Re: peculiar suspend/resume bug.

[Rafael J. Wysocki]

Hi,

On Thursday 17 August 2006 07:55, Nigel Cunningham wrote:
> Hi.
> 
> Thanks for the reply.
> 
> On Thu, 2006-08-17 at 07:44 +0200, Rafael J. Wysocki wrote:
> > 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. :-)
> 
> I think you'll find that the separate freezing of kernel space will
> help.

That certainly is possible, but will need some testing.

> We had SMP support in Suspend2 long before cpu hotplugging was 
> added, and it was stable and reliable. I'm reasonably certain that the
> switch to splitting freezing was pre-cpu hotplugging.
>
> > > 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.
> 
> Great. Sorry for my slowness. I just keep too many things on the go at
> once.
> 
> > > Alternatively, if it's viable, per-cpu kernel threads could perhaps be made
> > > NO_FREEZE. 
> > > 
> > > 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.
> 
> Mmm. My concern was that cpu hotplug might somehow deadlock if the
> process it was trying to migrate was frozen. You don't think that's a
> possibility?

No, I don't.  Of course it'll have to be tested anyway. :-)

> With affinities, would saving and restoring be a possibility?

I haven't thought about it yet.  Perhaps, but it will need to be done with
care.

Greetings,
Rafael
 

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