Public bug reported: [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. ** Affects: linux (Ubuntu) Importance: Undecided Status: New ** Tags: patch -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/2056227 Title: KVM: arm64: softlockups in stage2_apply_range To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/2056227/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
