linux-kernel - runaway latency detection

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Message-Id: <8E6107F3-E531-44C2-85B8-C83A35C924DF@gmail.com>
Date:   Sun, 21 Aug 2016 21:22:37 -0500
From:   "T. Smith" <tlsmith3777@...il.com>
To:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
        "irqbalance@...ts.infradead.org" <irqbalance@...ts.infradead.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 random spikes seem 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?   This is on ARM8 Cortex-A53 four core. Running the cyclictest on an isolated core 2 or 3 shows same behavior as do other platforms.  No Trusted OS configured.

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 on catching random latency spikes using ftrace or profiling when running the cyclictest?