Re: [PATCH 6/9] KVM: arm64: Split huge pages when dirty logging is enabled

[Oliver Upton <oliver.upton@linux.dev>]

Hi Ricardo,

On Fri, Jan 13, 2023 at 03:49:57AM +0000, 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 |  30 ++++++++
>  arch/arm64/kvm/mmu.c              | 110 +++++++++++++++++++++++++++++-
>  2 files changed, 138 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index 35a159d131b5..6ab37209b1d1 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -153,6 +153,36 @@ struct kvm_s2_mmu {
>  	/* The last vcpu id that ran on each physical CPU */
>  	int __percpu *last_vcpu_ran;
>  
> +	/*
> +	 * Memory cache used to split EAGER_PAGE_SPLIT_CHUNK_SIZE worth of huge
> +	 * pages. It is used to allocate stage2 page tables while splitting
> +	 * huge pages. Its capacity should be EAGER_PAGE_SPLIT_CACHE_CAPACITY.
> +	 * Note that the choice of EAGER_PAGE_SPLIT_CHUNK_SIZE influences both
> +	 * the capacity of the split page cache (CACHE_CAPACITY), 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 larger than
> +	 * all the available huge-page sizes, and be a multiple of all the
> +	 * other ones; for example, 1GB when all the available huge-page sizes
> +	 * are (1GB, 2MB, 32MB, 512MB).
> +	 *
> +	 * CACHE_CAPACITY should have enough pages to cover CHUNK_SIZE; for
> +	 * example, 1GB requires the following number of PAGE_SIZE-pages:
> +	 * - 512 when using 2MB hugepages with 4KB granules (1GB / 2MB).
> +	 * - 513 when using 1GB hugepages with 4KB granules (1 + (1GB / 2MB)).
> +	 * - 32 when using 32MB hugepages with 16KB granule (1GB / 32MB).
> +	 * - 2 when using 512MB hugepages with 64KB granules (1GB / 512MB).
> +	 * CACHE_CAPACITY below assumes the worst case: 1GB hugepages with 4KB
> +	 * granules.
> +	 *
> +	 * Protected by kvm->slots_lock.
> +	 */
> +#define EAGER_PAGE_SPLIT_CHUNK_SIZE		       SZ_1G
> +#define EAGER_PAGE_SPLIT_CACHE_CAPACITY					\
> +	(DIV_ROUND_UP_ULL(EAGER_PAGE_SPLIT_CHUNK_SIZE, SZ_1G) +		\
> +	 DIV_ROUND_UP_ULL(EAGER_PAGE_SPLIT_CHUNK_SIZE, SZ_2M))

Could you instead make use of the existing KVM_PGTABLE_MIN_BLOCK_LEVEL
as the batch size? 513 pages across all page sizes is a non-negligible
amount of memory that goes largely unused when PAGE_SIZE != 4K.

With that change it is a lot easier to correctly match the cache
capacity to the selected page size. Additionally, we continue to have a
single set of batching logic that we can improve later on.

> +	struct kvm_mmu_memory_cache split_page_cache;
> +
>  	struct kvm_arch *arch;
>  };
>  
> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index 700c5774b50d..41ee330edae3 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -31,14 +31,24 @@ 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)
> +bool __read_mostly eager_page_split = true;
> +module_param(eager_page_split, bool, 0644);
> +

Unless someone is really begging for it I'd prefer we not add a module
parameter for this.

> +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 +81,64 @@ static int stage2_apply_range(struct kvm *kvm, phys_addr_t addr,
>  	return ret;
>  }
>  
> +static inline bool need_topup(struct kvm_mmu_memory_cache *cache, int min)
> +{
> +	return kvm_mmu_memory_cache_nr_free_objects(cache) < min;
> +}

I don't think the helper is adding too much here.

> +static bool need_topup_split_page_cache_or_resched(struct kvm *kvm)
> +{
> +	struct kvm_mmu_memory_cache *cache;
> +
> +	if (need_resched() || rwlock_needbreak(&kvm->mmu_lock))
> +		return true;
> +
> +	cache = &kvm->arch.mmu.split_page_cache;
> +	return need_topup(cache, EAGER_PAGE_SPLIT_CACHE_CAPACITY);
> +}
> +
> +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;
> +	int cache_capacity = EAGER_PAGE_SPLIT_CACHE_CAPACITY;
> +
> +	lockdep_assert_held_write(&kvm->mmu_lock);

Rather than having the caller acquire the lock, can you instead do it
here? It would appear that the entire critical section is enclosed
within this function.

> +	lockdep_assert_held(&kvm->slots_lock);

This function doesn't depend on anything guarded by the slots_lock, can
you move this to kvm_mmu_split_memory_region()?

> +	cache = &kvm->arch.mmu.split_page_cache;
> +
> +	do {
> +		if (need_topup_split_page_cache_or_resched(kvm)) {
> +			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,
> +					       EAGER_PAGE_SPLIT_CHUNK_SIZE);
> +		ret = kvm_pgtable_stage2_split(pgt, addr, next - addr, cache);
> +		if (ret)
> +			break;
> +	} while (addr = next, addr != end);
> +
> +	return ret;
> +}

--
Thanks,
Oliver