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@xxxxxxxxxx>
> ---
> 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
