Re: [PATCH 21/46] hugetlb: use struct hugetlb_pte for walk_hugetlb_range

[James Houghton]

On Thu, Jan 26, 2023 at 12:31 PM Peter Xu <peterx@xxxxxxxxxx> wrote:
>
> James,
>
> On Thu, Jan 26, 2023 at 08:58:51AM -0800, James Houghton wrote:
> > It turns out that the THP-like scheme significantly slows down
> > MADV_COLLAPSE: decrementing the mapcounts for the 4K subpages becomes
> > the vast majority of the time spent in MADV_COLLAPSE when collapsing
> > 1G mappings. It is doing 262k atomic decrements, so this makes sense.
> >
> > This is only really a problem because this is done between
> > mmu_notifier_invalidate_range_start() and
> > mmu_notifier_invalidate_range_end(), so KVM won't allow vCPUs to
> > access any of the 1G page while we're doing this (and it can take like
> > ~1 second for each 1G, at least on the x86 server I was testing on).
>
> Did you try to measure the time, or it's a quick observation from perf?

I put some ktime_get()s in.

>
> IIRC I used to measure some atomic ops, it is not as drastic as I thought.
> But maybe it depends on many things.
>
> I'm curious how the 1sec is provisioned between the procedures.  E.g., I
> would expect mmu_notifier_invalidate_range_start() to also take some time
> too as it should walk the smally mapped EPT pgtables.

Somehow this doesn't take all that long (only like 10-30ms when
collapsing from 4K -> 1G) compared to hugetlb_collapse().

>
> Since we'll still keep the intermediate levels around - from application
> POV, one other thing to remedy this is further shrink the size of COLLAPSE
> so potentially for a very large page we can start with building 2M layers.
> But then collapse will need to be run at least two rounds.

That's exactly what I thought to do. :) I realized, too, that this is
actually how userspace *should* collapse things to avoid holding up
vCPUs too long. I think this is a good reason to keep intermediate
page sizes.

When collapsing 4K -> 1G, the mapcount scheme doesn't actually make a
huge difference: the THP-like scheme is about 30% slower overall.

When collapsing 4K -> 2M -> 1G, the mapcount scheme makes a HUGE
difference. For the THP-like scheme, collapsing 4K -> 2M requires
decrementing and then re-incrementing subpage->_mapcount, and then
from 2M -> 1G, we have to decrement all 262k subpages->_mapcount. For
the head-only scheme, for each 2M in the 4K -> 2M collapse, we
decrement the compound_mapcount 512 times (once per PTE), then
increment it once. And then for 2M -> 1G, for each 1G, we decrement
mapcount again by 512 (once per PMD), incrementing it once.

The mapcount decrements are about on par with how long it takes to do
other things, like updating page tables. The main problem is, with the
THP-like scheme (implemented like this [1]), there isn't a way to
avoid the 262k decrements when collapsing 1G. So if we want
MADV_COLLAPSE to be fast and we want a THP-like page_mapcount() API,
then I think something more clever needs to be implemented.

[1]: https://github.com/48ca/linux/blob/hgmv2-jan24/mm/hugetlb.c#L127-L178


- James