Re: [PATCH] selftests/rseq: take large C-state exit latency into consideration

["Chen, Zide" <zide.chen@intel.com>]

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.