A server running a multithreaded load is probably disabling turbo boost anyway because of the thermal load on the package already. Instead, you should disable speedstep and set your systems to maximum performance. However, this will increase the heat and your power bill considerably.
Mostly idle servers are a different story, obviously.
In that scenario, mixed loads won't help. You'll have at least one pinned core, and it can scale relative to the VMs load (considering you also pin hypervisor cores, etc). So, it's possible to hit that pinned core and execute the same timing attack.
I know it's a niche scenario, but it's not an impossible or implausible one. Another possibility is the servers which fill critical roles, but they're idle or in a constant low-load state to have headspace for high loads. Again, attacking these servers are plausible. Considering these servers are not open to internet most of the time, we're bordering on corporate espionage, but it's not the subject here.
Redhat Linux docs[1]:
The /proc/interrupts file lists the number of interrupts per CPU per I/O device. It displays the IRQ number, the number of that interrupt handled by each CPU core, the interrupt type, and a comma-delimited list of drivers that are registered to receive that interrupt.
The default value for smp_affinity is f, meaning that the IRQ can be serviced on any of the CPUs in the system. To view use cat /proc/irq/32/smp_affinity for interrupt 32 as example. Setting this value to 1, like echo 1 >/proc/irq/32/smp_affinity, means that only CPU 0 can service interrupt 32.
[1] https://access.redhat.com/documentation/en-us/red_hat_enterp...
A random hysteresis is bad from a system responsiveness aspect, because frequency scaling is not free (in terms of time) when observed from the CPU perspective.