On Tue, 07 Mar 2023 03:45:52 +0000,
Ricardo Koller <ricarkol@xxxxxxxxxx> 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>
> Reviewed-by: Shaoqin Huang <shahuang@xxxxxxxxxx>
> ---
> arch/arm64/kvm/mmu.c | 118 ++++++++++++++++++++++++++++++++++++++++++-
> 1 file changed, 116 insertions(+), 2 deletions(-)
>
> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index 898985b09321..b1b8da5f8b6c 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -31,14 +31,21 @@ static phys_addr_t __ro_after_init hyp_idmap_vector;
>
> static unsigned long __ro_after_init 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,
> @@ -75,6 +82,77 @@ static int stage2_apply_range(struct kvm_s2_mmu *mmu, phys_addr_t addr,
> #define stage2_apply_range_resched(mmu, addr, end, fn) \
> stage2_apply_range(mmu, addr, end, fn, true)
>
> +static bool need_topup_split_page_cache_or_resched(struct kvm *kvm, uint64_t min)
Please don't use the words "page cache", it triggers a painful
Pavlovian reflex. Something like "need_split_memcache_topup_or_reched"
makes me feel less anxious.
> +{
> + 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;
> +}
> +
> +/*
> + * Get the maximum number of page-tables needed to split a range of
nit: page-table pages.
> + * blocks into PAGE_SIZE PTEs. It assumes the range is already mapped
> + * at the PMD level, or at the PUD level if allowed.
> + */
> +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;
> +}
> +
> +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);
Please check for the lock being held early, even in the 0-sized chunk
condition.
> +
> + cache = &kvm->arch.mmu.split_page_cache;
> +
> + do {
> + if (need_topup_split_page_cache_or_resched(kvm,
> + cache_capacity)) {
Since cache_capacity is stored in the kvm struct, why not just passing
it to the helper function and let it deal with it?
> + 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;
> +
> + 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;
> +}
> +
> static bool memslot_is_logging(struct kvm_memory_slot *memslot)
> {
> return memslot->dirty_bitmap && !(memslot->flags & KVM_MEM_READONLY);
> @@ -773,6 +851,7 @@ 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 +1078,31 @@ 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);
You have already accessed the memslots by the time you check for the
lock. Not great.
> +
> + 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 +1894,16 @@ 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);
Would there be an advantage in merging these two operations somehow?
> + } 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,
M.
--
Without deviation from the norm, progress is not possible.
