** Also affects: linux (Ubuntu Jammy)
Importance: Undecided
Status: New
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https://bugs.launchpad.net/bugs/2056227
Title:
KVM: arm64: softlockups in stage2_apply_range
Status in linux package in Ubuntu:
New
Status in linux source package in Jammy:
New
Bug description:
[Impact]
Tearing down kvm VMs on arm64 can cause softlockups to appear on console.
When
terminating VMs with > 100Gb of memory and 4k pages, the memory unmap times
often exceed 20 seconds, which can trigger the softlockup detector. Portions
of
the unmap path also have interrupts disabled while tlb invalidation
instructions
run, which can further contribute to latency problems. My team has observed
networking latency problems if the cpu where the teardown is occurring is also
mapped to handle a NIC interrupt.
Fortunately, a solution has been in place since Linux 6.1. A small pair of
patches modify stage2_apply_range to operate on smaller memory ranges before
performing a cond_resched. With these patches applied, softlockups are no
longer observed when tearing down VMs with large amounts of memory.
Although I also submitted the patches to 5.15 LTS (link to LTS submission in
"Backport" section), I'd appreciate it if Ubuntu were willing to take this
submission in parallel since the impact has left us unable to utilize arm64
for
kvm until we can either migrate our hypervisors to hugepages, pick up this
fix,
or some combination of the two.
[Backport]
Backport the following fixes from linux 6.1:
3b5c082bbf KVM: arm64: Work out supported block level at compile time
5994bc9e05 KVM: arm64: Limit stage2_apply_range() batch size to largest block
The fix is in 5994bc9e05 and 3b5c082bbf is a dependency that was submitted as
part of the series. The original submission is here:
https://lore.kernel.org/all/20221007234151.461779-1-oliver.up...@linux.dev/
I've also submitted the patches to 5.15 LTS here:
https://lore.kernel.org/stable/cover.1709665227.git.k...@templeofstupid.com/
Both fixes cherry picked cleanly and there were no conflicts.
[Test]
Executed the test from 5994bc9e05 as well as my own run of kvm_page_table_test
on a VM with 4k pages and a memory size > 100Gb. Without the patches,
softlockups were observed in both tests. With the patches applied, the tests
ran without incident.
This was tested against both LTS 5.15.150 and linux-aws-5.15.0-1055.
[Potential Regression]
Regression potential is low. These patches have been present in Linux since
6.1
and appear to have needed no further maintenance.
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