linux-kernel - Re: [linux-pm] Re: Hibernation considerations

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Message-Id: <200707240010.35639.rjw@sisk.pl>
Date:	Tue, 24 Jul 2007 00:10:34 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Nigel Cunningham <nigel@...el.suspend2.net>
Cc:	Alan Stern <stern@...land.harvard.edu>, nigel@...pend2.net,
	Paul Mackerras <paulus@...ba.org>, david@...g.hm,
	Miklos Szeredi <miklos@...redi.hu>,
	linux-kernel@...r.kernel.org, miltonm@....com,
	ying.huang@...el.com, linux-pm@...ts.linux-foundation.org,
	Jeremy Maitin-Shepard <jbms@....edu>
Subject: Re: [linux-pm] Re: Hibernation considerations

On Monday, 23 July 2007 23:55, Nigel Cunningham wrote:
> Hi.
> 
> On Tuesday 24 July 2007 01:23:15 Alan Stern wrote:
> > On Mon, 23 Jul 2007, Nigel Cunningham wrote:
> > 
> > > Take a step back for a second.
> > > 
> > > The problem we're facing now is that we're getting some userspace threads, 
> > > used in processing I/O, that are functioning as exceptions to the "freeze 
> > > userspace, then freezeable kernel threads" rule. They are only exceptions 
> > > because of that role in processing I/O - because they're de facto kernel 
> > > threads. So, if we orient our thinking more in terms of I/O processing and 
> > > less in terms of the userspace/kernelspace distinction, we'll have a 
> > > solution:
> > > 
> > > 1) Freeze processes that aren't fs related (ie stop them generating I/O).
> > 
> > The problem here is that with things like FUSE, _every_ process is 
> > potentially fs related.  Nothing prevents a FUSE thread from doing IPC 
> > with any other thread.
> 
> Yes, but the fuse thread is going to know what other thread it's doing IPC 
> with, so it can get that thread flagged too.

Yes, but that thread may do IPC with yet another one and so on.

> > > 2) Flush pending I/O.
> > > 3) Freeze filesystems in reverse order of dependency, the primary purpose 
> > > being to stop them generating further I/O on their metadata.
> > > 
> > > Locks that are being held are only being held because work is being done. 
> If 
> > > we progressively focus on threads in terms of their create/process work 
> > > dependencies, we'll see that the problem isn't at all intractable.
> > 
> > As has been mentioned before, keeping track of all that dependency 
> > information would be very fragile and time-consuming.
> 
> I disagree. It's at least going to be less fragile and time-consuming then 
> maintaining new/extra code for kexec.

Well, I think the issue is real, so we need to find a solution (the simpler,
the better) and that need not be related to kexec. ;-)

Greetings,
Rafael


-- 
"Premature optimization is the root of all evil." - Donald Knuth
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