Message-ID: <3AB92AC1.DF054E2E@inet.com>
Date: Wed, 21 Mar 2001 16:27:13 -0600
From: Eli Carter <eli.carter@inet.com>
Organization: Inet Technologies, Inc.
MIME-Version: 1.0
To: Russell King <rmk@arm.linux.org.uk>,
        Jamie Lokier <lk@tantalophile.demon.co.uk>,
        linux-kernel@vger.kernel.org
Subject: Re: gettimeofday question
In-Reply-To: <200103192134.VAA01785@raistlin.arm.linux.org.uk> <3AB927D0.F152717D@inet.com>
Content-Type: text/plain; charset=us-ascii
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Sender: linux-kernel-owner@vger.kernel.org

Eli Carter wrote:
> 
> Russell King wrote:
> >
> > Eli Carter writes:
> > > What are you seeing that I'm missing?
> >
> > Ok, after sitting down and thinking again about this problem, its not
> > the 9.9999ms case, but the 10.000000001 case:
> [snip]
> > Like I say, this requires good timing to create, so may not be too much of
> > a problem, but it does seem to be a problem that could occur.
> 
> It appears that this problem is easier to create than we originally gave
> credit for....  All that is needed is for gettimeoffset() not to be
> called for a _minimum_ of >10ms, and for the timer to wrap during a call
> to do_gettimeofday() or during a period of time where interrupts are
> disabled and do_gettimeofday() is called.  Note that there is no upper
> limit to the time...
> 
> If we call gettimeoffset() after do_timer() returns (and there-by update
> the internal variables every 10ms), we should reduce the impact of this
> bug dramatically (in theory--in practice, disabling interrupts for long
> periods can also have some bad effects that this won't help, but I think
> that's another issue.)
> 
> One of the guys I'm working with did some testing on this, and he was
> seeing this problem (off by 10ms) every 5 to 10 minutes (on a modified
> ARM & kernel).  With the additional gettimeoffset() call, he no longer
> saw it (at least within ~3hrs.).
> 
> Questions, comments, etc.?

Another thought...

If we pull count_p and jiffies_p out of the various *_gettimeoffset()
functions, and added an updatetimeoffset() that only updated count_p and
jiffies_p, (and called it after every do_timer(),) we'd accomplish the
same thing, but with less overhead.

(This is based on looking at the ARM code; I haven't fully studied the
x86 & others.)

Questions, comments, etc?

Eli
-----------------------.           Rule of Accuracy: When working toward
Eli Carter             |            the solution of a problem, it always 
eli.carter(at)inet.com `------------------ helps if you know the answer.
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