Re: [PATCH v3 0/8] Create a userfaultfd demand paging test

[Ben Gardon]

If anyone has a chance to re-review this test patch series I'd be
grateful. I responded to most of the feedback I received in the first
series, and believe this test will be a useful performance benchmark
for future development.

On Mon, Dec 16, 2019 at 1:39 PM Ben Gardon <bgardon@xxxxxxxxxx> wrote:
>
> When handling page faults for many vCPUs during demand paging, KVM's MMU
> lock becomes highly contended. This series creates a test with a naive
> userfaultfd based demand paging implementation to demonstrate that
> contention. This test serves both as a functional test of userfaultfd
> and a microbenchmark of demand paging performance with a variable number
> of vCPUs and memory per vCPU.
>
> The test creates N userfaultfd threads, N vCPUs, and a region of memory
> with M pages per vCPU. The N userfaultfd polling threads are each set up
> to serve faults on a region of memory corresponding to one of the vCPUs.
> Each of the vCPUs is then started, and touches each page of its disjoint
> memory region, sequentially. In response to faults, the userfaultfd
> threads copy a static buffer into the guest's memory. This creates a
> worst case for MMU lock contention as we have removed most of the
> contention between the userfaultfd threads and there is no time required
> to fetch the contents of guest memory.
>
> This test was run successfully on Intel Haswell, Broadwell, and
> Cascadelake hosts with a variety of vCPU counts and memory sizes.
>
> This test was adapted from the dirty_log_test.
>
> The series can also be viewed in Gerrit here:
> https://linux-review.googlesource.com/c/virt/kvm/kvm/+/1464
> (Thanks to Dmitry Vyukov <dvyukov@xxxxxxxxxx> for setting up the Gerrit
> instance)
>
> Ben Gardon (9):
>   KVM: selftests: Create a demand paging test
>   KVM: selftests: Add demand paging content to the demand paging test
>   KVM: selftests: Add memory size parameter to the demand paging test
>   KVM: selftests: Pass args to vCPU instead of using globals
>   KVM: selftests: Support multiple vCPUs in demand paging test
>   KVM: selftests: Time guest demand paging
>   KVM: selftests: Add parameter to _vm_create for memslot 0 base paddr
>   KVM: selftests: Support large VMs in demand paging test
>   Add static flag
>
>  tools/testing/selftests/kvm/.gitignore        |   1 +
>  tools/testing/selftests/kvm/Makefile          |   4 +-
>  .../selftests/kvm/demand_paging_test.c        | 610 ++++++++++++++++++
>  tools/testing/selftests/kvm/dirty_log_test.c  |   2 +-
>  .../testing/selftests/kvm/include/kvm_util.h  |   3 +-
>  tools/testing/selftests/kvm/lib/kvm_util.c    |   7 +-
>  6 files changed, 621 insertions(+), 6 deletions(-)
>  create mode 100644 tools/testing/selftests/kvm/demand_paging_test.c
>
> --
> 2.23.0.444.g18eeb5a265-goog
>