Re: [PATCH v2 09/12] KVM: arm64: Split huge pages when dirty logging is enabled

[Gavin Shan <gshan@redhat.com>]

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@google.com>
> ---
>   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