linux-kernel - Re: [PATCH-v2] tracing: Fix wraparound problems in "uptime" tracer

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Message-ID: <CA+8MBb++z4px4wBCh6EzfgO6rQeLdBWNY+j8Aub352v5ONQiAw@mail.gmail.com>
Date:	Fri, 18 Jul 2014 10:05:00 -0700
From:	Tony Luck <tony.luck@...il.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	"<m.chehab@...sung.com> Xie XiuQi" <xiexiuqi@...wei.com>
Subject: Re: [PATCH-v2] tracing: Fix wraparound problems in "uptime" tracer

On Thu, Jul 17, 2014 at 7:08 PM, Steven Rostedt <rostedt@...dmis.org> wrote:
>> Do we really need to convert to nanoseconds? Couldn't we just return
>> jiffies:
>
> Sure, and we can make it a "counter". That is, the counters don't hide
> 1000 counts on output.

Can you explain that a bit more ... how do we mark it as a "counter"?

>> u64 notrace trace_clock_jiffies(void)
>> {
>>        return jiffies_64 - INITIAL_JIFFIES;
>> }

Hmm it seems that we make it hard for applications to see what HZ
value the kernel is actually using. All user-facing interfaces should
convert to USER_HZ (100).  So we should say:

u64 notrace trace_clock_jiffies(void)
{
        return jiffies_64_to_clock_t(jiffies_64 - INITIAL_JIFFIES);
}

Do we need to change anything in Documentation/trace/ftrace.txt?

Currently it says:

          uptime: This uses the jiffies counter and the time stamp
                  is relative to the time since boot up.

-Tony
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