Re: peculiar suspend/resume bug.

From: Rafael J. Wysocki
Date: Thu Aug 17 2006 - 01:38:14 EST

On Thursday 17 August 2006 03:44, Nigel Cunningham wrote:
> Hi.
> On Wed, 2006-08-16 at 03:41 +0100, Matthew Garrett wrote:
> > On Tue, Aug 15, 2006 at 08:37:28PM -0400, Dave Jones wrote:
> >
> > > cpufreq-applet crashes as soon as the cpu goes offline.
> > > Now, the applet should be written to deal with this scenario more
> > > gracefully, but I'm questioning whether or not userspace should
> > > *see* the unplug/replug that suspend does at all.
> >
> > As Nigel mentioned, cpu unplug happens just before processes are frozen,
> > so I guess there's a chance for it to be scheduled. On the other hand,
> > it's not unreasonable for CPUs to be unplugged during runtime anyway -
> > perhaps userspace should be able to deal with that?
> Agreed.
> I've spent a little more time thinking about this, and want to put a few
> thoughts forward for discussion/ignoring/flame bait/whatever.
> I see two main issues at the moment with freezing before hotplugging.
> The first is that we have cpu specific kernel threads that we're going
> to want to kill, and the second is that we have userspace threads that
> we want to migrate to another cpu. Have I missed anything?

I have bad memories from the time we were not using the CPU-hotplug and
tried to freeze tasks with all CPUs on-line. There were some very subtle
race conditions appearing between the freezer and the running tasks
which were a nightmare to figure out. I'm not sure that they will appear
now, but something tells me so. :-)

> The first issue could be helped by splitting the freezing of userspace
> processes from kernel space. The kernel threads could thus die without
> us having to worry about userspace seeing what's going on. I haven't
> looked at vanilla in a while; this might already be in.

Yes, it is.

> Alternatively, if it's viable, per-cpu kernel threads could perhaps be made
> The second issue is migrating userspace threads. I'm no scheduling
> expert, so I'll just speculate :>. I wondered if it's possible to make
> the migration happen lazily; in such a way that if, when we come to thaw
> userspace, the cpu has been hotplugged again, the migration never
> happens. Does that sound possible?

The CPU hotplug makes the tasks migrate automatically, but that's not
a problem, as I see it. The problem is some tasks may have specific CPU
affinities set and these should not change accross suspend/resume.


You never change things by fighting the existing reality.
R. Buckminster Fuller
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