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

[Peter Xu]

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?

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.

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.

-- 
Peter Xu