Re: /proc or ps tools bug? 2.6.3, time is off

[john stultz]

On Mon, 2004-05-03 at 23:12, Tim Schmielau wrote:
> On Mon, 3 May 2004, john stultz wrote:
> > 	This patch polishes up Tim Schmielau's (tim@xxxxxxxxxxxxxxxxxxxxxx) fix
> > for jiffies_to_clock_t() and jiffies_64_to_clock_t(). The issues
> > observed was w/ /proc output not matching up to wall time due to
> > accumulated error caused by HZ not being exactly 1000 on i386 systems.
> > The solution is to correct that error by using the more accurate
> > TICK_NSEC in our calculation. 
> 
> I wonder whether it's conceptually correct to use jiffies for accurate 
> long-time measurements at all. ntpd is there for a reason. Using both
> corrected, accurate and freely running clocks IMHO is calling for trouble. 
> This might be something to think about for 2.7.

Indeed. Moving away from jiffies as a time counter and more of an
interrupt counter is important. That allows for implementations of
variable HZ and other things the high-res timer folks want without
affecting the time keeping code. 

Roughly, I'd like to see the time code for all arches in 2.7 to look
like:

u64 system_time 	/* NTP adjusted nanosecs since boot */
u64 wall_time_offset	/* offset to system_time for time of day */
u64 offset_base		/* last read raw hw value */

ts_read(): 
        returns the raw cycle value from the hardware timesource
        (TSC/ACPI PM/HPET)
ts_delta(now, then):
        returns the difference between two raw cycle values
ts_cyc2ns(cycles):
        converts a cycle value to ns

monotonic_clock(): 
        returns NTP adjusted nanoseconds since boot
        ie: system_time +
        	NTP_GUNK(ts_cyc2ns(ts_delta(ts_read(),offset_base)))
gettimeofday(): 
        returns monotonic_clock() + sys_time_offset
settimeofday(): 
        adjusts only sys_time_offset
time_interrupt_hook(): 
        updates system_time. called by timer interrupt atleast once
        every hardware cycle (ie: before the hardware counter
        overflows), but otherwise unaffected by lost interrupts, etc.
        ie: 
                then = offset_base
                now = ts_read()
                system_time += NTP_GUNK(ts_cyc2ns(ts_delta(now, then)));
                DO_MORE_NTP_GUNK()

And ignoring the magic NTP_GUNK macros, that's all there is to it
(Although don't kid your self, the NTP_GUNK is nasty). 

Of course, with this approach, we actually have to be able to trust the
hardware 100%. With the current state of i386 hw having serious problems
w/ reliable timesources, this may be difficult. 

I've got a bigger proposal (with proper credits to Keith Mannthey and
George Anzinger for reviews and corrections) that I wrote up awhile
back, and I'll likely send it out if this sketch gathers any interest.  

thanks
-john

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