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

[Ricardo Koller <ricarkol@google.com>]

On Thu, Jan 26, 2023 at 10:45 AM Ricardo Koller <ricarkol@google.com> wrote:
>
> On Tue, Jan 24, 2023 at 2:45 PM Oliver Upton <oliver.upton@linux.dev> 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@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.
> >
>
> 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.
>

Ahh, you meant just the "lockdep" line. Yes, that makes sense. Will do.

> >
> > > +     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