linux-kernel - Re: [RFC PATCH(Experimental) 0/4] Freezer based Cpu-hotplug

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Date:	Sun, 18 Feb 2007 18:06:52 +0300
From:	Oleg Nesterov <oleg@...sign.ru>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
Cc:	ego@...ibm.com, akpm@...l.org, paulmck@...ibm.com, mingo@...e.hu,
	vatsa@...ibm.com, dipankar@...ibm.com,
	venkatesh.pallipadi@...el.com, linux-kernel@...r.kernel.org,
	Pavel Machek <pavel@....cz>
Subject: Re: [RFC PATCH(Experimental) 0/4] Freezer based Cpu-hotplug

On 02/18, Rafael J. Wysocki wrote:
>
> On Sunday, 18 February 2007 12:32, Oleg Nesterov wrote:
> > On 02/18, Rafael J. Wysocki wrote:
> > >
> > > On Sunday, 18 February 2007 00:42, Oleg Nesterov wrote:
> > > > On 02/17, Rafael J. Wysocki wrote:
> > > > >
> > > > > On Saturday, 17 February 2007 22:34, Oleg Nesterov wrote:
> > > > > > 
> > > > > > 	static inline int is_user_space(struct task_struct *p)
> > > > > > 	{
> > > > > > 		return p->mm && !(p->flags & PF_BORROWED_MM);
> > > > > > 	}
> > > > > > 
> > > > > > This doesn't look right. First, an exiting task has ->mm == NULL after
> > > > > > do_exit()->exit_mm(). Probably not a problem. However, PF_BORROWED_MM
> > > > > > check is racy without task_lock(), so we can have a false positive as
> > > > > > well. Is it ok? We can freeze aio_wq prematurely.
> > > > > 
> > > > > Right now aio_wq is not freezeable (PF_NOFREEZE).
> > > > 
> > > > Right now yes, but we are going to change this?
> > > 
> > > Well, is there any more reliable (and not racy) method of differentiating
> > > between kernel threads and user space processes?
> > 
> > Not that I know of. At least, we can take task_lock() to really rule out
> > kernel threads at FREEZER_USER_SPACE stage.
> 
> Something like this?

I think yes, as a first step.

In the long term, I think it would be really nice to have CLONE_KERNEL_THREAD
(filtered out in sys_clone). This also allows us to cleanup copy_process().
For example, we can then introduce CLONE_UNHASHED (currently denoted as pid==0)
and kill the ugly "int pid" copy_process's parameter.

Oleg.

> ---
>  kernel/power/process.c |    8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> Index: linux-2.6.20-mm2/kernel/power/process.c
> ===================================================================
> --- linux-2.6.20-mm2.orig/kernel/power/process.c
> +++ linux-2.6.20-mm2/kernel/power/process.c
> @@ -8,6 +8,7 @@
>  
>  #undef DEBUG
>  
> +#include <linux/sched.h>
>  #include <linux/smp_lock.h>
>  #include <linux/interrupt.h>
>  #include <linux/suspend.h>
> @@ -92,7 +93,12 @@ static void cancel_freezing(struct task_
>  
>  static inline int is_user_space(struct task_struct *p)
>  {
> -	return p->mm && !(p->flags & PF_BORROWED_MM);
> +	int ret;
> +
> +	task_lock(p);
> +	ret = p->mm && !(p->flags & PF_BORROWED_MM);
> +	task_unlock(p);
> +	return ret;
>  }
>  
>  static unsigned int try_to_freeze_tasks(int freeze_user_space)

-
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