[Bug 217380] New: Latency issues in creating kvm-nx-lpage-recovery kthread

[bugzilla-daemon@kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=217380

            Bug ID: 217380
           Summary: Latency issues in creating kvm-nx-lpage-recovery
                    kthread
           Product: Virtualization
           Version: unspecified
          Hardware: All
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P3
         Component: kvm
          Assignee: virtualization_kvm@kernel-bugs.osdl.org
          Reporter: zhuangel570@gmail.com
        Regression: No

Hi

We found some latency issue in high-density and high-concurrency scenarios, we
are using cloud hypervisor as vmm for lightweight VM, using VIRTIO net and
block for VM. In our test, we got about 50ms to 100ms+ latency in creating VM
and register irqfd, after trace with funclatency (a tool of bcc-tools,
https://github.com/iovisor/bcc), we found the latency introduced by following
functions:

- kvm_vm_create_worker_thread introduce tail latency more than 100ms.
  This function was called when create "kvm-nx-lpage-recovery" kthread when
  create a new VM, this patch was introduced to recovery large page to relief
  performance loss caused by software mitigation of ITLB_MULTIHIT, see
  b8e8c8303ff2 ("kvm: mmu: ITLB_MULTIHIT mitigation") and 1aa9b9572b10
  ("kvm: x86: mmu: Recovery of shattered NX large pages").

Here is a simple case, which can emulate the latency issue (the real latency
is lager). The case create 800 VM as background do nothing, then repeatedly
create 20 VM then destroy them after 400ms, just trace the two function
latency, 
you will reproduce such kind latency issue. Here is a trace log on Xeon(R)
Platinum 8255C server (96C, 2 sockets) with linux 6.2.20.

Reproduce Case
https://github.com/zhuangel/misc/blob/main/test/kvm_irqfd_fork/kvm_irqfd_fork.c
Reproduce log
https://github.com/zhuangel/misc/blob/main/test/kvm_irqfd_fork/test.log

To fix these latencies, I didn't have a graceful method, just simple ideas
is give user a chance to avoid these latencies, like a module parameter to
disable "kvm-nx-lpage-recovery" kthread.

Any suggestion to fix the issue if welcomed.

Thanks!

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