| Message ID | 20221019234050.3919566-3-dmatlack@google.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | KVM: Split huge pages mapped by the TDP MMU on fault | expand |
On Wed, Oct 19, 2022, David Matlack wrote: > Now that the TDP MMU has a mechanism to split huge pages, use it in the > fault path when a huge page needs to be replaced with a mapping at a > lower level. > > This change reduces the negative performance impact of NX HugePages. > Prior to this change if a vCPU executed from a huge page and NX > HugePages was enabled, the vCPU would take a fault, zap the huge page, > and mapping the faulting address at 4KiB with execute permissions > enabled. The rest of the memory would be left *unmapped* and have to be > faulted back in by the guest upon access (read, write, or execute). If > guest is backed by 1GiB, a single execute instruction can zap an entire > GiB of its physical address space. > > For example, it can take a VM longer to execute from its memory than to > populate that memory in the first place: > > $ ./execute_perf_test -s anonymous_hugetlb_1gb -v96 > > Populating memory : 2.748378795s > Executing from memory : 2.899670885s > > With this change, such faults split the huge page instead of zapping it, > which avoids the non-present faults on the rest of the huge page: > > $ ./execute_perf_test -s anonymous_hugetlb_1gb -v96 > > Populating memory : 2.729544474s > Executing from memory : 0.111965688s <--- > > This change also reduces the performance impact of dirty logging when > eager_page_split=N. eager_page_split=N (abbreviated "eps=N" below) can > be desirable for read-heavy workloads, as it avoids allocating memory to > split huge pages that are never written and avoids increasing the TLB > miss cost on reads of those pages. > > | Config: ept=Y, tdp_mmu=Y, 5% writes | > | Iteration 1 dirty memory time | > | --------------------------------------------- | > vCPU Count | eps=N (Before) | eps=N (After) | eps=Y | > ------------ | -------------- | ------------- | ------------ | > 2 | 0.332305091s | 0.019615027s | 0.006108211s | > 4 | 0.353096020s | 0.019452131s | 0.006214670s | > 8 | 0.453938562s | 0.019748246s | 0.006610997s | > 16 | 0.719095024s | 0.019972171s | 0.007757889s | > 32 | 1.698727124s | 0.021361615s | 0.012274432s | > 64 | 2.630673582s | 0.031122014s | 0.016994683s | > 96 | 3.016535213s | 0.062608739s | 0.044760838s | > > Eager page splitting remains beneficial for write-heavy workloads, but > the gap is now reduced. > > | Config: ept=Y, tdp_mmu=Y, 100% writes | > | Iteration 1 dirty memory time | > | --------------------------------------------- | > vCPU Count | eps=N (Before) | eps=N (After) | eps=Y | > ------------ | -------------- | ------------- | ------------ | > 2 | 0.317710329s | 0.296204596s | 0.058689782s | > 4 | 0.337102375s | 0.299841017s | 0.060343076s | > 8 | 0.386025681s | 0.297274460s | 0.060399702s | > 16 | 0.791462524s | 0.298942578s | 0.062508699s | > 32 | 1.719646014s | 0.313101996s | 0.075984855s | > 64 | 2.527973150s | 0.455779206s | 0.079789363s | > 96 | 2.681123208s | 0.673778787s | 0.165386739s | > > Further study is needed to determine if the remaining gap is acceptable > for customer workloads or if eager_page_split=N still requires a-priori > knowledge of the VM workload, especially when considering these costs > extrapolated out to large VMs with e.g. 416 vCPUs and 12TB RAM. > > Signed-off-by: David Matlack <dmatlack@google.com> > --- > arch/x86/kvm/mmu/tdp_mmu.c | 72 ++++++++++++++++++-------------------- > 1 file changed, 34 insertions(+), 38 deletions(-) > > diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c > index 4e5b3ae824c1..c53767104d5b 100644 > --- a/arch/x86/kvm/mmu/tdp_mmu.c > +++ b/arch/x86/kvm/mmu/tdp_mmu.c > @@ -1146,6 +1146,9 @@ static int tdp_mmu_link_sp(struct kvm *kvm, struct tdp_iter *iter, > return 0; > } > > +static int tdp_mmu_split_huge_page(struct kvm *kvm, struct tdp_iter *iter, > + struct kvm_mmu_page *sp, bool shared); > + > /* > * Handle a TDP page fault (NPT/EPT violation/misconfiguration) by installing > * page tables and SPTEs to translate the faulting guest physical address. > @@ -1171,49 +1174,42 @@ int kvm_tdp_mmu_map(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault) > if (iter.level == fault->goal_level) > break; > > - /* > - * If there is an SPTE mapping a large page at a higher level > - * than the target, that SPTE must be cleared and replaced > - * with a non-leaf SPTE. > - */ > + /* Step down into the lower level page table if it exists. */ > if (is_shadow_present_pte(iter.old_spte) && > - is_large_pte(iter.old_spte)) { > - if (tdp_mmu_zap_spte_atomic(vcpu->kvm, &iter)) > - break; > + !is_large_pte(iter.old_spte)) > + continue; > > - /* > - * The iter must explicitly re-read the spte here > - * because the new value informs the !present > - * path below. > - */ > - iter.old_spte = kvm_tdp_mmu_read_spte(iter.sptep); > - } > + /* > + * If SPTE has been frozen by another thread, just give up and > + * retry, avoiding unnecessary page table allocation and free. > + */ > + if (is_removed_spte(iter.old_spte)) > + break; > > - if (!is_shadow_present_pte(iter.old_spte)) { > - /* > - * If SPTE has been frozen by another thread, just > - * give up and retry, avoiding unnecessary page table > - * allocation and free. > - */ > - if (is_removed_spte(iter.old_spte)) > - break; > + /* > + * The SPTE is either non-present or points to a huge page that > + * needs to be split. > + */ > + sp = tdp_mmu_alloc_sp(vcpu); > + tdp_mmu_init_child_sp(sp, &iter); > > - sp = tdp_mmu_alloc_sp(vcpu); > - tdp_mmu_init_child_sp(sp, &iter); > + sp->nx_huge_page_disallowed = fault->huge_page_disallowed; > > - sp->nx_huge_page_disallowed = fault->huge_page_disallowed; > + if (is_shadow_present_pte(iter.old_spte)) > + ret = tdp_mmu_split_huge_page(kvm, &iter, sp, true); > + else > + ret = tdp_mmu_link_sp(kvm, &iter, sp, true); > > - if (tdp_mmu_link_sp(kvm, &iter, sp, true)) { > - tdp_mmu_free_sp(sp); > - break; > - } > + if (ret) { > + tdp_mmu_free_sp(sp); > + break; > + } > > - if (fault->huge_page_disallowed && > - fault->req_level >= iter.level) { > - spin_lock(&kvm->arch.tdp_mmu_pages_lock); > - track_possible_nx_huge_page(kvm, sp); > - spin_unlock(&kvm->arch.tdp_mmu_pages_lock); > - } > + if (fault->huge_page_disallowed && > + fault->req_level >= iter.level) { > + spin_lock(&kvm->arch.tdp_mmu_pages_lock); > + track_possible_nx_huge_page(kvm, sp); > + spin_unlock(&kvm->arch.tdp_mmu_pages_lock); > } > } > > @@ -1484,8 +1480,6 @@ static int tdp_mmu_split_huge_page(struct kvm *kvm, struct tdp_iter *iter, > const int level = iter->level; > int ret, i; > > - tdp_mmu_init_child_sp(sp, iter); > - David, thanks for the alignment with the precise nx huge page series. Nit: since this patch puts tdp_mmu_init_child_sp() out of the tdp_mmu_split_huge_page(), can we add a comment mentioned that, i.e., initialization of child sp is required before invoking the function? With that action done, Reviewed-by: Mingwei Zhang <mizhang@google.com> > /* > * No need for atomics when writing to sp->spt since the page table has > * not been linked in yet and thus is not reachable from any other CPU. > @@ -1561,6 +1555,8 @@ static int tdp_mmu_split_huge_pages_root(struct kvm *kvm, > continue; > } > > + tdp_mmu_init_child_sp(sp, &iter); > + > if (tdp_mmu_split_huge_page(kvm, &iter, sp, shared)) > goto retry; > > -- > 2.38.0.413.g74048e4d9e-goog >
diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c index 4e5b3ae824c1..c53767104d5b 100644 --- a/arch/x86/kvm/mmu/tdp_mmu.c +++ b/arch/x86/kvm/mmu/tdp_mmu.c @@ -1146,6 +1146,9 @@ static int tdp_mmu_link_sp(struct kvm *kvm, struct tdp_iter *iter, return 0; } +static int tdp_mmu_split_huge_page(struct kvm *kvm, struct tdp_iter *iter, + struct kvm_mmu_page *sp, bool shared); + /* * Handle a TDP page fault (NPT/EPT violation/misconfiguration) by installing * page tables and SPTEs to translate the faulting guest physical address. @@ -1171,49 +1174,42 @@ int kvm_tdp_mmu_map(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault) if (iter.level == fault->goal_level) break; - /* - * If there is an SPTE mapping a large page at a higher level - * than the target, that SPTE must be cleared and replaced - * with a non-leaf SPTE. - */ + /* Step down into the lower level page table if it exists. */ if (is_shadow_present_pte(iter.old_spte) && - is_large_pte(iter.old_spte)) { - if (tdp_mmu_zap_spte_atomic(vcpu->kvm, &iter)) - break; + !is_large_pte(iter.old_spte)) + continue; - /* - * The iter must explicitly re-read the spte here - * because the new value informs the !present - * path below. - */ - iter.old_spte = kvm_tdp_mmu_read_spte(iter.sptep); - } + /* + * If SPTE has been frozen by another thread, just give up and + * retry, avoiding unnecessary page table allocation and free. + */ + if (is_removed_spte(iter.old_spte)) + break; - if (!is_shadow_present_pte(iter.old_spte)) { - /* - * If SPTE has been frozen by another thread, just - * give up and retry, avoiding unnecessary page table - * allocation and free. - */ - if (is_removed_spte(iter.old_spte)) - break; + /* + * The SPTE is either non-present or points to a huge page that + * needs to be split. + */ + sp = tdp_mmu_alloc_sp(vcpu); + tdp_mmu_init_child_sp(sp, &iter); - sp = tdp_mmu_alloc_sp(vcpu); - tdp_mmu_init_child_sp(sp, &iter); + sp->nx_huge_page_disallowed = fault->huge_page_disallowed; - sp->nx_huge_page_disallowed = fault->huge_page_disallowed; + if (is_shadow_present_pte(iter.old_spte)) + ret = tdp_mmu_split_huge_page(kvm, &iter, sp, true); + else + ret = tdp_mmu_link_sp(kvm, &iter, sp, true); - if (tdp_mmu_link_sp(kvm, &iter, sp, true)) { - tdp_mmu_free_sp(sp); - break; - } + if (ret) { + tdp_mmu_free_sp(sp); + break; + } - if (fault->huge_page_disallowed && - fault->req_level >= iter.level) { - spin_lock(&kvm->arch.tdp_mmu_pages_lock); - track_possible_nx_huge_page(kvm, sp); - spin_unlock(&kvm->arch.tdp_mmu_pages_lock); - } + if (fault->huge_page_disallowed && + fault->req_level >= iter.level) { + spin_lock(&kvm->arch.tdp_mmu_pages_lock); + track_possible_nx_huge_page(kvm, sp); + spin_unlock(&kvm->arch.tdp_mmu_pages_lock); } } @@ -1484,8 +1480,6 @@ static int tdp_mmu_split_huge_page(struct kvm *kvm, struct tdp_iter *iter, const int level = iter->level; int ret, i; - tdp_mmu_init_child_sp(sp, iter); - /* * No need for atomics when writing to sp->spt since the page table has * not been linked in yet and thus is not reachable from any other CPU. @@ -1561,6 +1555,8 @@ static int tdp_mmu_split_huge_pages_root(struct kvm *kvm, continue; } + tdp_mmu_init_child_sp(sp, &iter); + if (tdp_mmu_split_huge_page(kvm, &iter, sp, shared)) goto retry;
