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Re: Xen balloon driver rewrite

On Thu, Apr 07, 2011 at 02:02:42PM +0100, Jean-Yves Migeon wrote:
> On Thu, 7 Apr 2011 12:34:00 +0200, Manuel Bouyer
> <bouyer%antioche.eu.org@localhost> wrote:
> >On Wed, Apr 06, 2011 at 10:21:11AM +0100, Jean-Yves Migeon wrote:
> >>Hi list,
> >>
> >>So, in an attempt to add most of the missing stuff in current Xen
> >>balloon driver, I ended up rewriting most of the logic behind. It's
> >>not yet finished, but really closed to it (FYI, I am attaching a
> >>patch). Only the workqueue part needs to be done, it is ~ one/two
> >>hours of coding, then testing. The balloon will be enabled by
> >>default for -6.
> >>
> >>The old design used a specific thread to queue balloon operations
> >>and handle inflating/deflating. The "new" driver will rather be
> >>workqueue(9) based, as it simplifies the locking and handling of
> >>errors from ballooning, especially for error feedback from
> >>balloon_thread. I will now simply log an error and terminate the
> >>worker.
> >
> >I'm not sure how this is easier than just logging an error
> >and going back to the thread's idle loop.
> It's still at paper level. ATM, I'm still not sure if workqueue(9)
> is necessary: at a locking level, both will check the same "target"
> variable. The only difference is that workqueue(9) will spawn a
> thread context when necessary, compared to a thread that stays
> "sleepy" most of the time (eventually with a timeout, but it rapidly
> gets amortized).
> The workqueue(9) has one advantage: I can simply return from it if
> it fails allocating memory, while for thread, I have to implement
> two levels of locking: one to protect ``target'' variable from being
> overwritten concurrently between feedback operation (in
> balloon_thread) and xenstore_watcher(), and one to wakeup
> balloon_thread.
> At a lower level though, I wonder what would happen with such a
> scenario with memoryalloactors(9):
> - suppose I want to inflate balloon, which will decrease domain's
> available memory. Eventually, it might start sleeping for certain
> allocations (see kmem_alloc() in reserve_pages() [1])
> - the balloon_thread() sleeps
> - now I want to deflate balloon, and give memory back to domain
> => how am I supposed to wake up the balloon_thread which is
> currently sleeping? are kmem_allocations interruptible when using

No, but you'll have the same problem with workqueues I guess:
you have only one worker active at a time; if it's sleeping other
requests will be queued.

Manuel Bouyer <bouyer%antioche.eu.org@localhost>
     NetBSD: 26 ans d'experience feront toujours la difference

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