Messages in this thread |  | | | From | "Rafael J. Wysocki" <> | | Subject | Re: peculiar suspend/resume bug. | | Date | Thu, 17 Aug 2006 08:30:48 +0200 |
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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
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