linux-kernel - Re: [RFC patch 7/8] timekeeping: Implement a shadow timekeeper

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Date:	Tue, 26 Feb 2013 13:17:07 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	John Stultz <john.stultz@...aro.org>
cc:	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	Eric Dumazet <dada1@...mosbay.com>,
	Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: [RFC patch 7/8] timekeeping: Implement a shadow timekeeper

On Fri, 22 Feb 2013, John Stultz wrote:

> On 02/21/2013 02:51 PM, Thomas Gleixner wrote:
> > Use the shadow timekeeper to do the update_wall_time() adjustments and
> > then copy it over to the real timekeeper.
> > 
> > Keep the shadow timekeeper in sync when updating stuff outside of
> > update_wall_time().
> > 
> > This allows us to limit the timekeeper_seq hold time to the update of
> > the real timekeeper and the vsyscall data in the next patch.
> > 
> > Signed-off-by: Thomas Gleixner<tglx@...utronix.de>
> > ---
> 
> So up to here it all looks ok to me (and not so different from my earlier
> attempts at the same).
> 
> The only gotcha here that I realized with my earlier patches, is that in order
> to do the shadow copy update properly, we are also going to need to merge the
> NTP state data into the timekeeper. Otherwise, we could run into odd cases
> where as we update the shadow copy, we change the NTP state which then would
> affect the non-shadow timekeeping state that is about to be updated. One
> example: A the leap second lands, and the tai offset gets bumped in the ntp
> state, while we do a similar counter adjustment to the shadow-copy. Then
> before the real/active timekeeper is updated, someone gets the tai offset and
> applies it to that pre-update timekeeper state, and gets an invalid tai time.
> 
> The down side is that the NTP state data is fairly large, and so adding it to
> the timekeeper will cause the memcopys to be a bit more painful.
> 
> I'm looking at the NTP code now to try to see if we can bound where the NTP
> state is accessed, so we can maybe thin out what ntp state is linked to
> timekeeper updates, and only move that data over to the timekeeper.

Hmm. Can we block the NTP data readout while we are doing the update ?

Thanks,

	tglx
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