On Tue, Jan 24, 2023 at 2:45 PM Oliver Upton <oliver.upton@xxxxxxxxx> wrote:
>
> 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.
>
Sounds good, will refine this for v2.
> 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.
It was mainly to match x86 and because it makes perf testing a bit
simpler. What do others think?
>
> > +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.
Will try how it looks without.
>
> > +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.
Sure. I will first double check things related to perf and correctness
just in case.
I'm not sure if the increase of acquire/releases makes any difference in perf.
Also, not sure if there's a correctness issue because of releasing the lock
between WP and split (I think it should be fine, but not 100% sure).
>
> > + 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()?
kvm_mmu_split_memory_region() takes a memslot.
That works in this case, eager splitting when enabling dirty logging, but won't
work in the next commit when spliting on the CLEAR ioctl.
>
> > + 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
