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SubjectRe: [PATCH RFCv2 0/9] kvm/arm64: Support Async Page Fault
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On 5/8/20 1:29 PM, Gavin Shan wrote:
> There are two stages of page faults and the stage one page fault is
> handled by guest itself. The guest is trapped to host when the page
> fault is caused by stage 2 page table, for example missing. The guest
> is suspended until the requested page is populated. There might be
> IO activities involved for host to populate the requested page. For
> instance, the requested page has been swapped out previously. In this
> case, the guest (vCPU) has to suspend for a few of milliseconds, which
> depends on the swapping media, regardless of the overall system load.
>
> The series adds asychornous page fault to improve the situation. A
> signal (PAGE_NOT_PRESENT) is sent from host to the guest if the requested
> page isn't absent immediately. In the mean while, a worker is started
> to populate the requested page in background. Guest either picks another
> available process to run or puts current (faulting) process to power
> saving mode when receiving the (PAGE_NOT_PRESENT) signal. After the
> requested page is populated by the worker, another signal (PAGE_READY)
> is sent from host to guest. Guest wakes up the (faulting) process when
> receiving the (PAGE_READY) signal.
>
> The signals are conveyed through control block. The control block physical
> address is passed from guest to host through dedicated KVM vendor specific
> hypercall. The control block is visible and accessible by host and guest
> in the mean while. The hypercall is also used to enable, disable, configure
> the functionality. Notifications, by injected abort data exception, are
> fired when there are pending signals. The exception handler will be invoked
> in guest kernel.
>
> Testing
> =======
> The tests are carried on the following machine. A guest with single vCPU
> and 4GB memory is started. Also, the QEMU process is put into memory cgroup
> (v1) whose memory limit is set to 2GB. In the guest, there are two threads,
> which are memory bound and CPU bound separately. The memory bound thread
> allocates all available memory, accesses and them free them. The CPU bound
> thread simply executes block of "nop". The test is carried out for 5 time
> continuously and the average number (per minute) of executed blocks in the
> CPU bound thread is taken as indicator of improvement.
>
> Vendor: GIGABYTE CPU: 224 x Cavium ThunderX2(R) CPU CN9975 v2.2 @ 2.0GHz
> Memory: 32GB Disk: Fusion-MPT SAS-3 (PCIe3.0 x8)
>
> Without-APF: 7029030180/minute = avg(7559625120 5962155840 7823208540
> 7629633480 6170527920)
> With-APF: 8286827472/minute = avg(8464584540 8177073360 8262723180
> 8095084020 8434672260)
> Outcome: +17.8%
>
> Another test case is to measure the time consumed by the application, but
> with the CPU-bound thread disabled.
>
> Without-APF: 40.3s = avg(40.6 39.3 39.2 41.6 41.2)
> With-APF: 40.8s = avg(40.6 41.1 40.9 41.0 40.7)
> Outcome: +1.2%
>
> I also have some code in the host to capture the number of async page faults,
> time used to do swapin and its maximal/minimal values when async page fault
> is enabled. During the test, the CPU-bound thread is disabled. There is about
> 30% of the time used to do swapin.
>
> Number of async page fault: 7555 times
> Total time used by application: 42.2 seconds
> Total time used by swapin: 12.7 seconds (30%)
> Minimal swapin time: 36.2 us
> Maximal swapin time: 55.7 ms
>

A kindly ping... Marc/Mark/Will, please let me know your comments
on this. thanks in advance!

> Changelog
> =========
> RFCv1 -> RFCv2
> * Rebase to 5.7.rc3
> * Performance data (Marc Zyngier)
> * Replace IMPDEF system register with KVM vendor specific hypercall (Mark Rutland)
> * Based on Will's KVM vendor hypercall probe mechanism (Will Deacon)
> * Don't use IMPDEF DFSC (0x43). Async page fault reason is conveyed
> by the control block (Mark Rutland)
> * Delayed wakeup mechanism in guest kernel (Gavin Shan)
> * Stability improvement in the guest kernel: delayed wakeup mechanism,
> external abort disallowed region, lazily clear async page fault,
> disabled interrupt on acquiring the head's lock and so on (Gavin Shan)
> * Stability improvement in the host kernel: serialized async page
> faults etc. (Gavin Shan)
> * Performance improvement in guest kernel: percpu sleeper head (Gavin Shan)
>
> Gavin Shan (7):
> kvm/arm64: Rename kvm_vcpu_get_hsr() to kvm_vcpu_get_esr()
> kvm/arm64: Detach ESR operator from vCPU struct
> kvm/arm64: Replace hsr with esr
> kvm/arm64: Export kvm_handle_user_mem_abort() with prefault mode
> kvm/arm64: Support async page fault
> kernel/sched: Add cpu_rq_is_locked()
> arm64: Support async page fault
>
> Will Deacon (2):
> arm64: Probe for the presence of KVM hypervisor services during boot
> arm/arm64: KVM: Advertise KVM UID to guests via SMCCC
>
> arch/arm64/Kconfig | 11 +
> arch/arm64/include/asm/exception.h | 3 +
> arch/arm64/include/asm/hypervisor.h | 11 +
> arch/arm64/include/asm/kvm_emulate.h | 83 +++--
> arch/arm64/include/asm/kvm_host.h | 47 +++
> arch/arm64/include/asm/kvm_para.h | 40 +++
> arch/arm64/include/uapi/asm/Kbuild | 2 -
> arch/arm64/include/uapi/asm/kvm_para.h | 22 ++
> arch/arm64/kernel/entry.S | 33 ++
> arch/arm64/kernel/process.c | 4 +
> arch/arm64/kernel/setup.c | 35 ++
> arch/arm64/kvm/Kconfig | 1 +
> arch/arm64/kvm/Makefile | 2 +
> arch/arm64/kvm/handle_exit.c | 48 +--
> arch/arm64/kvm/hyp/switch.c | 33 +-
> arch/arm64/kvm/hyp/vgic-v2-cpuif-proxy.c | 7 +-
> arch/arm64/kvm/inject_fault.c | 4 +-
> arch/arm64/kvm/sys_regs.c | 38 +-
> arch/arm64/mm/fault.c | 434 +++++++++++++++++++++++
> include/linux/arm-smccc.h | 32 ++
> include/linux/sched.h | 1 +
> kernel/sched/core.c | 8 +
> virt/kvm/arm/arm.c | 40 ++-
> virt/kvm/arm/async_pf.c | 335 +++++++++++++++++
> virt/kvm/arm/hyp/aarch32.c | 4 +-
> virt/kvm/arm/hyp/vgic-v3-sr.c | 7 +-
> virt/kvm/arm/hypercalls.c | 37 +-
> virt/kvm/arm/mmio.c | 27 +-
> virt/kvm/arm/mmu.c | 69 +++-
> 29 files changed, 1264 insertions(+), 154 deletions(-)
> create mode 100644 arch/arm64/include/asm/kvm_para.h
> create mode 100644 arch/arm64/include/uapi/asm/kvm_para.h
> create mode 100644 virt/kvm/arm/async_pf.c
>

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