Now that the TDP MMU has a mechanism to split huge pages, use it in the fault path when a huge page needs to be replaced with a mapping at a lower level. This change reduces the negative performance impact of NX HugePages. Prior to this change if a vCPU executed from a huge page and NX HugePages was enabled, the vCPU would take a fault, zap the huge page, and mapping the faulting address at 4KiB with execute permissions enabled. The rest of the memory would be left *unmapped* and have to be faulted back in by the guest upon access (read, write, or execute). If guest is backed by 1GiB, a single execute instruction can zap an entire GiB of its physical address space. For example, it can take a VM longer to execute from its memory than to populate that memory in the first place: $ ./execute_perf_test -s anonymous_hugetlb_1gb -v96 Populating memory : 2.748378795s Executing from memory : 2.899670885s With this change, such faults split the huge page instead of zapping it, which avoids the non-present faults on the rest of the huge page: $ ./execute_perf_test -s anonymous_hugetlb_1gb -v96 Populating memory : 2.729544474s Executing from memory : 0.111965688s <--- This change also reduces the performance impact of dirty logging when eager_page_split=N. eager_page_split=N (abbreviated "eps=N" below) can be desirable for read-heavy workloads, as it avoids allocating memory to split huge pages that are never written and avoids increasing the TLB miss cost on reads of those pages. | Config: ept=Y, tdp_mmu=Y, 5% writes | | Iteration 1 dirty memory time | | --------------------------------------------- | vCPU Count | eps=N (Before) | eps=N (After) | eps=Y | ------------ | -------------- | ------------- | ------------ | 2 | 0.332305091s | 0.019615027s | 0.006108211s | 4 | 0.353096020s | 0.019452131s | 0.006214670s | 8 | 0.453938562s | 0.019748246s | 0.006610997s | 16 | 0.719095024s | 0.019972171s | 0.007757889s | 32 | 1.698727124s | 0.021361615s | 0.012274432s | 64 | 2.630673582s | 0.031122014s | 0.016994683s | 96 | 3.016535213s | 0.062608739s | 0.044760838s | Eager page splitting remains beneficial for write-heavy workloads, but the gap is now reduced. | Config: ept=Y, tdp_mmu=Y, 100% writes | | Iteration 1 dirty memory time | | --------------------------------------------- | vCPU Count | eps=N (Before) | eps=N (After) | eps=Y | ------------ | -------------- | ------------- | ------------ | 2 | 0.317710329s | 0.296204596s | 0.058689782s | 4 | 0.337102375s | 0.299841017s | 0.060343076s | 8 | 0.386025681s | 0.297274460s | 0.060399702s | 16 | 0.791462524s | 0.298942578s | 0.062508699s | 32 | 1.719646014s | 0.313101996s | 0.075984855s | 64 | 2.527973150s | 0.455779206s | 0.079789363s | 96 | 2.681123208s | 0.673778787s | 0.165386739s | Further study is needed to determine if the remaining gap is acceptable for customer workloads or if eager_page_split=N still requires a-priori knowledge of the VM workload, especially when considering these costs extrapolated out to large VMs with e.g. 416 vCPUs and 12TB RAM. Signed-off-by: David Matlack <dmatlack@google.com> --- arch/x86/kvm/mmu/tdp_mmu.c | 72 ++++++++++++++++++-------------------- 1 file changed, 34 insertions(+), 38 deletions(-)