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Subject: Re: [PATCH][2.5] hangcheck-timer
From: john stultz <johnstul@us.ibm.com>
To: Joel Becker <Joel.Becker@oracle.com>
Cc: lkml <linux-kernel@vger.kernel.org>
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On Mon, 2003-01-20 at 18:00, Joel Becker wrote:
> On Mon, Jan 20, 2003 at 05:42:16PM -0800, john stultz wrote:
> > get_cycles() is a poor method for determining "real time". 
> > Please use do_gettimeofday().
> 
> 	Does do_gettimeofday() exist on all platforms?  Does it indeed
> give actual wall clock time, instead of the inaccurate time jiffies can
> give?

Yep, do_gettimeofday is called from generic code in sys_gettimeofday()
(kernel/time.c). It returns the same value userspace code would see
calling gettimeofday(). 

> > >  +      if (tsc_diff > hangcheck_tsc_margin) {
> > 
> > but now we're using it to compare cycles!  180sec != 180 cycles
> 
> 	Look at the calculations.  I'm comparing cycles to cycles,
> calculated from the original seconds.

Ah! Ok, I missed the conversion in hangcheck_init. Even so, the default
initializer is misleading. Yea, that's it... :)

> > Additionally, this code doesn't take systems that have unsync'ed TSCs,
> > or systems that change cpu frequency into account. Again, please use
> > do_gettimeofday(). Then you can then talk about the values returned in
> > secs and usecs, and I believe things will be much more clear. 
> 
> 	I'll look into it, but it must absolutely be in terms of wall
> clock time as measured from outside the system.

Completely understandable. do_gettimeofday will give you just that (w/o
the conversion muck w/ HZ and loops_per_jiffy). 

thanks
-john

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