Re: [PATCH RFC 0/4] mm: Remember young bit for migration entries

[David Hildenbrand <david@redhat.com>]

On 29.07.22 03:40, Peter Xu wrote:
> [Marking as RFC; only x86 is supported for now, plan to add a few more
>  archs when there's a formal version]
> 
> Problem
> =======
> 
> When migrate a page, right now we always mark the migrated page as old.
> The reason could be that we don't really know whether the page is hot or
> cold, so we could have taken it a default negative assuming that's safer.
> 
> However that could lead to at least two problems:
> 
>   (1) We lost the real hot/cold information while we could have persisted.
>       That information shouldn't change even if the backing page is changed
>       after the migration,
> 
>   (2) There can be always extra overhead on the immediate next access to
>       any migrated page, because hardware MMU needs cycles to set the young
>       bit again (as long as the MMU supports).
> 
> Many of the recent upstream works showed that (2) is not something trivial
> and actually very measurable.  In my test case, reading 1G chunk of memory
> - jumping in page size intervals - could take 99ms just because of the
> extra setting on the young bit on a generic x86_64 system, comparing to 4ms
> if young set.
> 
> This issue is originally reported by Andrea Arcangeli.
> 
> Solution
> ========
> 
> To solve this problem, this patchset tries to remember the young bit in the
> migration entries and carry it over when recovering the ptes.
> 
> We have the chance to do so because in many systems the swap offset is not
> really fully used.  Migration entries use swp offset to store PFN only,
> while the PFN is normally not as large as swp offset and normally smaller.
> It means we do have some free bits in swp offset that we can use to store
> things like young, and that's how this series tried to approach this
> problem.
> 
> One tricky thing here is even though we're embedding the information into
> swap entry which seems to be a very generic data structure, the number of
> bits that are free is still arch dependent.  Not only because the size of
> swp_entry_t differs, but also due to the different layouts of swap ptes on
> different archs.
> 
> Here, this series requires specific arch to define an extra macro called
> __ARCH_SWP_OFFSET_BITS represents the size of swp offset.  With this
> information, the swap logic can know whether there's extra bits to use,
> then it'll remember the young bits when possible.  By default, it'll keep
> the old behavior of keeping all migrated pages cold.
> 


I played with a similar idea when working on pte_swp_exclusive() but
gave up, because it ended up looking too hacky. Looking at patch #2, I
get the same feeling again. Kind of hacky.


If we mostly only care about x86_64, and it's a performance improvement
after all, why not simply do it like
pte_swp_mkexclusive/pte_swp_exclusive/ ... and reuse a spare PTE bit?


-- 
Thanks,

David / dhildenb