Subject: Re: PATCH [PPC64]: dead processes never reaped
From: Nick Piggin <nickpiggin@yahoo.com.au>
To: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Linas Vepstas <linas@austin.ibm.com>, Paul Mackerras <paulus@samba.org>,
       Anton Blanchard <anton@samba.org>, Andrew Morton <akpm@osdl.org>,
       linuxppc64-dev <linuxppc64-dev@ozlabs.org>,
       lkml <linux-kernel@vger.kernel.org>
In-Reply-To: <1113872821.5516.330.camel@gaston>
References: <20050418193833.GW15596@austin.ibm.com>
	 <1113872821.5516.330.camel@gaston>
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Date: Tue, 19 Apr 2005 11:25:15 +1000
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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 = &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.  
> 
> 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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