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    SubjectRe: [RFC PATCH v1 0/4] Reduce cost of ptep_get_lockless on arm64
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    On 27/03/2024 09:34, David Hildenbrand wrote:
    > On 26.03.24 18:51, Ryan Roberts wrote:
    >> On 26/03/2024 17:39, David Hildenbrand wrote:
    >>> On 26.03.24 18:32, Ryan Roberts wrote:
    >>>> On 26/03/2024 17:04, David Hildenbrand wrote:
    >>>>>>>>>
    >>>>>>>>> Likely, we just want to read "the real deal" on both sides of the
    >>>>>>>>> pte_same()
    >>>>>>>>> handling.
    >>>>>>>>
    >>>>>>>> Sorry I'm not sure I understand? You mean read the full pte including
    >>>>>>>> access/dirty? That's the same as dropping the patch, right? Of course if
    >>>>>>>> we do
    >>>>>>>> that, we still have to keep pte_get_lockless() around for this case. In an
    >>>>>>>> ideal
    >>>>>>>> world we would convert everything over to ptep_get_lockless_norecency() and
    >>>>>>>> delete ptep_get_lockless() to remove the ugliness from arm64.
    >>>>>>>
    >>>>>>> Yes, agreed. Patch #3 does not look too crazy and it wouldn't really affect
    >>>>>>> any
    >>>>>>> architecture.
    >>>>>>>
    >>>>>>> I do wonder if pte_same_norecency() should be defined per architecture
    >>>>>>> and the
    >>>>>>> default would be pte_same(). So we could avoid the mkold etc on all other
    >>>>>>> architectures.
    >>>>>>
    >>>>>> Wouldn't that break it's semantics? The "norecency" of
    >>>>>> ptep_get_lockless_norecency() means "recency information in the returned pte
    >>>>>> may
    >>>>>> be incorrect". But the "norecency" of pte_same_norecency() means "ignore the
    >>>>>> access and dirty bits when you do the comparison".
    >>>>>
    >>>>> My idea was that ptep_get_lockless_norecency() would return the actual
    >>>>> result on
    >>>>> these architectures. So e.g., on x86, there would be no actual change in
    >>>>> generated code.
    >>>>
    >>>> I think this is a bad plan... You'll end up with subtle differences between
    >>>> architectures.
    >>>>
    >>>>>
    >>>>> But yes, the documentation of these functions would have to be improved.
    >>>>>
    >>>>> Now I wonder if ptep_get_lockless_norecency() should actively clear
    >>>>> dirty/accessed bits to more easily find any actual issues where the bits still
    >>>>> matter ...
    >>>>
    >>>> I did a version that took that approach. Decided it was not as good as this way
    >>>> though. Now for the life of me, I can't remember my reasoning.
    >>>
    >>> Maybe because there are some code paths that check accessed/dirty without
    >>> "correctness" implications? For example, if the PTE is already dirty, no need to
    >>> set it dirty etc?
    >>
    >> I think I decided I was penalizing the architectures that don't care because all
    >> their ptep_get_norecency() and ptep_get_lockless_norecency() need to explicitly
    >> clear access/dirty. And I would have needed ptep_get_norecency() from day 1 so
    >> that I could feed its result into pte_same().
    >
    > True. With ptep_get_norecency() you're also penalizing other architectures.
    > Therefore my original thought about making the behavior arch-specific, but the
    > arch has to make sure to get the combination of
    > ptep_get_lockless_norecency()+ptep_same_norecency() is right.
    >
    > So if an arch decide to ignore bits in ptep_get_lockless_norecency(), it must
    > make sure to also ignore them in ptep_same_norecency(), and must be able to
    > handle access/dirty bit changes differently.
    >
    > Maybe one could have one variant for "hw-managed access/dirty" vs. "sw managed
    > accessed or dirty". Only the former would end up ignoring stuff here, the latter
    > would not.
    >
    > But again, just some random thoughts how this affects other architectures and
    > how we could avoid it. The issue I describe in patch #3 would be gone if
    > ptep_same_norecency() would just do a ptep_same() check on other architectures
    > -- and would make it easier to sell :)

    Perhaps - let me chew on that for a bit. It doesn't feel as easy as you suggest
    to me. But I can't put my finger on why...



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