Re: [Xen-devel] [PATCH 1 of 4] Improve ring management for memory events

[Olaf Hering <olaf@xxxxxxxxx>]

On Thu, Nov 24, Andres Lagar-Cavilla wrote:

> > On Wed, Nov 23, Andres Lagar-Cavilla wrote:
> >
> >> Well, we can tone down printk's to be debug level. I don't think they're
> >> unnecessary if they're made an optional debug tool.
> >
> > There is nothing to debug here, since the callers have to retry anyway.
> >
> >> Question: I have one vcpu, how do I fill up the ring quickly? (outside
> >> of
> >> foreign mappings)
> >
> > Have a balloon driver in the guest and balloon down more than
> > 64*PAGE_SIZE. This is the default at least in my setup where the kernel
> > driver releases some memory right away (I havent checked where this is
> > actually configured).
> 
> I see, a guest can call decrease_reservation with an extent_order large
> enough that it will overflow the ring. No matter the size of the ring.
> Isn't preemption of this hypercall a better tactic than putting the vcpu
> on a wait-queue? This won't preclude the need for wait queues, but it
> feels like a much cleaner solution.

Yes, yesterday I was thinking about this as well.
p2m_mem_paging_drop_page() should return -EBUSY. But currently not all
callers of guest_remove_page() look at the exit code. Perhaps that can
be fixed.

> With retrying of foreign mappings in xc_map_foreign_bulk (and grants), I
> wonder if we should put events in the ring due to foreign mappings *at
> all*, in the case of congestion. Eventually a retry will get to kick the
> pager.

What do you mean with that?

Olaf

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