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

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Sat, Feb 03, 2007 at 01:17:45AM +0100, Pavel Machek wrote:
> Hi!
> 
> > > Part of what I need to look at.  ;-)
> > 
> > OK.  This just might be feasible.  That said, there is a lot of code
> > containing PF_NOFREEZE that I am not familiar with.  That said, here
> > are my thoughts -- this is in addition to the changes to freeze_processes()
> > and thaw_processes() called out earlier.
> > 
> > Thoughts?
> 
> Looks ok to me.

Cool!

> > o	Introduce a mutex to prevent overlapping freezes -- or find
> > 	out what the heck prevents them at present!!!  (I don't see
> > 	anything.)  
> 
> swsusp is protected by some giant "doing suspend" mutex. Other users
> may be buggy :-).

Ah!  Any reason not to have locking at the level of the
freeze_processes()/thaw_processes() functions?

> > o	Replace all the "current->flags |= PF_NOFREEZE" statements with
> > 	"exempt_from_freeze(current, int pfe)" or some such.  This would
> > 	set the flags bit and also store the pfe argument into the pf_exempt
> > 	field.
> 
> I'd suggest step 0, remove as many PF_NOFREEZE as possible... ok, you
> seem to be doing that one.

Well, in my little corner of the kernel, anyway.  ;-)

> > 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!

> > o	kernel/softlockup.c line 88 watchdog().  Well, we wouldn't
> > 	want false alarms when freezing for hotplug.  Perhaps
> > 	temporarily disabling timestamp checking while doing hotplug
> > 	would do the trick.  But if hotplug takes the time required
> > 	to trigger softlockup (seconds!), we are broken anyway.
> > 	The fix would be to speed up the freezing process.
> 
> Freezing _can_ take seconds. We do sync in between freezing userspace
> and kernel, for example. We avoid freezing in some difficult situations
> by waiting for I/O to complete....

OK.  Point taken.

> > o	net/bluetooth/bnep/core.c line 476 bnep_session().  Suspending
> > 	to a bluetooth device???  These guys got -hair-!!!  I bet this
> > 	one can tolerate being frozen for hotplugging CPUs -- though
> > 	I could imagine the bluetooth protocol needing some TLC after
> > 	such an event.  But I don't know enough about bluetooth to do
> > 	more than raise the possibility.
> 
> Should be fixed. Someone was probably lazy.
> 
> > o	net/bluetooth/cmtp/core.c line 290 cmtp_session().  Same as
> > 	for bnep_session(), at least as far as I can tell.
> > 
> > o	net/bluetooth/hidp/core.c line 476 hidp_session().  Same as
> > 	for bnep_session(), AFAICT.
> > 
> > o	net/bluetooth/rfcomm/core.c line 1940 rfcomm_run(). Same as
> > 	for bnep_session(), AFAICT.
> 
> Someone was definitely lazy :-).
> 								Pavel

OK, so we should think in terms of moving these to try_to_freeze(),
then.

						Thanx, Paul