Re: [Xen-devel] [Memory Accounting] was: Re: PVH dom0 creation fails - the system freezes

[Juergen Gross <jgross@xxxxxxxx>]

On 26/07/18 13:11, Roger Pau Monné wrote:
> On Thu, Jul 26, 2018 at 10:45:08AM +0100, George Dunlap wrote:
>> On Thu, Jul 26, 2018 at 12:07 AM, Boris Ostrovsky
>> <boris.ostrovsky@xxxxxxxxxx> wrote:
>>> On 07/25/2018 02:56 PM, Andrew Cooper wrote:
>>>> On 25/07/18 17:29, Juergen Gross wrote:
>>>>> On 25/07/18 18:12, Roger Pau Monné wrote:
>>>>>> On Wed, Jul 25, 2018 at 05:05:35PM +0300, bercarug@xxxxxxxxxx wrote:
>>>>>>> On 07/25/2018 05:02 PM, Wei Liu wrote:
>>>>>>>> On Wed, Jul 25, 2018 at 03:41:11PM +0200, Juergen Gross wrote:
>>>>>>>>> On 25/07/18 15:35, Roger Pau Monné wrote:
>>>>>>>>>>> What could be causing the available memory loss problem?
>>>>>>>>>> That seems to be Linux aggressively ballooning out memory, you go 
>>>>>>>>>> from
>>>>>>>>>> 7129M total memory to 246M. Are you creating a lot of domains?
>>>>>>>>> This might be related to the tools thinking dom0 is a PV domain.
>>>>>>>> Good point.
>>>>>>>>
>>>>>>>> In that case, xenstore-ls -fp would also be useful. The output should
>>>>>>>> show the balloon target for Dom0.
>>>>>>>>
>>>>>>>> You can also try to set the autoballoon to off in /etc/xen/xl.cfg to 
>>>>>>>> see
>>>>>>>> if it makes any difference.
>>>>>>>>
>>>>>>>> Wei.
>>>>>>> Also tried setting autoballooning off, but it had no effect.
>>>>>> This is a Linux/libxl issue that I'm not sure what's the best way to
>>>>>> solve. Linux has the following 'workaround' in the balloon driver:
>>>>>>
>>>>>> err = xenbus_scanf(XBT_NIL, "memory", "static-max", "%llu",
>>>>>>                &static_max);
>>>>>> if (err != 1)
>>>>>>     static_max = new_target;
>>>>>> else
>>>>>>     static_max >>= PAGE_SHIFT - 10;
>>>>>> target_diff = xen_pv_domain() ? 0
>>>>>>             : static_max - balloon_stats.target_pages;
>>>>> Hmm, shouldn't PVH behave the same way as PV here? I don't think
>>>>> there is memory missing for PVH, opposed to HVM's firmware memory.
>>>>>
>>>>> Adding Boris for a second opinion.
>>>
>>> (Notwithstanding Andrews' rant below ;-))
>>>
>>> I am trying to remember --- what memory were we trying not to online for
>>> HVM here?
>>
>> My general memory of the situation is this:
>>
>> * Balloon drivers are told to reach a "target" value for max_pages.
>> * max_pages includes all memory assigned to the guest, including video
>> ram, "special" pages, ipxe ROMs, bios ROMs from passed-through
>> devices, and so on.
>> * Unfortunately, the balloon driver doesn't know what their max_pages
>> value is and can't read it.
>> * So what the balloon drivers do at the moment (as I understand it) is
>> look at the memory *reported as RAM*, and do a calculation:
>>   visible_ram - target_max_pages = pages_in_balloon
>>
>> You can probably see why this won't work -- the result is that the
>> guest balloons down to (target_max_pages + non_ram_pages).  This is
>> kind of messy for normal guests, but when you have a
>> populate-on-demand guest, that leaves non_ram_pages amount of PoD ram
>> in the guest.  The hypervisor then spends a huge amount of work
>> swapping the PoD pages around under the guest's feet, until it can't
>> find any more zeroed guest pages to use, and it crashes the guest.
>>
>> The kludge we have right now is to make up a number for HVM guests
>> which is slightly larger than non_ram_pages, and tell the guest to aim
>> for *that* instead.
>>
>> I think what we need is for the *toolstack* to calculate the size of
>> the balloon rather than the guest, and tell the balloon driver how big
>> to make its balloon, rather than the balloon driver trying to figure
>> that out on its own.
> 
> Maybe the best option would be for the toolstack to fetch the e820
> memory map and set the target based on the size of the RAM regions in
> there for PVH Dom0? That would certainly match the expectations of the
> guest.
> 
> Note that for DomUs if hvmloader (or any other component) inside of
> the guest changes the memory map it would also have to adjust the
> value in the xenstore 'target' node.

How would it do that later when the guest is already running?

I believe the right way would be to design a proper ballooning interface
suitable for all kinds of guests from scratch. This should include how
to deal with hotplug of memory or booting with mem < mem_max. Whether
PoD should be included should be discussed, too.

After defining that interface we can look after a proper way to select
the correct interface (old or new) in the gust and how to communicate
that selection to the host.


Juergen

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