linux-kernel - Re: [PATCH 2/3] ftrace: add tracepoint for xtime

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Date:	Wed, 16 Sep 2009 12:58:42 -0700
From:	john stultz <johnstul@...ibm.com>
To:	Zhaolei <zhaolei@...fujitsu.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/3] ftrace: add tracepoint for xtime

On Wed, Sep 16, 2009 at 12:56 PM, john stultz <johnstul@...ibm.com> wrote:
> On Tue, Aug 25, 2009 at 1:14 AM, Zhaolei <zhaolei@...fujitsu.com> wrote:
>> From: Xiao Guangrong <xiaoguangrong@...fujitsu.com>
>>  /* Structure holding internal timekeeping values. */
>>  struct timekeeper {
>>        /* Current clocksource used for timekeeping. */
>> @@ -338,6 +341,8 @@ int do_settimeofday(struct timespec *tv)
>>
>>        update_vsyscall(&xtime, timekeeper.clock);
>>
>> +       trace_gtod_update_xtime(&xtime, &wall_to_monotonic);
>> +
>
> So the only thing to watch out on here is that xtime doesn't hold the
> current time, but the accumulated time. There is some unaccumulated
> time still kept in the clocksource structure.
>
> You probably want (assuming you only need tick granularity time) to
> use current_kernel_time().
>
> As an aside, is there a reason you have to have update callbacks and
> duplicate the time storage instead of using the existing interfaces?
> (ie: Is this due to locking or something else?)

Doh. Sorry, you're actually tracing the timekeeping code. Not using
this to assist tracing. Got this confused.

So yea, I think this should be ok, plus or minus determining if you
really want xtime or xtime_cache.

-john
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