Re: [RFC PATCH 0/3] ARM64: Guest performance improvement during dirty

[Ricardo Koller <ricarkol@google.com>]

Hi Jing,

On Mon, Jan 10, 2022 at 09:04:38PM +0000, Jing Zhang wrote:
> This patch is to reduce the performance degradation of guest workload during
> dirty logging on ARM64. A fast path is added to handle permission relaxation
> during dirty logging. The MMU lock is replaced with rwlock, by which all
> permision relaxations on leaf pte can be performed under the read lock. This
> greatly reduces the MMU lock contention during dirty logging. With this
> solution, the source guest workload performance degradation can be improved
> by more than 60%.
> 
> Problem:
>   * A Google internal live migration test shows that the source guest workload
>   performance has >99% degradation for about 105 seconds, >50% degradation
>   for about 112 seconds, >10% degradation for about 112 seconds on ARM64.
>   This shows that most of the time, the guest workload degradtion is above
>   99%, which obviously needs some improvement compared to the test result
>   on x86 (>99% for 6s, >50% for 9s, >10% for 27s).
>   * Tested H/W: Ampere Altra 3GHz, #CPU: 64, #Mem: 256GB
>   * VM spec: #vCPU: 48, #Mem/vCPU: 4GB
> 
> Analysis:
>   * We enabled CONFIG_LOCK_STAT in kernel and used dirty_log_perf_test to get
>     the number of contentions of MMU lock and the "dirty memory time" on
>     various VM spec.
>     By using test command
>     ./dirty_log_perf_test -b 2G -m 2 -i 2 -s anonymous_hugetlb_2mb -v [#vCPU]
>     Below are the results:
>     +-------+------------------------+-----------------------+
>     | #vCPU | dirty memory time (ms) | number of contentions |
>     +-------+------------------------+-----------------------+
>     | 1     | 926                    | 0                     |
>     +-------+------------------------+-----------------------+
>     | 2     | 1189                   | 4732558               |
>     +-------+------------------------+-----------------------+
>     | 4     | 2503                   | 11527185              |
>     +-------+------------------------+-----------------------+
>     | 8     | 5069                   | 24881677              |
>     +-------+------------------------+-----------------------+
>     | 16    | 10340                  | 50347956              |
>     +-------+------------------------+-----------------------+
>     | 32    | 20351                  | 100605720             |
>     +-------+------------------------+-----------------------+
>     | 64    | 40994                  | 201442478             |
>     +-------+------------------------+-----------------------+
> 
>   * From the test results above, the "dirty memory time" and the number of
>     MMU lock contention scale with the number of vCPUs. That means all the
>     dirty memory operations from all vCPU threads have been serialized by
>     the MMU lock. Further analysis also shows that the permission relaxation
>     during dirty logging is where vCPU threads get serialized.
> 
> Solution:
>   * On ARM64, there is no mechanism as PML (Page Modification Logging) and
>     the dirty-bit solution for dirty logging is much complicated compared to
>     the write-protection solution. The straight way to reduce the guest
>     performance degradation is to enhance the concurrency for the permission
>     fault path during dirty logging.
>   * In this patch, we only put leaf PTE permission relaxation for dirty
>     logging under read lock, all others would go under write lock.
>     Below are the results based on the solution:
>     +-------+------------------------+
>     | #vCPU | dirty memory time (ms) |
>     +-------+------------------------+
>     | 1     | 803                    |
>     +-------+------------------------+
>     | 2     | 843                    |
>     +-------+------------------------+
>     | 4     | 942                    |
>     +-------+------------------------+
>     | 8     | 1458                   |
>     +-------+------------------------+
>     | 16    | 2853                   |
>     +-------+------------------------+
>     | 32    | 5886                   |
>     +-------+------------------------+
>     | 64    | 12190                  |
>     +-------+------------------------+

Just curious, do yo know why is time still doubling (roughly) with the
number of cpus? maybe you performed another experiment or have some
guess(es).

Thanks,
Ricardo

>     All "dirty memory time" have been reduced by more than 60% when the
>     number of vCPU grows.
>     
> ---
> 
> Jing Zhang (3):
>   KVM: arm64: Use read/write spin lock for MMU protection
>   KVM: arm64: Add fast path to handle permission relaxation during dirty
>     logging
>   KVM: selftests: Add vgic initialization for dirty log perf test for
>     ARM
> 
>  arch/arm64/include/asm/kvm_host.h             |  2 +
>  arch/arm64/kvm/mmu.c                          | 86 +++++++++++++++----
>  .../selftests/kvm/dirty_log_perf_test.c       | 10 +++
>  3 files changed, 80 insertions(+), 18 deletions(-)
> 
> 
> base-commit: fea31d1690945e6dd6c3e89ec5591490857bc3d4
> -- 
> 2.34.1.575.g55b058a8bb-goog
> 
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