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

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Message-ID: <CA+8MBbKgHqwzLbY17-6o_Y-EULatnXgTdgn2kUhhOabELYGV6w@mail.gmail.com>
Date:	Thu, 17 Jul 2014 16:02:26 -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>,
	"Luck, Tony" <tony.luck@...el.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 Mon, Jun 30, 2014 at 1:31 PM, Tony Luck <tony.luck@...el.com> wrote:
> +#else
> +       jiffy *= HZ_TO_NSEC_NUM;
> +       do_div(jiffy, HZ_TO_NSEC_DEN);
> +       return jiffy;
> +#endif

Well that didn't work either.  kernel/trace/trace_clock.c doesn't include
timeconst.h - so we fail the compile because HZ_TO_NSEC_{NUM,DEN}
aren't defined.  a #include looks dicey because timeconst.h is a generated
file up the the kernel/ level ... so getting Makefile dependencies right would
be ugly.

Do we really need to convert to nanoseconds? Couldn't we just return
jiffies:

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

and leave it as an exercise to userspace to interpret that?

-Tony

[Yes, this changes the UI . but there can't be any serious users
given that the existing UI only works for 1hr 11 minutes after each
boot]
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