Re: [PATCH v2] powernv: kvm: make _PAGE_NUMA take effect

[Alexander Graf <agraf@suse.de>]

On 10.04.14 05:28, Liu ping fan wrote:
> On Mon, Apr 7, 2014 at 4:36 PM, Alexander Graf <agraf@suse.de> wrote:
>> On 07.04.14 09:42, Aneesh Kumar K.V wrote:
>>> Alexander Graf <agraf@suse.com> writes:
>>>
>>>> On 03.04.14 04:36, Liu ping fan wrote:
>>>>> Hi Alex, could you help to pick up this patch? since  v3.14 kernel can
>>>>> enable numa fault for powerpc.
>>>> What bad happens without this patch? We map a page even though it was
>>>> declared to get NUMA migrated? What happens next?
>>> Nothing much, we won't be properly accounting the numa access in the
>>> host.  What we want to achieve is to convert a guest access of the page to
>>> a host fault so that we can do proper numa access accounting in the
>>> host. This would enable us to migrate the page to the correct numa
>>> node.
>>
>> Ok, so no breakages, just less performance. I wouldn't consider it stable
>> material then :).
>>
> Sorry to reply late, since I am half out of office during this period.
> I am puzzling about you reply.    Without this patch, the guest can
> NOT sense the numa changes and ask host to put the pages in right
> place.  So the pages which is used by guest will be always misplaced.
> The numa-fault method is inspired by real requirement to improve
> performance, so we should also consider the performance drop of guest.
> Right?

The patch will get into Linux, I just consider a non-working new feature 
not a regression that warrants us to CC stable@vger :). After all 
performance shouldn't be worse than without the numa migration feature, 
correct?


Alex