Re: [PATCH v1 0/9] mm/memory: optimize unmap/zap with PTE-mapped THP

[David Hildenbrand]

On 31.01.24 03:20, Yin Fengwei wrote:
> On 1/29/24 22:32, David Hildenbrand wrote:
> > This series is based on [1] and must be applied on top of it.
> > Similar to what we did with fork(), let's implement PTE batching
> > during unmap/zap when processing PTE-mapped THPs.
> >
> > We collect consecutive PTEs that map consecutive pages of the same large
> > folio, making sure that the other PTE bits are compatible, and (a) adjust
> > the refcount only once per batch, (b) call rmap handling functions only
> > once per batch, (c) perform batch PTE setting/updates and (d) perform TLB
> > entry removal once per batch.
> >
> > Ryan was previously working on this in the context of cont-pte for
> > arm64, int latest iteration [2] with a focus on arm6 with cont-pte only.
> > This series implements the optimization for all architectures, independent
> > of such PTE bits, teaches MMU gather/TLB code to be fully aware of such
> > large-folio-pages batches as well, and amkes use of our new rmap batching
> > function when removing the rmap.
> >
> > To achieve that, we have to enlighten MMU gather / page freeing code
> > (i.e., everything that consumes encoded_page) to process unmapping
> > of consecutive pages that all belong to the same large folio. I'm being
> > very careful to not degrade order-0 performance, and it looks like I
> > managed to achieve that.
>
> One possible scenario:
> If all the folio is 2M size folio, then one full batch could hold 510M memory.
> Is it too much regarding one full batch before just can hold (2M - 4096 * 2)
> memory?

Good point, we do have CONFIG_INIT_ON_FREE_DEFAULT_ON. I don't remember 
if init_on_free or init_on_alloc was used in production systems. In 
tlb_batch_pages_flush(), there is a cond_resched() to limit the number 
of entries we process.

So if that is actually problematic, we'd run into a soft-lockup and need 
another cond_resched() [I have some faint recollection that people are 
working on removing cond_resched() completely].

One could do some counting in free_pages_and_swap_cache() (where we 
iterate all entries already) and insert cond_resched+release_pages() for 
every (e.g., 512) pages.

--
Cheers,

David / dhildenb