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

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Message-ID: <alpine.LNX.2.00.1510310844360.17538@pobox.suse.cz>
Date:	Sat, 31 Oct 2015 09:19:33 +0100 (CET)
From:	Jiri Kosina <jikos@...nel.org>
To:	"Rafael J. Wysocki" <rjw@...ysocki.net>
cc:	Alan Stern <stern@...land.harvard.edu>,
	Pavel Machek <pavel@....cz>,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
	Dave Chinner <david@...morbit.com>, Jan Kara <jack@...e.cz>,
	Christoph Hellwig <hch@....de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Al Viro <viro@...iv.linux.org.uk>, Tejun Heo <tj@...nel.org>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-pm@...r.kernel.org
Subject: Re: [PATCH 0/3] PM, vfs: use filesystem freezing instead of kthread
 freezer

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.

Thanks,

-- 
Jiri Kosina
SUSE Labs

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