallest change possible.
Do you want this larger patch?


--linas

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