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
+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().
+
+ 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.
@@ -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
