On Tue, Aug 10, 2021 at 3:56 PM Oliver Upton <oupton@xxxxxxxxxx> wrote:
>
> Hi Jing,
>
> On Tue, Aug 10, 2021 at 3:32 PM Jing Zhang <jingzhangos@xxxxxxxxxx> wrote:
> >
> > Add a generic VM stats dirty_pages to record the number of dirty pages
> > reflected in dirty_bitmap at the moment.
>
> There can be multiple dirty bitmaps in a VM, one per memslot.
>
> > Original-by: Peter Feiner <pfeiner@xxxxxxxxxx>
> > Signed-off-by: Jing Zhang <jingzhangos@xxxxxxxxxx>
> > ---
> > arch/powerpc/kvm/book3s_64_mmu_hv.c | 8 ++++++--
> > arch/powerpc/kvm/book3s_64_mmu_radix.c | 1 +
> > arch/powerpc/kvm/book3s_hv_rm_mmu.c | 1 +
> > include/linux/kvm_host.h | 3 ++-
> > include/linux/kvm_types.h | 1 +
> > virt/kvm/kvm_main.c | 26 +++++++++++++++++++++++---
> > 6 files changed, 34 insertions(+), 6 deletions(-)
> >
> > diff --git a/arch/powerpc/kvm/book3s_64_mmu_hv.c b/arch/powerpc/kvm/book3s_64_mmu_hv.c
> > index c63e263312a4..e4aafa10efa1 100644
> > --- a/arch/powerpc/kvm/book3s_64_mmu_hv.c
> > +++ b/arch/powerpc/kvm/book3s_64_mmu_hv.c
> > @@ -1122,8 +1122,10 @@ long kvmppc_hv_get_dirty_log_hpt(struct kvm *kvm,
> > * since we always put huge-page HPTEs in the rmap chain
> > * corresponding to their page base address.
> > */
> > - if (npages)
> > + if (npages) {
> > set_dirty_bits(map, i, npages);
> > + kvm->stat.generic.dirty_pages += npages;
> > + }
> > ++rmapp;
> > }
> > preempt_enable();
> > @@ -1178,8 +1180,10 @@ void kvmppc_unpin_guest_page(struct kvm *kvm, void *va, unsigned long gpa,
> > gfn = gpa >> PAGE_SHIFT;
> > srcu_idx = srcu_read_lock(&kvm->srcu);
> > memslot = gfn_to_memslot(kvm, gfn);
> > - if (memslot && memslot->dirty_bitmap)
> > + if (memslot && memslot->dirty_bitmap) {
> > set_bit_le(gfn - memslot->base_gfn, memslot->dirty_bitmap);
> > + ++kvm->stat.generic.dirty_pages;
> > + }
> > srcu_read_unlock(&kvm->srcu, srcu_idx);
> > }
> >
> > diff --git a/arch/powerpc/kvm/book3s_64_mmu_radix.c b/arch/powerpc/kvm/book3s_64_mmu_radix.c
> > index b5905ae4377c..3a6cb3854a44 100644
> > --- a/arch/powerpc/kvm/book3s_64_mmu_radix.c
> > +++ b/arch/powerpc/kvm/book3s_64_mmu_radix.c
> > @@ -1150,6 +1150,7 @@ long kvmppc_hv_get_dirty_log_radix(struct kvm *kvm,
> > j = i + 1;
> > if (npages) {
> > set_dirty_bits(map, i, npages);
> > + kvm->stat.generic.dirty_pages += npages;
> > j = i + npages;
> > }
> > }
> > diff --git a/arch/powerpc/kvm/book3s_hv_rm_mmu.c b/arch/powerpc/kvm/book3s_hv_rm_mmu.c
> > index 632b2545072b..16806bc473fa 100644
> > --- a/arch/powerpc/kvm/book3s_hv_rm_mmu.c
> > +++ b/arch/powerpc/kvm/book3s_hv_rm_mmu.c
> > @@ -109,6 +109,7 @@ void kvmppc_update_dirty_map(const struct kvm_memory_slot *memslot,
> > npages = (psize + PAGE_SIZE - 1) / PAGE_SIZE;
> > gfn -= memslot->base_gfn;
> > set_dirty_bits_atomic(memslot->dirty_bitmap, gfn, npages);
> > + kvm->stat.generic.dirty_pages += npages;
> > }
> > EXPORT_SYMBOL_GPL(kvmppc_update_dirty_map);
> >
> > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> > index f50bfcf225f0..1e8e66fb915b 100644
> > --- a/include/linux/kvm_host.h
> > +++ b/include/linux/kvm_host.h
> > @@ -1421,7 +1421,8 @@ struct _kvm_stats_desc {
> > KVM_STATS_BASE_POW10, -9)
> >
> > #define KVM_GENERIC_VM_STATS() \
> > - STATS_DESC_COUNTER(VM_GENERIC, remote_tlb_flush)
> > + STATS_DESC_COUNTER(VM_GENERIC, remote_tlb_flush), \
> > + STATS_DESC_COUNTER(VM_GENERIC, dirty_pages)
> >
> > #define KVM_GENERIC_VCPU_STATS() \
> > STATS_DESC_COUNTER(VCPU_GENERIC, halt_successful_poll), \
> > diff --git a/include/linux/kvm_types.h b/include/linux/kvm_types.h
> > index ed6a985c5680..6c05df00aebf 100644
> > --- a/include/linux/kvm_types.h
> > +++ b/include/linux/kvm_types.h
> > @@ -78,6 +78,7 @@ struct kvm_mmu_memory_cache {
> >
> > struct kvm_vm_stat_generic {
> > u64 remote_tlb_flush;
> > + u64 dirty_pages;
> > };
> >
> > struct kvm_vcpu_stat_generic {
> > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > index a438a7a3774a..93f0ca2ea326 100644
> > --- a/virt/kvm/kvm_main.c
> > +++ b/virt/kvm/kvm_main.c
> > @@ -1228,6 +1228,19 @@ static int kvm_alloc_dirty_bitmap(struct kvm_memory_slot *memslot)
> > return 0;
> > }
> >
> > +static inline unsigned long hweight_dirty_bitmap(
> > + struct kvm_memory_slot *memslot)
> > +{
> > + unsigned long i;
> > + unsigned long count = 0;
> > + unsigned long n = kvm_dirty_bitmap_bytes(memslot);
> > +
> > + for (i = 0; i < n / sizeof(long); ++i)
> > + count += hweight_long(memslot->dirty_bitmap[i]);
> > +
> > + return count;
> > +}
>
> Hrm, this seems like a decent amount of work for a statistic.
