On Fri, Mar 22, 2024, Zide Chen wrote: > Currently, the migration worker delays 1-10 us, assuming that one > KVM_RUN iteration only takes a few microseconds. But if C-state exit > latencies are large enough, for example, hundreds or even thousands > of microseconds on server CPUs, it may happen that it's not able to > bring the target CPU out of C-state before the migration worker starts > to migrate it to the next CPU. > > If the system workload is light, most CPUs could be at a certain level > of C-state, and the vCPU thread may waste milliseconds before it can > actually migrate to a new CPU.
Well fudge. That's definitely not on my bingo sheet. > Thus, the tests may be inefficient in such systems, and in some cases > it may fail the migration/KVM_RUN ratio sanity check. > > Since we are not able to turn off the cpuidle sub-system in run time, > this patch creates an idle thread on every CPU to prevent them from > entering C-states. First off, huge thanks for debugging this! That must have been quite the task (no pun intended). While spinning up threads on every CPU is a clever way to ensure they don't go into a deep sleep state, I'm not exactly excited about the idea of putting every reachable CPU into a busy loop. And while this doesn't add _that_ much complexity, I'm not sure the benefit (preserving the assert for all systems) is worth it. I also don't want to arbitrarily prevent idle task (as in, the kernel's idle task) interactions. E.g. it's highly (highly) unlikely, but not impossible for there to be a bug that's unique to idle tasks, or C-states, or other edge case. Are there any metrics/stats that can be (easily) checked to grant an exception to the sanity check? That's a very hand-wavy question, as I'm not even sure what type of stat we'd want to look at. Actual runtime of a task, maybe? If that's not easy, what if we add an off-by-default command line option to skip the sanity check? I was resistant to simply deleting the assert in the past, but that was mainly because I didn't want to delete it without understanding what was causing problems. That would allow CI environments to opt-out as needed, while still keeping the sanity check alive for enough systems to make it useful.
