Re: [PATCH 1/5] mm: Add support for unaccepted memory

Dave Hansen
2021-08-10T19:46:37Z

On 8/10/21 12:23 PM, Andi Kleen wrote:
>> But, not everyone is going to agree with me.
>
> Both the Intel TDX and the AMD SEV side independently came to opposite
> conclusions. In general people care a lot about boot time of VM guests.

I was also saying that getting to userspace fast is important to me.
Almost everyone agrees there.

>> This also begs the question of how folks know when this "blip" is over.
>>   Do we have a counter for offline pages?  Is there any way to force page
>> acceptance?  Or, are we just stuck allocating a bunch of memory to warm
>> up the system?
>>
>> How do folks who care about these new blips avoid them?
>
> It's not different than any other warmup. At warmup time you always have
> lots of blips until the working set stabilizes. For example in
> virtualization first touch of a new page is usually an EPT violation
> handled to the host. Or in the native case you may need to do IO or free
> memory. Everybody who based their critical latency percentiles around a
> warming up process would be foolish, the picture would be completely
> distorted.
>
> So the basic operation is adding some overhead, but I don't think
> anything is that unusual compared to the state of the art.

Except that today, you can totally avoid the allocation latency (not
sure about the EPT violation/fill latency) from things like QEMU's
-mem-prealloc.

> Now perhaps the locking might be a problem if the other operations all
> run in parallel, causing unnecessary serialization If that's really a
> problem I guess we can optimize later. I don't think there's anything
> fundamental about the current locking.

These boot blips are not the biggest issue in the world. But, it is
fully under the guest's control and I think the guest has some
responsibility to provide *some* mitigation for it.

1. Do background acceptance, as opposed to relying 100% on demand-driven
acceptance. Guarantees a limited window in which blips can occur.
2. Do acceptance based on user input, like from sysfs.
3. Add a command-line argument to accept everything up front, or at
least before userspace runs.
4. Add some statistic for how much unaccepted memory remains.

I can think of at least four ways we could mitigate it. A sysfs
statistic file would probably take ~30 lines of code to loop over the
bitmap. A command-line option would probably be <10 lines of code to
just short-circuit the bitmap and accept everything up front. A file to
force acceptance would probably be pretty quick too.

Nothing there seem too onerous.