Re: [RFC PATCH v1 0/4] Reduce cost of ptep_get_lockless on arm64

[David Hildenbrand]

On 03.04.24 14:59, Ryan Roberts wrote:
> 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 :)
>
> I've been thinking some more about this. I think your proposal is the following:
>
>
> // ARM64
> ptep_get_lockless_norecency()
> {
> 	- returned access/dirty may be incorrect
> 	- returned access/dirty may be differently incorrect between 2 calls
> }
> pte_same_norecency()
> {
> 	- ignore access/dirty when doing comparison
> }
> ptep_set_access_flags(ptep, pte)
> {
> 	- must not assume access/dirty in pte are "more permissive" than
> 	  access/dirty in *ptep
> 	- must only set access/dirty in *ptep, never clear
> }
>
>
> // Other arches: no change to generated code
> ptep_get_lockless_norecency()
> {
> 	return ptep_get_lockless();
> }
> pte_same_norecency()
> {
> 	return pte_same();
> }
> ptep_set_access_flags(ptep, pte)
> {
> 	- may assume access/dirty in pte are "more permissive" than access/dirty
> 	  in *ptep
> 	- if no HW access/dirty updates, "*ptep = pte" always results in "more
> 	  permissive" change
> }
>
> An arch either specializes all 3 or none of them.
>
> This would allow us to get rid of ptep_get_lockless().
>
> And it addresses the bug you found with ptep_set_access_flags().
>
>
> BUT, I still have a nagging feeling that there are likely to be other similar
> problems caused by ignoring access/dirty during pte_same_norecency(). I can't
> convince myself that its definitely all safe and robust.

Right, we'd have to identify the other possible cases and document what 
an arch + common code must stick to to make it work.

Some rules would be: if an arch implements ptep_get_lockless_norecency():

(1) Passing the result from ptep_get_lockless_norecency() to pte_same()
    is wrong.
(2) Checking pte_young()/pte_old/pte_dirty()/pte_clean() after
    ptep_get_lockless_norecency() is very likely wrong.


> So I'm leaning towards dropping patch 3 and therefore keeping
> ptep_get_lockless() around.
>
> Let me know if you have any insight that might help me change my mind :)

I'm wondering if it would help if we could find a better name (or 
concept) for "norecency" here, that expresses that only on some archs 
we'd have that fuzzy handling.

Keeping ptep_get_lockless() around for now sounds like the best alternative.

--
Cheers,

David / dhildenb