| Message ID | 20210202185734.1680553-1-bgardon@google.com (mailing list archive) |
|---|---|
| Headers | show |
| Series | Allow parallel MMU operations with TDP MMU | expand |
On 02/02/21 19:57, Ben Gardon wrote: > The TDP MMU was implemented to simplify and improve the performance of > KVM's memory management on modern hardware with TDP (EPT / NPT). To build > on the existing performance improvements of the TDP MMU, add the ability > to handle vCPU page faults, enabling and disabling dirty logging, and > removing mappings, in parallel. In the current implementation, > vCPU page faults (actually EPT/NPT violations/misconfigurations) are the > largest source of MMU lock contention on VMs with many vCPUs. This > contention, and the resulting page fault latency, can soft-lock guests > and degrade performance. Handling page faults in parallel is especially > useful when booting VMs, enabling dirty logging, and handling demand > paging. In all these cases vCPUs are constantly incurring page faults on > each new page accessed. > > Broadly, the following changes were required to allow parallel page > faults (and other MMU operations): > -- Contention detection and yielding added to rwlocks to bring them up to > feature parity with spin locks, at least as far as the use of the MMU > lock is concerned. > -- TDP MMU page table memory is protected with RCU and freed in RCU > callbacks to allow multiple threads to operate on that memory > concurrently. > -- The MMU lock was changed to an rwlock on x86. This allows the page > fault handlers to acquire the MMU lock in read mode and handle page > faults in parallel, and other operations to maintain exclusive use of > the lock by acquiring it in write mode. > -- An additional lock is added to protect some data structures needed by > the page fault handlers, for relatively infrequent operations. > -- The page fault handler is modified to use atomic cmpxchgs to set SPTEs > and some page fault handler operations are modified slightly to work > concurrently with other threads. > > This series also contains a few bug fixes and optimizations, related to > the above, but not strictly part of enabling parallel page fault handling. > > Correctness testing: > The following tests were performed with an SMP kernel and DBX kernel on an > Intel Skylake machine. The tests were run both with and without the TDP > MMU enabled. > -- This series introduces no new failures in kvm-unit-tests > SMP + no TDP MMU no new failures > SMP + TDP MMU no new failures > DBX + no TDP MMU no new failures > DBX + TDP MMU no new failures What's DBX? Lockdep etc.? > -- All KVM selftests behave as expected > SMP + no TDP MMU all pass except ./x86_64/vmx_preemption_timer_test > SMP + TDP MMU all pass except ./x86_64/vmx_preemption_timer_test > (./x86_64/vmx_preemption_timer_test also fails without this patch set, > both with the TDP MMU on and off.) Yes, it's flaky. It depends on your host. > DBX + no TDP MMU all pass > DBX + TDP MMU all pass > -- A VM can be booted running a Debian 9 and all memory accessed > SMP + no TDP MMU works > SMP + TDP MMU works > DBX + no TDP MMU works > DBX + TDP MMU works > > This series can be viewed in Gerrit at: > https://linux-review.googlesource.com/c/linux/kernel/git/torvalds/linux/+/7172 Looks good! I'll wait for a few days of reviews, but I'd like to queue this for 5.12 and I plan to make it the default in 5.13 or 5.12-rc (depending on when I can ask Red Hat QE to give it a shake). It also needs more documentation though. I'll do that myself based on your KVM Forum talk so that I can teach myself more of it. Paolo
On Wed, Feb 03, 2021, Paolo Bonzini wrote: > Looks good! I'll wait for a few days of reviews, I guess I know what I'm doing this afternoon :-) > but I'd like to queue this for 5.12 and I plan to make it the default in 5.13 > or 5.12-rc (depending on when I can ask Red Hat QE to give it a shake). Hmm, given that kvm/queue doesn't seem to get widespread testing, I think it should be enabled by default in rc1 for whatever kernel it targets. Would it be too heinous to enable it by default in 5.12-rc1, knowing full well that there's a good possibility it would get reverted?
