On Thu, Feb 23, 2023 at 10:43 AM Sean Christopherson <seanjc@xxxxxxxxxx> wrote: > > On Thu, Feb 16, 2023, Yu Zhao wrote: > > An existing selftest can quickly demonstrate the effectiveness of this > > patch. On a generic workstation equipped with 128 CPUs and 256GB DRAM: > > Not my area of maintenance, but a non-existent changelog (for all intents and > purposes) for a change of this size and complexity is not acceptable. Will fix. > > $ sudo max_guest_memory_test -c 64 -m 250 -s 250 > > > > MGLRU run2 > > --------------- > > Before ~600s > > After ~50s > > Off ~250s > > > > kswapd (MGLRU before) > > 100.00% balance_pgdat > > 100.00% shrink_node > > 100.00% shrink_one > > 99.97% try_to_shrink_lruvec > > 99.06% evict_folios > > 97.41% shrink_folio_list > > 31.33% folio_referenced > > 31.06% rmap_walk_file > > 30.89% folio_referenced_one > > 20.83% __mmu_notifier_clear_flush_young > > 20.54% kvm_mmu_notifier_clear_flush_young > > => 19.34% _raw_write_lock > > > > kswapd (MGLRU after) > > 100.00% balance_pgdat > > 100.00% shrink_node > > 100.00% shrink_one > > 99.97% try_to_shrink_lruvec > > 99.51% evict_folios > > 71.70% shrink_folio_list > > 7.08% folio_referenced > > 6.78% rmap_walk_file > > 6.72% folio_referenced_one > > 5.60% lru_gen_look_around > > => 1.53% __mmu_notifier_test_clear_young > > Do you happen to know how much of the improvement is due to batching, and how > much is due to using a walkless walk? No. I have three benchmarks running at the moment: 1. Windows SQL server guest on x86 host, 2. Apache Spark guest on arm64 host, and 3. Memcached guest on ppc64 host. If you are really interested in that, I can reprioritize -- I need to stop 1) and use that machine to get the number for you. > > @@ -5699,6 +5797,9 @@ static ssize_t show_enabled(struct kobject *kobj, struct kobj_attribute *attr, c > > if (arch_has_hw_nonleaf_pmd_young() && get_cap(LRU_GEN_NONLEAF_YOUNG)) > > caps |= BIT(LRU_GEN_NONLEAF_YOUNG); > > > > + if (kvm_arch_has_test_clear_young() && get_cap(LRU_GEN_SPTE_WALK)) > > + caps |= BIT(LRU_GEN_SPTE_WALK); > > 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 fully understand why you don't want MM to call into KVM directly. No acceptable ways to set up a clear interface between MM and KVM other than the MMU notifier?
