On Fri, Apr 12, 2024, Zide Chen wrote: > On 4/5/2024 4:01 PM, Sean Christopherson wrote: > > 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. > > Sorry for not replying earlier. I overlooked your email from my inbox. :) > > Alternative to the busy loop, how about using the /dev/cpu_dma_latency > interface to disable c-states (I wish I had learned this before writing > the initial patch)? The good thing is it can do automatic cleanup when > it closes the fd. It's probably not practical to touch /dev/cpu_dma_latency in code, e.g. on my system it's fully root-only. And forcing rseq_test to run as root, or be bookended with script commands to toggle /dev/cpu_dma_latency, is not a net positive. Lastly, fiddling with a system-wide knob in a KVM selftests is opening a can of worms I don't want to open. However, we could have the failing TEST_ASSERT() explicitly call out /dev/cpu_dma_latency as a knob to try changing if the assert is failing. If we do that *and* add a command line option to skip the sanity check, that seems like it would give users sufficient flexibility to avoid false positives, while still maintaining good coverage. > The reason why I still think of disabling c-states is, even in the low > c-states exit latency setup, it can still increase the chances of > successful migration. > > Otherwise, I can implement a command line option to skip the sanity > check, as I was not able to find out a metrics/stats that is easy and > reliable to indicate that the scheduler is not able to wake up the > target CPU before the task is scheduled to another CPU.
