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

> 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 :-).

> 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.

> 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?

> 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....
> 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

--
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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