linux-kernel - Re: [PATCH] timekeeping: Add tracepoints for xtime changes

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Message-ID: <515A072A.3080307@linaro.org>
Date:	Mon, 01 Apr 2013 15:16:10 -0700
From:	John Stultz <john.stultz@...aro.org>
To:	David Ahern <dsahern@...il.com>
CC:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH] timekeeping: Add tracepoints for xtime changes - v2

On 04/01/2013 02:58 PM, David Ahern wrote:
> On 4/1/13 12:55 PM, John Stultz wrote:
>
>> This all looks reasonable. Though do we need to be more explicit in what
>> we're tracing here? ie: CLOCK_REALTIME timestamps?
>
> The tracepoints don't care about the what and the tp names follow the 
> convention of trace_<function_name> so you know where it is triggering.
>
>>
>> I'd someday eventually like to rework the timekeeping core to be mostly
>> ktime_t based, building time in a more logical method up from
>> CLOCK_MONOTONIC rather then using CLOCK_REALTIME as our base and
>> subtracting time from that. I'm just worried about what sort of
>> constraints these tracepoints may put on a larger rework in the future.
>
> Understood. And my comment above is not going to help -- ie., telling 
> perf specific tracepoints which include function names. Should I 
> consolidate this into a single trace_tod_update() that gets invoked in 
> various places? The locations can move without affecting perf. I just 
> want the tod update; where it happens should not matter.
>

I guess what I'm getting at is: What ABI are we creating here?  Can 
these tracepoints come and go without any consequence? Or would changing 
them in the future cause application breakage?

I'm somewhat worried even trace_tod_update() is maybe too vague (again 
not that the name specifically is critical, but that the semantics we're 
specifying are clear). In other words, I think you're wanting a 
tracepoint at any time CLOCK_REALTIME is updated by anything other then 
the normal progression of time? Is that right?

You may want to also include the leapsecond modification in the tracing 
as well.

thanks
-john
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