linux-kernel - Re: [PATCH 1/8] Enhance process freezer interface for usage beyond software suspend

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Message-Id: <200704022248.25948.rjw@sisk.pl>
Date:	Mon, 2 Apr 2007 22:48:24 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Pavel Machek <pavel@....cz>
Cc:	Gautham R Shenoy <ego@...ibm.com>, akpm@...ux-foundation.org,
	paulmck@...ibm.com, torvalds@...ux-foundation.org,
	linux-kernel@...r.kernel.org, vatsa@...ibm.com,
	Oleg Nesterov <oleg@...sign.ru>, mingo@...e.hu,
	dipankar@...ibm.com, dino@...ibm.com,
	masami.hiramatsu.pt@...achi.com
Subject: Re: [PATCH 1/8] Enhance process freezer interface for usage beyond software suspend

On Monday, 2 April 2007 15:56, Pavel Machek wrote:
> Hi!
> 
> >  This patch provides an interface to extend the use of the process
> >  freezer beyond Suspend.
> > 
> > The tasks can selectively mark themselves to be exempted from specific
> > freeze events like SUSPEND /KPROBES/CPU_HOTPLUG.
> > 
> > This patch however, *does not* sort non freezable threads into
> > different categories based on the freeze events. Thus all 
> > tasks which were previously marked PF_NOFREEZE are now
> > exempted from freezer using 
> > 	freezer_exempt(FE_ALL);
> > which means exempt from all kinds of freezes.
> > 
> > Signed-off-by: Gautham R Shenoy <ego@...ibm.com>
> > Cc: Pavel Machek <pavel@....cz>
> > Cc: Rafael J. Wysocki <rjw@...k.pl>
> > Cc: Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
> 
> Actually no, I was not in cc. 
> 
> > +/* Per process freezer specific flags */
> > +#define PF_FE_SUSPEND	0x00008000	/* This thread should not be frozen
> > +					 * for suspend
> > +					 */
> > +
> > +#define PF_FE_KPROBES	0x00000010	/* This thread should not be frozen
> > +					 * for Kprobes
> > +					 */
> 
> Just put the comment before the define for long comments?

Agreed.

> > -#ifdef CONFIG_PM
> > +#if defined(CONFIG_PM) || defined(CONFIG_HOTPLUG_CPU) || \
> > +					defined(CONFIG_KPROBES)
> 
> Should we create CONFIG_FREEZER?

Why do you think so?  I think the freezer should be compiled automatically
if any of the above is set, which is what this directive really means.

> > Index: linux-2.6.21-rc5/kernel/softlockup.c
> > ===================================================================
> > --- linux-2.6.21-rc5.orig/kernel/softlockup.c
> > +++ linux-2.6.21-rc5/kernel/softlockup.c
> > @@ -96,7 +96,7 @@ static int watchdog(void * __bind_cpu)
> >  	struct sched_param param = { .sched_priority = MAX_RT_PRIO-1 };
> >  
> >  	sched_setscheduler(current, SCHED_FIFO, &param);
> > -	current->flags |= PF_NOFREEZE;
> > +	freezer_exempt(FE_ALL);
> >  
> >  	/*
> >  	 * Run briefly once per second to reset the softlockup timestamp.
> 
> Hmmm, I do not really like softlockup watchdog running during suspend.
> Can we make this freezeable and make watchdog shut itself off while
> suspending?

Generally, I agree, but this patch only replaces the existing instances
of PF_NOFREEZE with the new mechanism.  The changes you're talking about
require a separate patch series (or at least one separate patch), I think, and
they need not be so simple to make.

> > Index: linux-2.6.21-rc5/kernel/rcutorture.c
> > ===================================================================
> > --- linux-2.6.21-rc5.orig/kernel/rcutorture.c
> > +++ linux-2.6.21-rc5/kernel/rcutorture.c
> > @@ -559,7 +559,7 @@ rcu_torture_fakewriter(void *arg)
> >  
> >  	VERBOSE_PRINTK_STRING("rcu_torture_fakewriter task started");
> >  	set_user_nice(current, 19);
> > -	current->flags |= PF_NOFREEZE;
> > +	freezer_exempt(FE_ALL);
> 
> 
> Fix rcutorture instead. It has no business running while suspending.
> 
> >  
> >  	do {
> >  		schedule_timeout_uninterruptible(1 + rcu_random(&rand)%10);
> > @@ -590,7 +590,7 @@ rcu_torture_reader(void *arg)
> >  
> >  	VERBOSE_PRINTK_STRING("rcu_torture_reader task started");
> >  	set_user_nice(current, 19);
> > -	current->flags |= PF_NOFREEZE;
> > +	freezer_exempt(FE_ALL);
> >  
> 
> Same here.
> 
> Eventually, we should fix apm, too.
> 
> > Index: linux-2.6.21-rc5/init/do_mounts_initrd.c
> > ===================================================================
> > --- linux-2.6.21-rc5.orig/init/do_mounts_initrd.c
> > +++ linux-2.6.21-rc5/init/do_mounts_initrd.c
> > @@ -55,7 +55,7 @@ static void __init handle_initrd(void)
> >  	sys_mount(".", "/", NULL, MS_MOVE, NULL);
> >  	sys_chroot(".");
> >  
> > -	current->flags |= PF_NOFREEZE;
> > +	freezer_exempt(FE_ALL);
> >  	pid = kernel_thread(do_linuxrc, "/linuxrc", SIGCHLD);
> >  	if (pid > 0) {
> >  		while (pid != sys_wait4(-1, NULL, 0, NULL))
> 
> Does this mean we have userland /linuxrc running with PF_NOFREEZE?
> That would be very bad...

No, actually it is _required_ for the userland resume to work.  Well, perhaps
I should place a comment in there so that I don't have to explain this again
and again. :-)

> > --- linux-2.6.21-rc5.orig/kernel/kprobes.c
> > +++ linux-2.6.21-rc5/kernel/kprobes.c
> > @@ -104,7 +104,7 @@ static int __kprobes check_safety(void)
> >  {
> >  	int ret = 0;
> >  #if defined(CONFIG_PREEMPT) && defined(CONFIG_PM)
> 
> Eh? Why does kprobes code depend on config_pm?

Because it uses the freezer? ;-)

Greetings,
Rafael
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