Subject: Re: [PATCH 4/5] KVM: selftests: Add a test for KVM_RUN+rseq to detect task migration bugs

From: Sean Christopherson <[email-masked]>
Date: Fri, 20 Aug 2021 22:25:11 +0000

On Fri, Aug 20, 2021, Mathieu Desnoyers wrote:
> I still really hate flakiness in tests, because then people stop caring when they
> fail once in a while. And with the nature of rseq, a once-in-a-while failure is a
> big deal. Let's see if we can use other tricks to ensure stability of the cpu id
> without changing timings too much.

Yeah, zero agrument regarding flaky tests.

> One idea would be to use a seqcount lock.

A sequence counter did the trick!  Thanks much!

> But even if we use that, I'm concerned that the very long writer critical
> section calling sched_setaffinity would need to be alternated with a sleep to
> ensure the read-side progresses. The sleep delay could be relatively small
> compared to the duration of the sched_setaffinity call, e.g. ratio 1:10.

I already had an arbitrary usleep(10) to let the reader make progress between
sched_setaffinity() calls.  Dropping it down to 1us didn't affect reproducibility,
so I went with that to shave those precious cycles :-)  Eliminating the delay
entirely did result in no repro, which was a nice confirmation that it's needed
to let the reader get back into KVM_RUN.

Thanks again!