linux-kernel - Re: Fw: Re: [mm PATCH 4/6] RCU: (now) CPU hotplug

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Message-Id: <200702041446.18482.rjw@sisk.pl>
Date:	Sun, 4 Feb 2007 14:46:17 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Pavel Machek <pavel@....cz>
Cc:	paulmck@...ux.vnet.ibm.com, Andrew Morton <akpm@...l.org>,
	Ingo Molnar <mingo@...e.hu>, dipankar@...ibm.com,
	Gautham Shenoy <ego@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org
Subject: Re: Fw: Re: [mm PATCH 4/6] RCU: (now) CPU hotplug

Hi,

On Sunday, 4 February 2007 13:53, Pavel Machek wrote:
> Hi!
> 
> > > > > o	init/do_mounts_initrd.c line 57 handle_initrd().
> > > > > 	This looks to be short term anyway, so OK to leave.
> > > > > 	But does kernel_execve() clear PF_NOFREEZE?
> > > > > 
> > > > > 	But it should be OK to freeze the init process when doing CPU
> > > > > 	hotplug ops, right?
> > > > 
> > > > That looks bogus. If it is short term, it can as well live _without_
> > > > PF_NOFREEZE. Noone should suspend system at that stage, right?
> > >
> > > I agree that any attempt to freeze that early in boot would be
> > > at best an act of extreme bravery!
> > 
> > This is needed so that the _resume_ works, when it's handled from the user land
> > by our resume tool.  Currently, the resume code calls
> > freeze_processes() too.
> 
> I do not understand... freeze_processes() always leaves curent process
> running... why is it needed?

IIRC, the do_linuxrc thread cannot be frozen (doesn't call try_to_freeze()),
so the freeze_processes() during the resume fails and the resume fails as a
result.

Still, I have an idea:

Instead of hunting for PF_NOFREEZE and wondering if the suspend/resume fails
when we remove them or replace them with try_to_freeze(), why don't we add
an "ignore_pf_nofreeze" argument to freeze_processes() and make it regard
_all_ tasks as if they haven't set PF_NOFREEZE when this "ignore_pf_nofreeze"
is set?  Of course, additionally we'll have to make everyone call
try_to_freeze(), even if they set PF_NOFREEZE anyway.

Then, if freeze_processes() is called with "ignore_pf_nofreeze = 0", it will
work just as it does now.  However, if it's called with
"ignore_pf_nofreeze = 1", it will try to make all prcesses enter the
refrigerator.  The "ignore_pf_nofreeze = 0" version will be suitable for us
(ie. suspend etc.) and the "ignore_pf_nofreeze = 1" version will be suitable
for the CPU hotplug and such things.

You think?

Rafael
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