linux-kernel - Re: [PATCH] [RFC] Potential fix for leapsecond caused futex related load spikes

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Message-ID: <4FF1D33D.90703@us.ibm.com>
Date:	Mon, 02 Jul 2012 09:58:37 -0700
From:	John Stultz <johnstul@...ibm.com>
To:	Richard Cochran <richardcochran@...il.com>
CC:	Prarit Bhargava <prarit@...hat.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] [RFC] Potential fix for leapsecond caused futex related
 load spikes

On 07/02/2012 03:16 AM, Richard Cochran wrote:
> On Sun, Jul 01, 2012 at 10:28:25AM -0700, John Stultz wrote:
>> Reworking the patch now.
> John,
>
> I know you didn't like my (originally Michael Hack's) idea of keeping
> time in TAI, but wouldn't changing to an internal, continuous time
> scale (not necessary TAI) solve these sorts of timer issues?
So first, I don't think keeping a different time base would have avoided 
this particular issue.
Its really an issue where the hrtimer code has in-effect a cache of 
timekeeping state that, since clock_was_set() wasn't called, didn't get 
updated when we applied the leapsecond.

Second, I'm not opposed to reworking how the internal system keeps track 
of time.  I just wasn't fond of specifics in your implementation (mostly 
around mixing cleanups with behavioural changes).

I wouldn't be opposed to something like:
     CLOCK_TAI = CLOCK_MONOTONIC + monotonic_to_tai
     CLOCK_REALTIME = CLOCK_TAI + tai_to_utc

Also, some of your suggested changes to move some of the NTP state into 
the timekeeper struct made sense as well, but just needed  some slight 
tweaks.

> There have been a number of clock/timer/leap bugs over the last
> years. Some of these might have been avoided by using a continuous
> scale, since no special timer actions would be needed during a leap
> second.
Unfortunately the other issues have been locking related, so I don't 
think changing the internal time scale would have helped.

Regardless, I do hope you rework and resend your proposed changes. 
Clearly we could use more eyes in this area.

thanks
-john


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