Re: [Qemu-devel] [PATCH] set VIRTIO_BALLOON_F_MUST_TELL_HOST unconditionally

[Dave Hansen <dave@linux.vnet.ibm.com>]

On Fri, 2011-04-15 at 14:20 -0500, Anthony Liguori wrote: 
> On 04/15/2011 02:15 PM, Dave Hansen wrote:
> > On Fri, 2011-04-15 at 12:17 -0500, Anthony Liguori wrote:
> >> If you're in OOM and you need memory, you can't ask the host for more
> >> and wait for a response.  You have to reclaim it immediately.
> > Why not?  The call in to the notifier chain the s390 case is
> > synchronous.  The OOM only affects one task at a time and won't proceed
> > elsewhere while this is going on.
> 
> Because if we tell the host, we have to wait for the host to ack which 
> means we'd sleep waiting for an interrupt.  Can you do this in the OOM path?

Sure.  One of the s390 handlers sleeps today.

> > Why do we even _tell_ qemu, though?  The MADV_WILLNEED is nice, but far
> > from being necessary.  We could just skip the entire notification in OOM
> > situations.
> 
> There's really no particular reason to other than symmetry.

When we get there, it sounds like we can simply _skip_ the notification
in the qemu case.  

-- Dave