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

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

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.

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.