From: Sean Christopherson <seanjc@google•com>
To: Mathieu Desnoyers <mathieu.desnoyers@efficios•com>
Cc: KVM list <kvm@vger•kernel.org>,
Peter Zijlstra <peterz@infradead•org>,
linux-kernel <linux-kernel@vger•kernel.org>,
Will Deacon <will@kernel•org>, Guo Ren <guoren@kernel•org>,
linux-kselftest <linux-kselftest@vger•kernel.org>,
Ben Gardon <bgardon@google•com>, shuah <shuah@kernel•org>,
Paul Mackerras <paulus@samba•org>,
linux-s390 <linux-s390@vger•kernel.org>, gor <gor@linux•ibm.com>,
"Russell King, ARM Linux" <linux@armlinux•org.uk>,
linux-csky <linux-csky@vger•kernel.org>,
Christian Borntraeger <borntraeger@de•ibm.com>,
Ingo Molnar <mingo@redhat•com>,
Catalin Marinas <catalin.marinas@arm•com>,
linux-mips <linux-mips@vger•kernel.org>,
Boqun Feng <boqun.feng@gmail•com>, paulmck <paulmck@kernel•org>,
Heiko Carstens <hca@linux•ibm.com>, rostedt <rostedt@goodmis•org>,
Shakeel Butt <shakeelb@google•com>,
Andy Lutomirski <luto@kernel•org>,
Thomas Gleixner <tglx@linutronix•de>,
Peter Foley <pefoley@google•com>,
linux-arm-kernel <linux-arm-kernel@lists•infradead.org>,
Thomas Bogendoerfer <tsbogend@alpha•franken.de>,
Oleg Nesterov <oleg@redhat•com>,
Paolo Bonzini <pbonzini@redhat•com>,
linuxppc-dev <linuxppc-dev@lists•ozlabs.org>
Subject: Re: [PATCH 4/5] KVM: selftests: Add a test for KVM_RUN+rseq to detect task migration bugs
Date: Fri, 20 Aug 2021 22:25:11 +0000 [thread overview]
Message-ID: <YSArx+ppjIH+6/uK@google.com> (raw)
In-Reply-To: <407716135.20250.1629484298288.JavaMail.zimbra@efficios.com>
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!
next prev parent reply other threads:[~2021-08-20 22:26 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-08-18 0:12 [PATCH 0/5] KVM: rseq: Fix and a test for a KVM+rseq bug Sean Christopherson
2021-08-18 0:12 ` [PATCH 1/5] KVM: rseq: Update rseq when processing NOTIFY_RESUME on xfer to KVM guest Sean Christopherson
2021-08-19 21:39 ` Mathieu Desnoyers
2021-08-19 23:48 ` Sean Christopherson
2021-08-20 18:51 ` Mathieu Desnoyers
2021-08-20 22:26 ` Sean Christopherson
2021-09-06 10:28 ` Paolo Bonzini
2021-09-07 14:38 ` Sean Christopherson
2021-08-18 0:12 ` [PATCH 2/5] entry: rseq: Call rseq_handle_notify_resume() in tracehook_notify_resume() Sean Christopherson
2021-08-19 21:41 ` Mathieu Desnoyers
2021-08-18 0:12 ` [PATCH 3/5] tools: Move x86 syscall number fallbacks to .../uapi/ Sean Christopherson
2021-08-18 0:12 ` [PATCH 4/5] KVM: selftests: Add a test for KVM_RUN+rseq to detect task migration bugs Sean Christopherson
2021-08-19 21:52 ` Mathieu Desnoyers
2021-08-19 23:33 ` Sean Christopherson
2021-08-20 18:31 ` Mathieu Desnoyers
2021-08-20 22:25 ` Sean Christopherson [this message]
2021-08-18 0:12 ` [PATCH 5/5] KVM: selftests: Remove __NR_userfaultfd syscall fallback Sean Christopherson
2021-09-22 14:12 ` [PATCH 0/5] KVM: rseq: Fix and a test for a KVM+rseq bug Paolo Bonzini
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