On 08/05/2009 10:58 AM, Avi Kivity wrote:
On 08/05/2009 05:40 AM, Wu Fengguang wrote:
Greetings,
Jeff Dike found that many KVM pages are being refaulted in 2.6.29:
"Lots of pages between discarded due to memory pressure only to be
faulted back in soon after. These pages are nearly all stack pages.
This is not consistent - sometimes there are relatively few such pages
and they are spread out between processes."
The refaults can be drastically reduced by the following patch, which
respects the referenced bit of all anonymous pages (including the KVM
pages).
However it risks reintroducing the problem addressed by commit 7e9cd4842
(fix reclaim scalability problem by ignoring the referenced bit,
mainly the pte young bit). I wonder if there are better solutions?
How do you distinguish between kvm pages and non-kvm anonymous pages?
More importantly, why should you?
Jeff, do you see the refaults on Nehalem systems? If so, that's
likely due to the lack of an accessed bit on EPT pagetables. It would
be interesting to compare with Barcelona (which does).
If that's indeed the case, we can have the EPT ageing mechanism give
pages a bit more time around by using an available bit in the EPT PTEs
to return accessed on the first pass and not-accessed on the second.
The attached patch implements this.
--
error compiling committee.c: too many arguments to function
diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index 7b53614..310938a 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -195,6 +195,7 @@ static u64 __read_mostly shadow_x_mask; /* mutual exclusive with nx_mask */
static u64 __read_mostly shadow_user_mask;
static u64 __read_mostly shadow_accessed_mask;
static u64 __read_mostly shadow_dirty_mask;
+static int __read_mostly shadow_accessed_shift;
static inline u64 rsvd_bits(int s, int e)
{
@@ -219,6 +220,8 @@ void kvm_mmu_set_mask_ptes(u64 user_mask, u64 accessed_mask,
{
shadow_user_mask = user_mask;
shadow_accessed_mask = accessed_mask;
+ shadow_accessed_shift
+ = find_first_bit((void *)&shadow_accessed_mask, 64);
shadow_dirty_mask = dirty_mask;
shadow_nx_mask = nx_mask;
shadow_x_mask = x_mask;
@@ -817,11 +820,11 @@ static int kvm_age_rmapp(struct kvm *kvm, unsigned long *rmapp)
while (spte) {
int _young;
u64 _spte = *spte;
- BUG_ON(!(_spte & PT_PRESENT_MASK));
- _young = _spte & PT_ACCESSED_MASK;
+ BUG_ON(!(_spte & shadow_accessed_mask));
+ _young = _spte & shadow_accessed_mask;
if (_young) {
young = 1;
- clear_bit(PT_ACCESSED_SHIFT, (unsigned long *)spte);
+ clear_bit(shadow_accessed_shift, (unsigned long *)spte);
}
spte = rmap_next(kvm, rmapp, spte);
}
@@ -2572,7 +2575,7 @@ static void kvm_mmu_access_page(struct kvm_vcpu *vcpu, gfn_t gfn)
&& shadow_accessed_mask
&& !(*spte & shadow_accessed_mask)
&& is_shadow_present_pte(*spte))
- set_bit(PT_ACCESSED_SHIFT, (unsigned long *)spte);
+ set_bit(shadow_accessed_shift, (unsigned long *)spte);
}
void kvm_mmu_pte_write(struct kvm_vcpu *vcpu, gpa_t gpa,
diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 0ba706e..bc99367 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -4029,7 +4029,7 @@ static int __init vmx_init(void)
bypass_guest_pf = 0;
kvm_mmu_set_base_ptes(VMX_EPT_READABLE_MASK |
VMX_EPT_WRITABLE_MASK);
- kvm_mmu_set_mask_ptes(0ull, 0ull, 0ull, 0ull,
+ kvm_mmu_set_mask_ptes(0ull, 1ull << 63, 0ull, 0ull,
VMX_EPT_EXECUTABLE_MASK);
kvm_enable_tdp();
} else