>
> > +
> > /*
> > * Delete a memslot by decrementing the number of used slots and shifting all
> > * other entries in the array forward one spot.
> > @@ -1612,6 +1625,7 @@ static int kvm_delete_memslot(struct kvm *kvm,
> > if (r)
> > return r;
> >
> > + kvm->stat.generic.dirty_pages -= hweight_dirty_bitmap(old);
> > kvm_free_memslot(kvm, old);
> > return 0;
> > }
> > @@ -1733,8 +1747,10 @@ int __kvm_set_memory_region(struct kvm *kvm,
> > if (r)
> > goto out_bitmap;
> >
> > - if (old.dirty_bitmap && !new.dirty_bitmap)
> > + if (old.dirty_bitmap && !new.dirty_bitmap) {
> > + kvm->stat.generic.dirty_pages -= hweight_dirty_bitmap(&old);
> > kvm_destroy_dirty_bitmap(&old);
> > + }
>
> Races to increment by a few pages might be OK, so long as imprecision
> is acceptable, but decrementing by an entire bitmap could cause the
> stat to get waaay off from the state of the VM.
My original use case was to know the rate at which memory was being
dirtied to predict how expensive a live migration would be. I didn't
need full precision, but I would have needed a bound on the slop. A
"few pages" isn't a bound :-)
IMO, this patch isn't worth figuring out without a use case. It's
complex and has perf overhead. Maybe just drop it?
>
> What if the statistic was 'dirtied_pages', which records the number of
> pages dirtied in the lifetime of a VM? Userspace could just record the
> value each time it blows away the dirty bitmaps and subtract that
> value next time it reads the stat. It would circumvent the need to
> walk the entire dirty bitmap to keep the statistic sane.
>
> > return 0;
> >
> > out_bitmap:
> > @@ -1895,6 +1911,7 @@ static int kvm_get_dirty_log_protect(struct kvm *kvm, struct kvm_dirty_log *log)
> > offset = i * BITS_PER_LONG;
> > kvm_arch_mmu_enable_log_dirty_pt_masked(kvm, memslot,
> > offset, mask);
> > + kvm->stat.generic.dirty_pages -= hweight_long(mask);
> > }
> > KVM_MMU_UNLOCK(kvm);
> > }
> > @@ -2012,6 +2029,7 @@ static int kvm_clear_dirty_log_protect(struct kvm *kvm,
> > flush = true;
> > kvm_arch_mmu_enable_log_dirty_pt_masked(kvm, memslot,
> > offset, mask);
> > + kvm->stat.generic.dirty_pages -= hweight_long(mask);
> > }
> > }
> > KVM_MMU_UNLOCK(kvm);
> > @@ -3062,11 +3080,13 @@ void mark_page_dirty_in_slot(struct kvm *kvm,
> > unsigned long rel_gfn = gfn - memslot->base_gfn;
> > u32 slot = (memslot->as_id << 16) | memslot->id;
> >
> > - if (kvm->dirty_ring_size)
> > + if (kvm->dirty_ring_size) {
> > kvm_dirty_ring_push(kvm_dirty_ring_get(kvm),
> > slot, rel_gfn);
> > - else
> > + } else {
> > set_bit_le(rel_gfn, memslot->dirty_bitmap);
> > + ++kvm->stat.generic.dirty_pages;
> > + }
>
> Aren't pages being pushed out to the dirty ring just as dirty? :-)
>
> > }
> > }
> > EXPORT_SYMBOL_GPL(mark_page_dirty_in_slot);
> >
> > base-commit: d0732b0f8884d9cc0eca0082bbaef043f3fef7fb
> > --
> > 2.32.0.605.g8dce9f2422-goog
> >
