linux-kernel - Re: [PATCH] time: Provide full featured jiffies_to

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Date:	Fri, 16 May 2014 22:24:35 +0800
From:	Shy Shuky <rousya@...il.com>
To:	Tony Luck <tony.luck@...el.com>
Cc:	linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...hat.com>,
	Mauro Chehab <m.chehab@...sung.com>,
	"xiexiuqi@...wei.com" <xiexiuqi@...wei.com>
Subject: Re: [PATCH] time: Provide full featured jiffies_to_nsecs() function

2014-04-09 1:32 GMT+08:00 Tony Luck <tony.luck@...el.com>:
> The "uptime" tracer added in:
>     commit 8aacf017b065a805d27467843490c976835eb4a5
>     tracing: Add "uptime" trace clock that uses jiffies
> has wraparound problems when the system has been up more
> than 1 hour 11 minutes and 34 seconds. It converts jiffies
> to nanoseconds using:
>         (u64)jiffies_to_usecs(jiffy) * 1000ULL
> but since jiffies_to_usecs() only returns a 32-bit value, it
> truncates at 2^32 microseconds.  An additional problem on 32-bit
> systems is that the argument is "unsigned long", so fixing the
> return value only helps until 2^32 jiffies (49.7 days on a HZ=1000
> system).
>
> So we provide a full featured jiffies_to_nsec() function that
> takes a "u64" argument and provides a "u64" result.  To avoid
> cries of rage from the other user of this: scheduler_tick_max_deferment()
> we check whether the argument is small enough that we can do
> the calculations in 32-bit operations.

Hi Tony,
How about get uptime by get_monotonic_boottime() directly, which's
the same as /proc/uptime.

Then trace_clock_jiffies() would be like bellow:

u64 notrace trace_clock_jiffies(void)
        struct timespec uptime;
        get_monotonic_boottime(&uptime);

        return uptime.tv_sec * NSEC_PER_SEC + uptime.tv_nsec;
}

--
XiuQi

>
> Signed-off-by: Tony Luck <tony.luck@...el.com>
> ---
>  include/linux/jiffies.h    |  6 +-----
>  kernel/time.c              | 24 ++++++++++++++++++++++++
>  kernel/timeconst.bc        | 12 ++++++++++++
>  kernel/trace/trace_clock.c |  2 +-
>  4 files changed, 38 insertions(+), 6 deletions(-)
...
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