linux-kernel - Re: [PATCH 0/6][RFC] Rework vsyscall to avoid truncation/rounding issue in timekeeping core

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Message-ID: <20120918180200.GA2281@netboy.at.omicron.at>
Date:	Tue, 18 Sep 2012 20:02:00 +0200
From:	Richard Cochran <richardcochran@...il.com>
To:	John Stultz <john.stultz@...aro.org>
Cc:	Andy Lutomirski <luto@...capital.net>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Tony Luck <tony.luck@...el.com>,
	Paul Mackerras <paulus@...ba.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Paul Turner <pjt@...gle.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Prarit Bhargava <prarit@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 0/6][RFC] Rework vsyscall to avoid truncation/rounding
 issue in timekeeping core

On Mon, Sep 17, 2012 at 05:20:41PM -0700, John Stultz wrote:
> On 09/17/2012 04:49 PM, Andy Lutomirski wrote:
> >2. There's nothing vsyscall-specific about the code in
> >vclock_gettime.c.  In fact, the VVAR macro should work just fine in
> >kernel code.  If you moved all this code into a header, then in-kernel
> >uses could use it, and maybe even other arches could use it.  Last
> >time I checked, it seemed like vclock_gettime was considerably faster
> >than whatever the in-kernel equivalent did.
> I like the idea of unifying the implementations, but I'd want to
> know more about why vclock_gettime was faster then the in-kernel
> getnstimeofday(), since it might be due to the more limited locking
> (we only update vsyscall data under the vsyscall lock, where as the
> timekeeper lock is held for the entire execution of
> update_wall_time()), or some of the optimizations in the vsyscall
> code is focused on providing timespecs to userland, where as
> in-kernel we also have to provide ktime_ts.

This there a valid technical reason why each arch has its own vdso
implementation?

If not, I would suggest that the first step would be to refactor these
into one C-language header. If this can be shared with kernel code,
then all the better.

It would make it a lot easier to fix the leap second thing, too.

Thanks,
Richard

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