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Re: [Xen-devel] Detecting deadlocks with hypervisor..



From: Anthony Liguori <aliguori@xxxxxxxxxx>
To: T S <thileepan_@xxxxxxxxxxx>
CC: xen-devel@xxxxxxxxxxxxxxxxxxx
Subject: Re: [Xen-devel] Detecting deadlocks with hypervisor..
Date: Fri, 24 Mar 2006 13:24:46 -0600

T S wrote:
This may sound a silly question (pardon me because i am relatively new to linux kernel) .. will it be possible to continue running reboot.c (or for that matter any kernel thread) when the kernel is deadlocked ? In Linux, is the kernel a single process or a bunch of parallelly executing entities? If later, then during a kernel deadlock (eg: by loading a faulty module that disables interrupts and do something silly) there can still be some other processes/threads run, right?

Sorry for not making this more clear previously. You cannot restore a dead-locked domain if a normal xm save doesn't work. One thing that makes Xen unique is that guests actually are aware of what physical pages are assigned to them. When one does a save/restore, the guest has to canonicalize all of it's internal references to physical pages. When it's restored, it then remaps it's newly assigned physical pages to all the old places where it needed to know about them for some reason or another.

Thank you for the reply. Do you mean to say that the canonicalize..() functions in the xc_linux_save.c are actually invoked in the guest OS' context?

If the guest isn't responsive when you do a save, then it will never canonicalize itself and there is no way to restore the domain.

Regards,

Anthony Liguori

thanks
TS


If a suspend completes correctly, Xend will see it (another watch will fire),
and xc_linux_save will be free to complete the save.

> Also, does it seem viable to clone a copy of a deadlocked guest OS in the
> first place?

If you have a byte-for-byte copy of a deadlocked guest, even if you could
suspend it, surely it will be deadlocked when it is resumed.  How do you
intend to break the deadlock, and how is it easier to do that from outside
than it is to perform deadlock detection in the guest?

Ewan.


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