On Mon, Jun 3, 2024 at 4:03 PM Sean Christopherson <sea...@google.com> wrote: > > On Mon, Jun 03, 2024, James Houghton wrote: > > On Thu, May 30, 2024 at 11:06 PM Yu Zhao <yuz...@google.com> wrote: > > > What I don't think is acceptable is simplifying those optimizations > > > out without documenting your justifications (I would even call it a > > > design change, rather than simplification, from v3 to v4). > > > > I'll put back something similar to what you had before (like a > > test_clear_young() with a "fast" parameter instead of "bitmap"). I > > like the idea of having a new mmu notifier, like > > fast_test_clear_young(), while leaving test_young() and clear_young() > > unchanged (where "fast" means "prioritize speed over accuracy"). > > Those two statements are contradicting each other, aren't they?
I guess it depends on how you define "similar". :) > Anyways, I vote > for a "fast only" variant, e.g. test_clear_young_fast_only() or so. gup() has > already established that terminology in mm/, so hopefully it would be familiar > to readers. We could pass a param, but then the MGLRU code would likely end > up > doing a bunch of useless indirect calls into secondary MMUs, whereas a > dedicated > hook allows implementations to nullify the pointer if the API isn't supported > for whatever reason. > > And pulling in Oliver's comments about locking, I think it's important that > the > mmu_notifier API express it's requirement that the operation be "fast", not > that > it be lockless. E.g. if a secondary MMU can guarantee that a lock will be > contented only in rare, slow cases, then taking a lock is a-ok. Or a > secondary > MMU could do try-lock and bail if the lock is contended. > > That way KVM can honor the intent of the API with an implementation that works > best for KVM _and_ for MGRLU. I'm sure there will be future adjustments and > fixes, > but that's just more motivation for using something like "fast only" instead > of > "lockless". Yes, thanks, this is exactly what I meant. I really should have "only" in the name to signify that it is a requirement that it be fast. Thanks for wording it so clearly. > > > > > I made this logic change as part of removing batching. > > > > > > > > I'd really appreciate guidance on what the correct thing to do is. > > > > > > > > In my mind, what would work great is: by default, do aging exactly > > > > when KVM can do it locklessly, and then have a Kconfig to always have > > > > MGLRU to do aging with KVM if a user really cares about proactive > > > > reclaim (when the feature bit is set). The selftest can check the > > > > Kconfig + feature bit to know for sure if aging will be done. > > > > > > I still don't see how that Kconfig helps. Or why the new static branch > > > isn't enough? > > > > Without a special Kconfig, the feature bit just tells us that aging > > with KVM is possible, not that it will necessarily be done. For the > > self-test, it'd be good to know exactly when aging is being done or > > not, so having a Kconfig like LRU_GEN_ALWAYS_WALK_SECONDARY_MMU would > > help make the self-test set the right expectations for aging. > > > > The Kconfig would also allow a user to know that, no matter what, > > we're going to get correct age data for VMs, even if, say, we're using > > the shadow MMU. > > Heh, unless KVM flushes, you won't get "correct" age data. > > > This is somewhat important for me/Google Cloud. Is that reasonable? Maybe > > there's a better solution. > > Hmm, no? There's no reason to use a Kconfig, e.g. if we _really_ want to > prioritize > accuracy over speed, then a KVM (x86?) module param to have KVM walk nested > TDP > page tables would give us what we want. > > But before we do that, I think we need to perform due dilegence (or provide > data) > showing that having KVM take mmu_lock for write in the "fast only" API > provides > better total behavior. I.e. that the additional accuracy is indeed worth the > cost. That sounds good to me. I'll drop the Kconfig. I'm not really sure what to do about the self-test, but that's not really all that important.
