linux-kernel - Re: [PATCH 4/5] KVM: selftests: Add a test for KVM

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Message-ID: <YSArx+ppjIH+6/uK@google.com>
Date:   Fri, 20 Aug 2021 22:25:11 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     "Russell King, ARM Linux" <linux@...linux.org.uk>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, Guo Ren <guoren@...nel.org>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Michael Ellerman <mpe@...erman.id.au>,
        Heiko Carstens <hca@...ux.ibm.com>, gor <gor@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Oleg Nesterov <oleg@...hat.com>, rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        paulmck <paulmck@...nel.org>, Boqun Feng <boqun.feng@...il.com>,
        Paolo Bonzini <pbonzini@...hat.com>, shuah <shuah@...nel.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-csky <linux-csky@...r.kernel.org>,
        linux-mips <linux-mips@...r.kernel.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        linux-s390 <linux-s390@...r.kernel.org>,
        KVM list <kvm@...r.kernel.org>,
        linux-kselftest <linux-kselftest@...r.kernel.org>,
        Peter Foley <pefoley@...gle.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Ben Gardon <bgardon@...gle.com>
Subject: Re: [PATCH 4/5] KVM: selftests: Add a test for KVM_RUN+rseq to
 detect task migration bugs

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!