linux-kernel - Re: [RFC][PATCH][EXPERIMENTAL] Make kernel threads nonfreezable by default

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Message-Id: <1180345596.14749.40.camel@nigel.suspend2.net>
Date:	Mon, 28 May 2007 19:46:36 +1000
From:	Nigel Cunningham <nigel@...el.suspend2.net>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Gautham R Shenoy <ego@...ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Oleg Nesterov <oleg@...sign.ru>, Pavel Machek <pavel@....cz>
Subject: Re: [RFC][PATCH][EXPERIMENTAL] Make kernel threads nonfreezable by
	default

Hello!

In reply to your more recent message, I had looked but not tried, so
didn't feel in a position to reply yet.

On Sun, 2007-05-27 at 00:12 +0200, Rafael J. Wysocki wrote:
>  63 files changed, 78 insertions(+), 138 deletions(-)

Well, that looks good, for a start :)

> Index: linux-2.6.22-rc3/kernel/exit.c
> ===================================================================
> --- linux-2.6.22-rc3.orig/kernel/exit.c
> +++ linux-2.6.22-rc3/kernel/exit.c
> @@ -389,6 +389,11 @@ void daemonize(const char *name, ...)
>  	 * they would be locked into memory.
>  	 */
>  	exit_mm(current);
> +	/*
> +	 * We don't want to have TIF_FREEZE set if the system-wide hibernation
> +	 * or suspend transision begins right now.
> +	 */
> +	current->flags |= PF_NOFREEZE;

s/transision/transition
 
>  	set_special_pids(1, 1);
>  	proc_clear_tty(current);
> Index: linux-2.6.22-rc3/include/linux/freezer.h
> ===================================================================
> --- linux-2.6.22-rc3.orig/include/linux/freezer.h
> +++ linux-2.6.22-rc3/include/linux/freezer.h
> @@ -118,6 +118,14 @@ static inline int freezer_should_skip(st
>  	return !!(p->flags & PF_FREEZER_SKIP);
>  }
>  
> +/*
> + * Tell the freezer that the current task should be frozen by it
> + */
> +static inline void set_freezable(void)
> +{
> +	current->flags &= ~PF_NOFREEZE;
> +}
> +

Given the clearing of the flag above, should we just have a
set_unfreezeable here that's used above (and potentially elsewhere)...
(reads more)... or more generic set_[un]freezeable(task_struct *p)
routines that could also be used in copy_flags below?

>  #else
>  static inline int frozen(struct task_struct *p) { return 0; }
>  static inline int freezing(struct task_struct *p) { return 0; }
> @@ -134,6 +142,7 @@ static inline int try_to_freeze(void) { 
>  static inline void freezer_do_not_count(void) {}
>  static inline void freezer_count(void) {}
>  static inline int freezer_should_skip(struct task_struct *p) { return 0; }
> +static inline void set_freezable_current(void) {}
>  #endif
>  
>  #endif /* LINUX_FREEZER_H */
> Index: linux-2.6.22-rc3/kernel/fork.c
> ===================================================================
> --- linux-2.6.22-rc3.orig/kernel/fork.c
> +++ linux-2.6.22-rc3/kernel/fork.c
> @@ -920,7 +920,7 @@ static inline void copy_flags(unsigned l
>  {
>  	unsigned long new_flags = p->flags;
>  
> -	new_flags &= ~(PF_SUPERPRIV | PF_NOFREEZE);
> +	new_flags &= ~PF_SUPERPRIV;
>  	new_flags |= PF_FORKNOEXEC;
>  	if (!(clone_flags & CLONE_PTRACE))
>  		p->ptrace = 0;
> Index: linux-2.6.22-rc3/arch/i386/kernel/io_apic.c
> ===================================================================
> --- linux-2.6.22-rc3.orig/arch/i386/kernel/io_apic.c
> +++ linux-2.6.22-rc3/arch/i386/kernel/io_apic.c
> @@ -669,7 +669,6 @@ static int balanced_irq(void *unused)
>  
>  	for ( ; ; ) {
>  		time_remaining = schedule_timeout_interruptible(time_remaining);
> -		try_to_freeze();
>  		if (time_after(jiffies,
>  				prev_balance_time+balanced_irq_interval)) {
>  			preempt_disable();

I'm the one who is confused, aren't I? If I'm reading this right,
io_apic used to be frozen. After this patch, it will not be frozen. If
that's the intended behaviour, shouldn't this be two patches - one to
make kernel threads unfreezeable by default, and one to make threads
that were formerly freezeable unfreezeable?

[...]

> Index: linux-2.6.22-rc3/Documentation/power/swsusp.txt
> ===================================================================
> --- linux-2.6.22-rc3.orig/Documentation/power/swsusp.txt
> +++ linux-2.6.22-rc3/Documentation/power/swsusp.txt
> @@ -140,22 +140,6 @@ should be sent to the mailing list avail
>  website, and not to the Linux Kernel Mailing List. We are working
>  toward merging suspend2 into the mainline kernel.
>  
> -Q: A kernel thread must voluntarily freeze itself (call 'refrigerator').
> -I found some kernel threads that don't do it, and they don't freeze
> -so the system can't sleep. Is this a known behavior?
> -
> -A: All such kernel threads need to be fixed, one by one. Select the
> -place where the thread is safe to be frozen (no kernel semaphores
> -should be held at that point and it must be safe to sleep there), and
> -add:
> -
> -       try_to_freeze();
> -
> -If the thread is needed for writing the image to storage, you should
> -instead set the PF_NOFREEZE process flag when creating the thread (and
> -be very careful).
> -
> -
>  Q: What is the difference between "platform" and "shutdown"?
>  
>  A:

Perhaps it would be good to keep a variant of this question, along the
lines of:

Q: I have a kernel thread that needs to be frozen during hibernation.
How do I make that happen?

Regards,

Nigel

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