On Wed, Feb 8, 2023 at 10:26 PM Gavin Shan <gshan@xxxxxxxxxx> wrote: > > Hi Ricardo, > > On 2/7/23 3:58 AM, Ricardo Koller wrote: > > Split huge pages eagerly when enabling dirty logging. The goal is to > > avoid doing it while faulting on write-protected pages, which > > negatively impacts guest performance. > > > > A memslot marked for dirty logging is split in 1GB pieces at a time. > > This is in order to release the mmu_lock and give other kernel threads > > the opportunity to run, and also in order to allocate enough pages to > > split a 1GB range worth of huge pages (or a single 1GB huge page). > > Note that these page allocations can fail, so eager page splitting is > > best-effort. This is not a correctness issue though, as huge pages > > can still be split on write-faults. > > > > The benefits of eager page splitting are the same as in x86, added > > with commit a3fe5dbda0a4 ("KVM: x86/mmu: Split huge pages mapped by > > the TDP MMU when dirty logging is enabled"). For example, when running > > dirty_log_perf_test with 64 virtual CPUs (Ampere Altra), 1GB per vCPU, > > 50% reads, and 2MB HugeTLB memory, the time it takes vCPUs to access > > all of their memory after dirty logging is enabled decreased by 44% > > from 2.58s to 1.42s. > > > > Signed-off-by: Ricardo Koller <ricarkol@xxxxxxxxxx> > > --- > > arch/arm64/include/asm/kvm_host.h | 16 +++++ > > arch/arm64/kvm/mmu.c | 113 +++++++++++++++++++++++++++++- > > 2 files changed, 127 insertions(+), 2 deletions(-) > > > > diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h > > index a69a815719cf..eab62d8b3ad4 100644 > > --- a/arch/arm64/include/asm/kvm_host.h > > +++ b/arch/arm64/include/asm/kvm_host.h > > @@ -154,6 +154,22 @@ struct kvm_s2_mmu { > > int __percpu *last_vcpu_ran; > > > > #define KVM_ARM_EAGER_SPLIT_CHUNK_SIZE_DEFAULT PMD_SIZE > > + /* > > + * Memory cache used to split > > + * KVM_CAP_ARM_EAGER_SPLIT_CHUNK_SIZE worth of huge pages. It > > + * is used to allocate stage2 page tables while splitting huge > > + * pages. Note that the choice of EAGER_PAGE_SPLIT_CHUNK_SIZE > > + * influences both the capacity of the split page cache, and > > + * how often KVM reschedules. Be wary of raising CHUNK_SIZE > > + * too high. > > + * > > + * A good heuristic to pick CHUNK_SIZE is that it should be > > + * the size of huge-page to be split. > > + * > > + * Protected by kvm->slots_lock. > > + */ > > + struct kvm_mmu_memory_cache split_page_cache; > > + uint64_t split_page_chunk_size; > > > > struct kvm_arch *arch; > > }; > > diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c > > index e2ada6588017..73f8b3953f6a 100644 > > --- a/arch/arm64/kvm/mmu.c > > +++ b/arch/arm64/kvm/mmu.c > > @@ -31,14 +31,21 @@ static phys_addr_t hyp_idmap_vector; > > > > static unsigned long io_map_base; > > > > -static phys_addr_t stage2_range_addr_end(phys_addr_t addr, phys_addr_t end) > > +static phys_addr_t __stage2_range_addr_end(phys_addr_t addr, phys_addr_t end, > > + phys_addr_t size) > > { > > - phys_addr_t size = kvm_granule_size(KVM_PGTABLE_MIN_BLOCK_LEVEL); > > phys_addr_t boundary = ALIGN_DOWN(addr + size, size); > > > > return (boundary - 1 < end - 1) ? boundary : end; > > } > > > > +static phys_addr_t stage2_range_addr_end(phys_addr_t addr, phys_addr_t end) > > +{ > > + phys_addr_t size = kvm_granule_size(KVM_PGTABLE_MIN_BLOCK_LEVEL); > > + > > + return __stage2_range_addr_end(addr, end, size); > > +} > > + > > /* > > * Release kvm_mmu_lock periodically if the memory region is large. Otherwise, > > * we may see kernel panics with CONFIG_DETECT_HUNG_TASK, > > @@ -71,6 +78,72 @@ static int stage2_apply_range(struct kvm *kvm, phys_addr_t addr, > > return ret; > > } > > > > +static bool need_topup_split_page_cache_or_resched(struct kvm *kvm, uint64_t min) > > +{ > > + struct kvm_mmu_memory_cache *cache; > > + > > + if (need_resched() || rwlock_needbreak(&kvm->mmu_lock)) > > + return true; > > + > > + cache = &kvm->arch.mmu.split_page_cache; > > + return kvm_mmu_memory_cache_nr_free_objects(cache) < min; > > +} > > + > > +static int kvm_mmu_split_nr_page_tables(u64 range) > > +{ > > + int n = 0; > > + > > + if (KVM_PGTABLE_MIN_BLOCK_LEVEL < 2) > > + n += DIV_ROUND_UP_ULL(range, PUD_SIZE); > > + n += DIV_ROUND_UP_ULL(range, PMD_SIZE); > > + return n; > > +} > > + > > I think it needs comments to explain how the number of page tables are calculated, > similar to what have been done for stage2_block_get_nr_page_tables() in pgtable.c Will add a comment. > > > +static int kvm_mmu_split_huge_pages(struct kvm *kvm, phys_addr_t addr, > > + phys_addr_t end) > > +{ > > + struct kvm_mmu_memory_cache *cache; > > + struct kvm_pgtable *pgt; > > + int ret; > > + u64 next; > > + u64 chunk_size = kvm->arch.mmu.split_page_chunk_size; > > + int cache_capacity = kvm_mmu_split_nr_page_tables(chunk_size); > > + > > + if (chunk_size == 0) > > + return 0; > > + > > + lockdep_assert_held_write(&kvm->mmu_lock); > > + > > + cache = &kvm->arch.mmu.split_page_cache; > > + > > + do { > > + if (need_topup_split_page_cache_or_resched(kvm, > > + cache_capacity)) { > > + write_unlock(&kvm->mmu_lock); > > + cond_resched(); > > + /* Eager page splitting is best-effort. */ > > + ret = __kvm_mmu_topup_memory_cache(cache, > > + cache_capacity, > > + cache_capacity); > > + write_lock(&kvm->mmu_lock); > > + if (ret) > > + break; > > + } > > + > > + pgt = kvm->arch.mmu.pgt; > > + if (!pgt) > > + return -EINVAL; > > I don't think the check to see @pgt is existing or not because the VM can't be > created with its page-table isn't allocated and set in kvm_init_stage2_mmu(). GIven that the lock is released/acquired every chunk, the intent was to check that the page-table wasn't freed in between. > > > + > > + next = __stage2_range_addr_end(addr, end, chunk_size); > > + ret = kvm_pgtable_stage2_split(pgt, addr, next - addr, > > + cache, cache_capacity); > > + if (ret) > > + break; > > + } while (addr = next, addr != end); > > + > > + return ret; > > +} > > + > > #define stage2_apply_range_resched(kvm, addr, end, fn) \ > > stage2_apply_range(kvm, addr, end, fn, true) > > > > I'm wandering if stage2_apply_range() can be reused to avoid invent another similar > function. the gap are the granularity and conditions to reschedule. Will try and see what it looks like and report back. > > > @@ -772,6 +845,7 @@ int kvm_init_stage2_mmu(struct kvm *kvm, struct kvm_s2_mmu *mmu, unsigned long t > > void kvm_uninit_stage2_mmu(struct kvm *kvm) > > { > > kvm_free_stage2_pgd(&kvm->arch.mmu); > > + kvm_mmu_free_memory_cache(&kvm->arch.mmu.split_page_cache); > > } > > > > static void stage2_unmap_memslot(struct kvm *kvm, > > @@ -999,6 +1073,31 @@ static void kvm_mmu_write_protect_pt_masked(struct kvm *kvm, > > stage2_wp_range(&kvm->arch.mmu, start, end); > > } > > > > +/** > > + * kvm_mmu_split_memory_region() - split the stage 2 blocks into PAGE_SIZE > > + * pages for memory slot > > + * @kvm: The KVM pointer > > + * @slot: The memory slot to split > > + * > > + * Acquires kvm->mmu_lock. Called with kvm->slots_lock mutex acquired, > > + * serializing operations for VM memory regions. > > + */ > > +static void kvm_mmu_split_memory_region(struct kvm *kvm, int slot) > > +{ > > + struct kvm_memslots *slots = kvm_memslots(kvm); > > + struct kvm_memory_slot *memslot = id_to_memslot(slots, slot); > > + phys_addr_t start, end; > > + > > + lockdep_assert_held(&kvm->slots_lock); > > + > > + start = memslot->base_gfn << PAGE_SHIFT; > > + end = (memslot->base_gfn + memslot->npages) << PAGE_SHIFT; > > + > > + write_lock(&kvm->mmu_lock); > > + kvm_mmu_split_huge_pages(kvm, start, end); > > + write_unlock(&kvm->mmu_lock); > > +} > > + > > /* > > * kvm_arch_mmu_enable_log_dirty_pt_masked - enable dirty logging for selected > > * dirty pages. > > @@ -1790,6 +1889,16 @@ void kvm_arch_commit_memory_region(struct kvm *kvm, > > return; > > > > kvm_mmu_wp_memory_region(kvm, new->id); > > + kvm_mmu_split_memory_region(kvm, new->id); > > + } else { > > + /* > > + * Free any leftovers from the eager page splitting cache. Do > > + * this when deleting, moving, disabling dirty logging, or > > + * creating the memslot (a nop). Doing it for deletes makes > > + * sure we don't leak memory, and there's no need to keep the > > + * cache around for any of the other cases. > > + */ > > + kvm_mmu_free_memory_cache(&kvm->arch.mmu.split_page_cache); > > } > > } > > > > > > Thanks, > Gavin >