Date: Wed, 4 Aug 2010 09:32:16 -0700
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Arjan van de Ven <arjan@infradead.org>
Cc: Arve =?iso-8859-1?B?SGr4bm5lduVn?= <arve@android.com>,
        linux-pm@lists.linux-foundation.org, linux-kernel@vger.kernel.org,
        mjg59@srcf.ucam.org, pavel@ucw.cz, florian@mickler.org, rjw@sisk.pl,
        stern@rowland.harvard.edu, swetland@google.com, peterz@infradead.org,
        tglx@linutronix.de, alan@lxorguk.ukuu.org.uk
Subject: Re: Attempted summary of suspend-blockers LKML thread
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On Tue, Aug 03, 2010 at 06:34:47PM -0700, Arjan van de Ven wrote:
> On Mon, 2 Aug 2010 21:56:10 -0700
> Arve Hj?nnev?g <arve@android.com> wrote:
> 
> > > non-obvious dependencies.
> > >
> > 
> > The dependencies is what made this solution uninteresting to us. For
> > instance, we currently use cgroup scheduling to reduce the impact of
> > some background tasks, but we occasionally saw a watchdog restart of
> > the system process were critical services were waiting on a kernel
> > mutex owned by a background task for more than 20 seconds. If we froze
> > a cgroup instead, we would not hit this particular problem since tasks
> > cannot be frozen while executing kernel code the same way they can be
> > preempted, but nothing prevents a task from being frozen while holding
> > a user-space resource.
> 
> one of the solutions we're looking at to solve this is to unfreeze the
> cgroup on a regular basis (say, every 15 to 30 seconds) briefly to avoid
> this kind of deadlock...

Cute!!!  ;-)

If this doesn't work for the Android folks for whatever reason, another
approach would be to do the freeze in user code, which could track
whether any user-level resources (pthread mutexes, SysV semas, whatever)
where held, and do the freeze on a thread-by-thread basis within each
"victim" application as the threads reach safe points.

I must of course defer to the Android folks as to whether or not this
would actually work for Android.  But if your approach works for them,
why worry?

							Thanx, Paul
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