linux-kernel - runaway latency detection

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Date:   Sat, 20 Aug 2016 12:03:37 -0500
From:   "T. Smith" <tlsmith3777@...il.com>
To:     Daniel Bristot de Oliveira <bristot@...hat.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Jon Masters <jcm@...hat.com>, Carsten Emde <C.Emde@...dl.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        Clark Williams <williams@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Daniel Wagner <wagi@...om.org>
Cc:     linux-kernel@...r.kernel.org
Subject: runaway latency detection

Trying to isolate PREEMPT_RT (full preemption) runaway latency conditions seen when using the cyclictest on the 4.1.8 SMP PREEMPT_RT kernel.  Seen average latency of 3uS, but runaway or outlier max latency seems indeterminate.  Noted 72uS with one run of the cyclictest for example. Is this a known issue, the cause understood, and is there a patch for this?  

The goal is to isolate causes of indeterminism when using the PREEMPT_RT kernel configuration with full preemption and to characterize latency and jitter using ftrace, any recommendations?