linux-kernel - Re: [RFC 0/2] ABI for clock_gettime

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Date:	Wed, 14 Dec 2011 09:31:55 -0800
From:	john stultz <johnstul@...ibm.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Richard Cochran <richardcochran@...il.com>,
	linux-kernel@...r.kernel.org, Kumar Sundararajan <kumar@...com>,
	Arun Sharma <asharma@...com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [RFC 0/2] ABI for clock_gettime_ns

On Wed, 2011-12-14 at 09:15 -0800, Andy Lutomirski wrote:
> On Wed, Dec 14, 2011 at 8:48 AM, john stultz <johnstul@...ibm.com> wrote:
> > On Wed, 2011-12-14 at 08:46 +0100, Richard Cochran wrote:
> >>
> >> What about this sort of time value?
> >>
> >> struct sys_timeval {
> >>       __s64 nanoseconds;
> >>       __u32 fractional_ns;
> >> };
> >>
> >> The second field can just be zero, for now.
> >
> > I'm mixed on this.
> >
> > We could do this, as the kernel keeps track of sub-ns granularity.
> > However, its not stored in a decimal format. So I worry the extra math
> > needed to convert it to something usable might add extra overhead,
> > removing the gain of the proposed clock_gettime_ns() interface.
> >
> 
> I would actually prefer units of 2^-32 ns over .  I have no attachment
> to SI picoseconds so long as the units are constant.

2^-32ns would be much easier to do.


> Windows sidesteps this issue by returning arbitrary units and telling
> the user what those units are.  This adds a lot of unpleasantness (try
> relating the timestamps to actual wall time) and we need to rescale
> the time anyway for NTP.
> 
> What about:
> 
> struct sys_timeval {
>     u64 nanoseconds;  /* unsigned.  the current time will always be
> after 1970, and those extra 290 years might be nice. */

I'd suspect we will still need this to be signed if it goes to userland.
In-kernel u64 for nanoseconds is fine because it doesn't have to deal
with anything that far in the past. But for userland we probably should
use s64. 

>     u64 padding;  /* for later.  currently always zero. */
> 
> That way, once there's both an implementation and a use case, we can
> implement it.  In the mean time, the overhead is probably immeasurably
> low -- it's a single assignment.

This sounds good to me. 

Kumar, Arun, I know we've strayed a bit from your original patch, but
any objections here?

thanks
-john




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