On 9/22/23 08:48, Ryan Roberts wrote:
...
I never had any feedback on the below; I'm not sure if that means everyone is
happy or that nobody read it??
One can never really know: zero or more people read it, and of those, no
one hated it enough to send out a quick NAK. So that's a *possible*,
lukewarm endorsement of sorts. Success! :)
...
BUT I've had yet another idea on the controls front, which would enable exposing
this to user space as an extension to transparent_hugepage, while continuing to
support THP as is and also be able to control THP and ALF (anon large folio)
The new ALF / ANON_LARGE_FOLIO naming looks good to me. The grep aspect
is a nice touch.
...
Add 2 controls to sysfs:
/sys/kernel/mm/transparent_hugepage/anon_orders
- bitfield where set bits are orders that will be tried during allocation
- defaults to 1<<PMD_ORDER, which gives current THP behaviour with no ALF
- For now, 1<<PMD_ORDER is highest settable bit, but easy to expand in future
- To enable ALF, set the appropriate lower bits
- To disable THP, clear 1<<PMD_ORDER
- (In future we could add an "auto" option too)
/sys/kernel/mm/transparent_hugepage/anon_always_mask
- orders in (anon_orders & anon_always_mask) are not subject to madvise
- so when enabled=madvise, still try (anon_orders & anon_always_mask) orders
as if enabled=always
- defaults to 0 (all subject to madvise)
I *think* I like this a lot, although I have some clarifying question
below. It seems to address the key things that have been complicating
the discussions: the API is now looking more flexible, and yet still
easy to understand and reason about. Nice.
A couple of questions about how this works:
The defaults for those controls give you "legacy THP". But you can modify the
controls to generate policies like this:
For these tables, a small key or legend would help. I've forgotten already
what "S" means, and am also vague about exactly what "THP>ALF>S" behavior
means, too.
THP only - existing behaviour (default):
----------------------------------------
anon_orders = 1<<PMD_ORDER
anon_always_mask = 0
thp prctl: | dis | ena | ena | ena
All I see in the prctl(2) man page is PR_SET_THP_DISABLE, I don't
see any _ENABLE. What does the above refer to?
thp sysfs: | X | never | madvise | always
----------------------|-----------|-----------|-----------|-------------
no hint | S | S | S | THP>S
MADV_HUGEPAGE | S | S | THP>S | THP>S
MADV_NOHUGEPAGE | S | S | S | S
...
It does have the disadvantage that ALF is tied to MADV_HUGEPAGE, whereas the
Right, that is a little awkward. But maybe less so now, with this new proposal,
which leaves THP a little closer to ALF.
thanks,
--
John Hubbard
NVIDIA
