On 4/12/2024 11:52 AM, Sean Christopherson wrote: > 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. Make sense, will do it in V2.
