kvm_create performance regression for Linux 6.x

[andreiltd]

We observed the regression in KVM startup times when switching from Kernel 5.x to 6.x. This regression is captured by running create_sandbox benchmark tests:

• Linux 5.15

sandboxes/create_sandbox
                        time:   [1.8367 ms 1.8681 ms 1.9014 ms]

• LInux 6.9.3

 sandboxes/create_sandbox
                        time:   [7.8108 ms 8.7506 ms 9.7426 ms]

A similar issue has been seen in Firecracker: Linux 6.1 boot time regression

We should try proposed mitigation strategies from the link above and see if it improves the performance.

[danbugs]

• Benchmarks from KVM on Ubuntu Server 20.04 (using Linux 5.15):

• Benchmarks from KVM on Ubuntu Server 24.04 (using Linux 6.8):

[danbugs]

After running the first mitigation suggested here (i.e., sudo mount -o remount,favordynmods /sys/fs/cgroup), we see 18% improvement on Ubuntu 24.04:

[LTBT2530]

Thanks you