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

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Date:	Wed, 14 Dec 2011 19:30:25 +0100
From:	Richard Cochran <richardcochran@...il.com>
To:	john stultz <johnstul@...ibm.com>
Cc:	Andy Lutomirski <luto@...capital.net>,
	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, Dec 14, 2011 at 08:48:30AM -0800, john stultz wrote:
> On Wed, 2011-12-14 at 08:46 +0100, Richard Cochran wrote:
> > On Mon, Dec 12, 2011 at 11:09:29PM -0800, Andy Lutomirski wrote:
> > > On Mon, Dec 12, 2011 at 7:43 PM, john stultz <johnstul@...ibm.com> wrote:
> > > >> - New name, to distance ourselves from POSIX (clock_ns_get?)
> > > 
> > > I will defer to the bikeshedding consensus :)
> > > 
> > > >> - Family of calls, with set/get
> > > 
> > > Setting the time is a big can of worms.  adjtimex is rather
> > > incomprehensible (without reading lots of source and/or the rfc) and
> > > IMO puts a lot of NTP magic into the kernel, where it doesn't belong.
> 
> Honestly, I don't really see how we jumped to adjtimex from setting the
> time, nor the complexity hinted at. First, the rational for getting
> clock_gettime_ns is to avoid the overhead of userland translating from
> timespec to ns.   I doubt there are similar performance needs for
> settimeofday().  Even if it was needed, it shouldn't be more complex
> then the unit conversion done in this abi patch. Am I missing something?

So, you agree on adding new syscalls as a performance tweek?

I am not against it, but I do think syscalls should try to satisfy a
large number of user cases.

> But again, the hard part with in-kernel TAI (possibly as the base of
> time)is that initialization of the TAI/UTC offset needs to be able to be
> phased in slowly, as we also have to preserve legacy interfaces and
> behavior. 

With brand new syscall, there are no legacy uses.

> Why do we need a new interface for TAI? clock_gettime(CLOCK_TAI,...)
> should be achievable. I do think it would be interesting, but I also
> think its separate from the goal of this proposal.

I mean to define an interface that always returns TAI values, no matter
what the clock device.

Richard
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