Re: [PATCH 0/3] PM, vfs: use filesystem freezing instead of kthread freezer

From: Jiri Kosina
Date: Sat Oct 31 2015 - 04:19:59 EST

On Sat, 31 Oct 2015, Rafael J. Wysocki wrote:

> > > > > > I would say instead "no I/O is allowed from now on". Maybe that's an
> > > > > > overstatement, but I think it comes closer to the truth.
> > > >
> > > > But that's what PM callbacks are for.
> Not really. In fact, PM callbacks may not be suitable for some use cases even
> in theory (that's if you want something to stop running before any PM callbacks
> are executed during suspend).

If that's the case, wouldn't then it be better to have .pre_suspend
callback as well, instead of having to do this kind of polling?

> > > The most natural implementation would be for the callback routine to set
> > > a flag; at various strategic points the kthread would check the flag and
> > > if it was set, call a routine that sits around and waits for the suspend
> > > to be over.
> >
> > Could you name at least some existing kthreads that would actually *need*
> > such complex handling, instead of just waiting in schedule() until
> > suspend-resume cycle is over, given that PM callbacks do all the necessary
> > cleanup (putting HW to sleep, cancelling timers, etc) anyway?
> >
> > PM callback can always explicitly do kthread_stop() on a particular
> > kthread if really necessary.
> Runtime PM uses a freezable workqueue, allocated in pm_start_workqueue().

Yeah, agreed, as I said in the original mail, freezable workqueues I am
not covering for the moment, there might be a additional issues with them
compared to kthreads. That needs to be further looked into.

> > The main point of freezer is to reach quiescent state wrt. filesystems
> > (metadata in memory need to be absolutely in sync with what's on disk).
> > That's no different between hibernation and s2ram, is it?
> >
> > BTW, a quite some of this has been already "pre-discussed" in
> > Documentation/power/freezing-of-tasks.txt (which has BTW been written
> > before we've had the possibility to freeze filesystems, and this fact is
> > even point there out).
> That is somewhat outdated in my view. At least my list of reasons for using
> the freezer is now somewhat different from the one given in that file.

Well, so what is the list then, and how do you know that kthread_run()
users are behaving according to that list? That's basically exactly what
this effort is about -- making some sense out of current situation.


Jiri Kosina

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