[Devel] [RFC][PATCH 07/16] Move alloc_pid call to copy_process

[sukadev at us.ibm.com]

Pavel Emelianov [xemul at sw.ru] wrote:
| > Index: lx26-21-mm2/kernel/pid.c
| > ===================================================================
| > --- lx26-21-mm2.orig/kernel/pid.c	2007-05-22 16:59:46.000000000 -0700
| > +++ lx26-21-mm2/kernel/pid.c	2007-05-22 17:06:48.000000000 -0700
| > @@ -216,6 +216,10 @@ fastcall void free_pid(struct pid *pid)
| >  	/* We can be called with write_lock_irq(&tasklist_lock) held */
| >  	unsigned long flags;
| >  
| > +	/* check this here to keep copy_process() cleaner */
| > +	if (unlikely(pid == &init_struct_pid))
| > +		return;
| > +
| 
| This looks ugly to me.

I agree about the ugly part :-) but we need to distinguish
between idle thread and normal thread at some point in do_fork().

| That's the same as if we put 
|    if (ns == &init_pid_ns)
|         return;
| in put_pid_ns() call.
| 
| Such small struts of their own do not introduce any noticeable
| effect, but when we have them in many places (and on fast patsh
| like alloc_pid()) the performance hurts...

I agree and we have been trying to keep the impact as low as possible.
| 
| 
| >  	spin_lock_irqsave(&pidmap_lock, flags);
| >  	hlist_del_rcu(&pid->pid_chain);
| >  	spin_unlock_irqrestore(&pidmap_lock, flags);
| > @@ -224,12 +228,16 @@ fastcall void free_pid(struct pid *pid)
| >  	call_rcu(&pid->rcu, delayed_put_pid);
| >  }
| >  
| > -struct pid *alloc_pid(void)
| > +struct pid *alloc_pid(enum copy_process_type copy_src)
| >  {
| >  	struct pid *pid;
| >  	enum pid_type type;
| >  	int nr = -1;
| >  
| > +	/* check this here to keep copy_process() cleaner */
| > +	if (unlikely(copy_src == COPY_IDLE_PROCESS))
| > +		return &init_struct_pid;
| > +
| >  	pid = kmem_cache_alloc(pid_cachep, GFP_KERNEL);
| >  	if (!pid)
| >  		goto out;
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