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Subject: Re: [PATCH v6 4/4] tracing: Histogram for delayed hrtimer offsets
To: Linux kernel mailing list <linux-kernel@vger.kernel.org>
References: <1473246835-30075-1-git-send-email-binoy.jayan@linaro.org>
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Cc: Thomas Gleixner <tglx@linutronix.de>,
        Binoy Jayan <binoy.jayan@linaro.org>,
        "Steven Rostedt (Red Hat)" <rostedt@goodmis.org>,
        Ingo Molnar <mingo@redhat.com>,
        Daniel Wagner <daniel.wagner@bmw-carit.de>,
        Arnd Bergmann <arnd@arndb.de>, Masami <masami.hiramatsu@linaro.org>
From: Carsten Emde <C.Emde@osadl.org>
Organization: Open Source Automation Development Lab (OSADL)
Message-ID: <4877f67c-bd18-1811-2932-1c2fda47c0e4@osadl.org>
Date: Fri, 23 Sep 2016 16:20:44 +0200
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On 09/09/2016 03:30 PM, Thomas Gleixner wrote:
> On Thu, 8 Sep 2016, Binoy Jayan wrote:
>> On 8 September 2016 at 12:40, Thomas Gleixner <tglx@linutronix.de> wrote:
> [..]
>>>> +     latency = ktime_to_ns(ktime_sub(basenow,
>>>> +                           ktime_to_ns(timer->praecox) ?
>>>> +                           timer->praecox : hrtimer_get_expires(timer)));
>>>> +     task = timer->function == hrtimer_wakeup ?
>>>> +                     container_of(timer, struct hrtimer_sleeper,
>>>> +                                  timer)->task : NULL;
>>>
>>> This is a complete horrible hack. You're tying the task evaluation into a
>>> single instance of hrtimer users. What's the justification for this and why
>>> do you need task at all?
Makes only sense, if any, if wakeup latency histograms that store the task with
the highest latency so far are available. Initially, I wanted to give a short
explanation here why such internal histograms may be useful, but I found it
easier in a format where pictures of example latency plots and recordings can
be shown. So I wrote a short article on "Preemption latency of real-time Linux
systems" -> https://www.osadl.org/Single-View.111+M5246ebc55e3.0.html.

Hope this helps,

	-Carsten.