[Bug 218259] High latency in KVM guests

[bugzilla-daemon@kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=218259

--- Comment #2 from Joern Heissler (kernelbugs2012@joern-heissler.de) ---
Hi,

1. KSM is already disabled. Didn't try to enable it.
2. NUMA autobalancing was enabled on the host (value 1), not in the guest. When
disabled, I can't see the issue anymore.
3. tdp_mmu was "Y", disabling it seems to make no difference.

So might be related to NUMA. On older kernels, the flag is 1 as well.

There's one difference in the kernel messages that I hadn't noticed before. The
newer one prints "pci_bus 0000:7f: Unknown NUMA node; performance will be
reduced" (same with ff again). The older ones don't. No idea what this means,
if it's important, and can't find info on the web regarding it.


I think the kernel is preemptible:

"uname -a" shows: "Linux vm123 6.1.0-15-amd64 #1 SMP PREEMPT_DYNAMIC Debian
6.1.66-1 (2023-12-09) x86_64 GNU/Linux"

"grep -i" on the config shows:
CONFIG_PREEMPT_BUILD=y
# CONFIG_PREEMPT_NONE is not set
CONFIG_PREEMPT_VOLUNTARY=y
# CONFIG_PREEMPT is not set
CONFIG_PREEMPT_COUNT=y
CONFIG_PREEMPTION=y
CONFIG_PREEMPT_DYNAMIC=y
CONFIG_PREEMPT_RCU=y
CONFIG_HAVE_PREEMPT_DYNAMIC=y
CONFIG_HAVE_PREEMPT_DYNAMIC_CALL=y
CONFIG_PREEMPT_NOTIFIERS=y
CONFIG_DRM_I915_PREEMPT_TIMEOUT=640
# CONFIG_DEBUG_PREEMPT is not set
# CONFIG_PREEMPT_TRACER is not set
# CONFIG_PREEMPTIRQ_DELAY_TEST is not set

Attaching output of "dmesg" and "lspci -v". Perhaps there's something useful in
there.

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