On Thu, Feb 23, 2023 at 1:29 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote: > > On Thu, Feb 23, 2023, Yu Zhao wrote: > > On Thu, Feb 23, 2023 at 12:58 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote: > > > > > > On Thu, Feb 23, 2023, Yu Zhao wrote: > > > > On Thu, Feb 23, 2023 at 12:11 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote: > > > > > > > > > > On Thu, Feb 23, 2023, Yu Zhao wrote: > > > > > > > As alluded to in patch 1, unless batching the walks even if KVM does _not_ support > > > > > > > a lockless walk is somehow _worse_ than using the existing mmu_notifier_clear_flush_young(), > > > > > > > I think batching the calls should be conditional only on LRU_GEN_SPTE_WALK. Or > > > > > > > if we want to avoid batching when there are no mmu_notifier listeners, probe > > > > > > > mmu_notifiers. But don't call into KVM directly. > > > > > > > > > > > > I'm not sure I fully understand. Let's present the problem on the MM > > > > > > side: assuming KVM supports lockless walks, batching can still be > > > > > > worse (very unlikely), because GFNs can exhibit no memory locality at > > > > > > all. So this option allows userspace to disable batching. > > > > > > > > > > I'm asking the opposite. Is there a scenario where batching+lock is worse than > > > > > !batching+lock? If not, then don't make batching depend on lockless walks. > > > > > > > > Yes, absolutely. batching+lock means we take/release mmu_lock for > > > > every single PTE in the entire VA space -- each small batch contains > > > > 64 PTEs but the entire batch is the whole KVM. > > > > > > Who is "we"? > > > > Oops -- shouldn't have used "we". > > > > > I don't see anything in the kernel that triggers walking the whole > > > VMA, e.g. lru_gen_look_around() limits the walk to a single PMD. I feel like I'm > > > missing something... > > > > walk_mm() -> walk_pud_range() -> walk_pmd_range() -> walk_pte_range() > > -> test_spte_young() -> mmu_notifier_test_clear_young(). > > > > MGLRU takes two passes: during the first pass, it sweeps entire VA > > space on each MM (per MM/KVM); during the second pass, it uses the rmap on each > > folio (per folio). > > Ah. IIUC, userspace can use LRU_GEN_SPTE_WALK to control whether or not to walk > secondary MMUs, and the kernel further restricts LRU_GEN_SPTE_WALK to secondary > MMUs that implement a lockless walk. And if the answer is "no", secondary MMUs > are simply not consulted. Sorry for the bad naming -- probably LRU_GEN_SPTE_BATCH_WALK would be less confusing. MGLRU always consults the secondary MMU for each page it's going to reclaim (during the second pass), i.e., it checks the A-bit in the SPTE mapping a page (by the rmap) it plans to reclaim so that it won't take a hot page away from KVM. If the lockless walk is supported, MGLRU doesn't need to work at page granularity: (physical) pages on the LRU list may have nothing in common (e.g., from different processes), checking their PTEs/SPTEs one by one is expensive. Instead, it sweeps the entire KVM spaces in the first pass and checks the *adjacent SPTEs* of a page it plans to reclaim in the second pass. Both rely on the *assumption* there would be some spatial locality to exploit. This assumption can be wrong, and LRU_GEN_SPTE_WALK disables it. > If that's correct, then the proper way to handle this is by extending mmu_notifier_ops > to query (a) if there's at least one register listeners that implements > test_clear_young() and (b) if all registered listeners that implement test_clear_young() > support lockless walks. That avoids direct dependencies on KVM, and avoids making > assumptions that may not always hold true, e.g. that KVM is the only mmu_notifier > user that supports the young APIs. > > P.S. all of this info absolutely belongs in documentation and/or changelogs. Will do.