On 03/02/21 18:54, Sean Christopherson wrote: > On Wed, Feb 03, 2021, Paolo Bonzini wrote: >> Looks good! I'll wait for a few days of reviews, > > I guess I know what I'm doing this afternoon :-) > >> but I'd like to queue this for 5.12 and I plan to make it the default in 5.13 >> or 5.12-rc (depending on when I can ask Red Hat QE to give it a shake). > > Hmm, given that kvm/queue doesn't seem to get widespread testing, I think it > should be enabled by default in rc1 for whatever kernel it targets. > > Would it be too heinous to enable it by default in 5.12-rc1, knowing full well > that there's a good possibility it would get reverted? Absolutely not. However, to clarify my plan: - what is now kvm/queue and has been reviewed will graduate to kvm/next in a couple of days, and then to 5.12-rc1. Ben's patches are already in kvm/queue, but there's no problem in waiting another week before moving them to kvm/next because it's not enabled by default. (Right now even CET is in kvm/queue, but it will not move to kvm/next until bare metal support is in). - if this will not have been tested by Red Hat QE by say 5.12-rc3, I would enable it in kvm/next instead, and at that point the target would become the 5.13 merge window (and release). Paolo
The TDP MMU was implemented to simplify and improve the performance of KVM's memory management on modern hardware with TDP (EPT / NPT). To build on the existing performance improvements of the TDP MMU, add the ability to handle vCPU page faults, enabling and disabling dirty logging, and removing mappings, in parallel. In the current implementation, vCPU page faults (actually EPT/NPT violations/misconfigurations) are the largest source of MMU lock contention on VMs with many vCPUs. This contention, and the resulting page fault latency, can soft-lock guests and degrade performance. Handling page faults in parallel is especially useful when booting VMs, enabling dirty logging, and handling demand paging. In all these cases vCPUs are constantly incurring page faults on each new page accessed. Broadly, the following changes were required to allow parallel page faults (and other MMU operations): -- Contention detection and yielding added to rwlocks to bring them up to feature parity with spin locks, at least as far as the use of the MMU lock is concerned. -- TDP MMU page table memory is protected with RCU and freed in RCU callbacks to allow multiple threads to operate on that memory concurrently. -- The MMU lock was changed to an rwlock on x86. This allows the page fault handlers to acquire the MMU lock in read mode and handle page faults in parallel, and other operations to maintain exclusive use of the lock by acquiring it in write mode. -- An additional lock is added to protect some data structures needed by the page fault handlers, for relatively infrequent operations. -- The page fault handler is modified to use atomic cmpxchgs to set SPTEs and some page fault handler operations are modified slightly to work concurrently with other threads. This series also contains a few bug fixes and optimizations, related to the above, but not strictly part of enabling parallel page fault handling. Correctness testing: The following tests were performed with an SMP kernel and DBX kernel on an Intel Skylake machine. The tests were run both with and without the TDP MMU enabled. -- This series introduces no new failures in kvm-unit-tests SMP + no TDP MMU no new failures SMP + TDP MMU no new failures DBX + no TDP MMU no new failures DBX + TDP MMU no new failures -- All KVM selftests behave