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

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Date:	Sun, 4 Feb 2007 14:59:48 +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

On Sunday, 4 February 2007 14:50, Pavel Machek wrote:
> Hi!
> 
> > > > 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.
> 
> Aha, ok. (We still may want to add try_to_freeze there; there's no
> reason to have that running while resuming).
> 
> > 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.
> 
> Yep, something like that will be needed. Probably more finegrained
> (with flags), because CPU hotplug and kprobes may want their own sets
> of unfreezeable processes.

Yes, but for this purpose we'll need to use one more task flag at least, I
think.  Or transform PF_NOFREEZE into a 2-bit field with values from 0 to 3
indicating the "freezing level" at which given task should be frozen and
call freeze_processes() with the level as an argument?

Paul, what is your opnion?

Rafael
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