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

[Peter Xu]

On Tue, Aug 02, 2022 at 02:06:37PM +0200, David Hildenbrand wrote:
> On 02.08.22 00:35, Peter Xu wrote:
> > On Mon, Aug 01, 2022 at 10:21:32AM +0200, David Hildenbrand wrote:
> >> 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.
> > 
> > Could you explain what's the "hacky" part you mentioned?
> 
> SWP_PFN_OFFSET_FREE_BITS :)
> 
> It's a PFN offset and we're mangling in random other bits. That's hacky
> IMHO.
> 
> I played with the idea of converting all code to store bits in addition
> to the type + offset. But that requires digging through a lot of arch
> code to teach that code about additional flags, so I discarded that idea
> when working on the COW fixes.

Having SWP_PFN_OFFSET_FREE_BITS was the cleanest approach I could think of
before I know the max_swapfile_size() trick. It only needs the arch to
define this one macro and swapops.h will take action accordingly.

OTOH, I don't want to use swap pte bits not only because there aren't a lot
left for some archs (x86_64 only has bit 4 left, afaict), but also since
this is a page migration issue it'll be nicer to me if can be solved in the
swp entry level, not pte.

> 
> > 
> > I used swap entry to avoid per-arch operations. I failed to figure out a
> > common way to know swp offset length myself so unluckily in this RFC I
> > still needed one macro per-arch.  Ying's suggestion seems to be a good fit
> > here to me to remove the last arch-specific dependency.
> 
> Instead of mangling this into the PFN offset and let the arch tell you
> which bits of the PFN offset are unused ... rather remove the bits from
> the offset and define them manually to have a certain meaning. That's
> exactly how pte_swp_mkexclusive/pte_swp_exclusive/ is supposed to be
> handled on architectures that want to support it.
> 
> I hope I could make it clearer what the hacky part is IMHO :)
> 
> > 
> >>
> >>
> >> 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?
> > 
> > Page migration works for most archs, I want to have it work for all archs
> > that can easily benefit from it.
> 
> Yet we only care about x86-64 IIUC regarding performance, just the way
> the dirty bit is handled?

I don't think we only care about x86_64?  Should other archs have the same
issue as long as there's the hardware young bit?

Even without it, it'll affect page reclaim logic too, and that's also not
x86 only.

> 
> > 
> > Besides I actually have a question on the anon exclusive bit in the swap
> > pte: since we have that anyway, why we need a specific migration type for
> > anon exclusive pages?  Can it be simply read migration entries with anon
> > exclusive bit set?
> 
> Not before all arch support pte_swp_mkexclusive/pte_swp_exclusive/.
> 
> As pte_swp_mkexclusive/pte_swp_exclusive/ only applies to actual swap
> PTEs, you could even reuse that bit for migration entries and get at
> alteast the most relevant 64bit architectures supported easily.

Yes, but I think having two mechanisms for the single problem can confuse
people.

IIUC the swap bit is already defined in major archs anyway, and since anon
exclusive bit is best-effort (or am I wrong?..), I won't worry too much on
archs outside x86/arm/ppc/s390 on having anon exclusive bit lost during
migrations, because afaict the whole swap type of ANON_EXCLUSIVE_READ is
only servicing that very minority.. which seems to be a pity to waste the
swp type on all archs even if the archs defined swp pte bits just for anon
exclusive.

Thanks,

-- 
Peter Xu