as expected SMP + no TDP MMU all pass except ./x86_64/vmx_preemption_timer_test SMP + TDP MMU all pass except ./x86_64/vmx_preemption_timer_test (./x86_64/vmx_preemption_timer_test also fails without this patch set, both with the TDP MMU on and off.) DBX + no TDP MMU all pass DBX + TDP MMU all pass -- A VM can be booted running a Debian 9 and all memory accessed SMP + no TDP MMU works SMP + TDP MMU works DBX + no TDP MMU works DBX + TDP MMU works This series can be viewed in Gerrit at: https://linux-review.googlesource.com/c/linux/kernel/git/torvalds/linux/+/7172 Changelog v1 -> v2: - Removed the MMU lock union + using a spinlock when the TDP MMU is disabled - Merged RCU commits - Extended additional MMU operations to operate in parallel - Ammended dirty log perf test to cover newly parallelized code paths - Misc refactorings (see changelogs for individual commits) - Big thanks to Sean and Paolo for their thorough review of v1 Ben Gardon (28): KVM: x86/mmu: change TDP MMU yield function returns to match cond_resched KVM: x86/mmu: Add comment on __tdp_mmu_set_spte KVM: x86/mmu: Add lockdep when setting a TDP MMU SPTE KVM: x86/mmu: Don't redundantly clear TDP MMU pt memory KVM: x86/mmu: Factor out handling of removed page tables locking/rwlocks: Add contention detection for rwlocks sched: Add needbreak for rwlocks sched: Add cond_resched_rwlock KVM: x86/mmu: Fix braces in kvm_recover_nx_lpages KVM: x86/mmu: Fix TDP MMU zap collapsible SPTEs KVM: x86/mmu: Merge flush and non-flush tdp_mmu_iter_cond_resched KVM: x86/mmu: Rename goal_gfn to next_last_level_gfn KVM: x86/mmu: Ensure forward progress when yielding in TDP MMU iter KVM: x86/mmu: Yield in TDU MMU iter even if no SPTES changed KVM: x86/mmu: Skip no-op changes in TDP MMU functions KVM: x86/mmu: Clear dirtied pages mask bit before early break KVM: x86/mmu: Protect TDP MMU page table memory with RCU KVM: x86/mmu: Use an rwlock for the x86 MMU KVM: x86/mmu: Factor out functions to add/remove TDP MMU pages KVM: x86/mmu: Use atomic ops to set SPTEs in TDP MMU map KVM: x86/mmu: Flush TLBs after zap in TDP MMU PF handler KVM: x86/mmu: Mark SPTEs in disconnected pages as removed KVM: x86/mmu: Allow parallel page faults for the TDP MMU KVM: x86/mmu: Allow zap gfn range to operate under the mmu read lock KVM: x86/mmu: Allow zapping collapsible SPTEs to use MMU read lock KVM: x86/mmu: Allow enabling / disabling dirty logging under MMU read lock KVM: selftests: Add backing src parameter to dirty_log_perf_test KVM: selftests: Disable dirty logging with vCPUs running arch/x86/include/asm/kvm_host.h | 15 + arch/x86/kvm/mmu/mmu.c | 120 +-- arch/x86/kvm/mmu/mmu_internal.h | 9 +- arch/x86/kvm/mmu/page_track.c | 8 +- arch/x86/kvm/mmu/paging_tmpl.h | 8 +- arch/x86/kvm/mmu/spte.h | 21 +- arch/x86/kvm/mmu/tdp_iter.c | 46 +- arch/x86/kvm/mmu/tdp_iter.h | 21 +- arch/x86/kvm/mmu/tdp_mmu.c | 741 ++++++++++++++---- arch/x86/kvm/mmu/tdp_mmu.h | 5 +- arch/x86/kvm/x86.c | 4 +- include/asm-generic/qrwlock.h | 24 +- include/linux/kvm_host.h | 5 + include/linux/rwlock.h | 7 + include/linux/sched.h | 29 + kernel/sched/core.c | 40 + .../selftests/kvm/demand_paging_test.c | 3 +- .../selftests/kvm/dirty_log_perf_test.c | 25 +- .../testing/selftests/kvm/include/kvm_util.h | 6 - .../selftests/kvm/include/perf_test_util.h | 3 +- .../testing/selftests/kvm/include/test_util.h | 14 + .../selftests/kvm/lib/perf_test_util.c | 6 +- tools/testing/selftests/kvm/lib/test_util.c | 29 + virt/kvm/dirty_ring.c | 10 + virt/kvm/kvm_main.c | 46 +- 25 files changed, 963 insertions(+), 282 deletions(-)