linux-kernel - Re: runaway latency detection

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Date:   Sat, 20 Aug 2016 23:45:53 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Tracy Smith <tlsmith3777@...il.com>,
        Nikolay Borisov <n.borisov.lkml@...il.com>
CC:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Clark Williams <williams@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Carsten Emde <C.Emde@...dl.org>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jon Masters <jcm@...hat.com>, Daniel Wagner <wagi@...om.org>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: runaway latency detection



On August 20, 2016 3:04:43 PM EDT, Tracy Smith 


>Using ftace it is difficult to catch the latency since it can occur at
>any
>point during the cyclictest. It is an uncomfortable level of
>indeterminism
>without knowing the cause.
>
>Is there a way to trace random latency jumps on a single core using
>trace,
>or any other suggestion to isolate it? I know the functions and tasks
>running on the core using ftrace, but can't catch the random latency
>jumps.

Have you tried using the -b option of cyclictest?  It stops the tracing when it detects a latency. It shows what happened up to that point. I can discuss more about what to do later but I'll be traveling to LinuxCon tomorrow.

-- Steve


>
>Thx, Tracy
>On Aug 20, 2016 12:54 PM, "Nikolay Borisov" <n.borisov.lkml@...il.com>
>wrote:
>
>>
>>
>> On 20.08.2016 20:03, T. Smith wrote:
>> > 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?
>>
>> What hardware is it ? If it's x86 it's entirely possible you are
>seeing
>> SMI. Perhaps you should look into disabling those if you are on x86?
>>

-- 
Sent from my Android device with K-9 Mail. Please excuse my brevity.