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

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Message-Id: <200704071133.42269.rjw@sisk.pl>
Date:	Sat, 7 Apr 2007 11:33:40 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	nigel@...el.suspend2.net
Cc:	Pavel Machek <pavel@....cz>, 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 Saturday, 7 April 2007 00:20, Nigel Cunningham wrote:
> Hi.
> 
> On Fri, 2007-04-06 at 16:34 +0200, Rafael J. Wysocki wrote:
> > On Monday, 2 April 2007 22:51, Pavel Machek wrote:
> > > Hi!
> > > 
> > > > > > +/* 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.
> > > 
> > > (Actually it would be nice to say
> > > 
> > > /* This thread should not be frozen for suspend, becuase it is needed
> > >    for getting image saved to disk */
> > > 
> > > > > > -#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.
> > > 
> > > Kconfig can do that. ("select statement"). If we have one such ifdef,
> > > it is okay, but if it would be more of them.
> > > 
> > > > > 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.
> > > 
> > > Agreed about separate patch series.
> > > 
> > > > > > -	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. :-)
> > > 
> > > Can you put big bold comment there?
> > >
> > > Why is it needed? Freezer never freezes _current_ task.
> > 
> > No, it doesn't, but this task spawns linuxrc and then calls sys_wait4() in a
> > loop.
> > 
> > Well, actually, I'll try to plant try_to_freeze() in this loop and see if that
> > works.  If it doesn't, I'll add a comment.
> 
> It works. I've had:
> 
>                 while (pid != sys_wait4(-1, NULL, 0, NULL)) {
>                         yield();
>                         try_to_freeze();
>                 }
> 
> there for ages for Suspend2.

OK, thanks.  Is there any particular reason to place try_to_freeze() after
yield()?

Greetings,
Rafael
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