Re: PATCH [PPC64]: dead processes never reaped

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

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...

Ben.


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