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SubjectRe: [RFC] possible erronous use of tick_usec in do_gettimeofday
Joe Korty wrote:
> On Tue, Nov 25, 2003 at 11:57:55AM -0800, George Anzinger wrote:
>
>>Joe Korty wrote:
>>
>>>test10's version of do_gettimeofday is using tick_usec which is
>>>defined in terms of USER_HZ not HZ.
>>
>>We still have the problem that we are doing this calculation in usecs while
>>the wall clock uses nsecs. This would be fine if there were an even number
>>of usecs in tick_nsec, but in fact it is somewhat less than (USEC_PER_SEC /
>>HZ). This means that this correction (if we are behind by 7 or more ticks)
>>will push the clock past current time. Here are the numbers:
>>
>>tick_nsec =999849 or 1ms less 151 ns. So if we are behind 7 or more ticks
>>we will report the time out 1 us too high. (7 * 151 = 1057 or 1.057 usec).
>>
>>Question is, do we care? Will we ever be 7ms late in updating the wall
>>clock? As I recall, the wall clock is updated in the interrupt handler for
>>the tick so, to be this late, we would need to suffer a long interrupt hold
>>off AND the tick recovery code would need to have done its thing. But this
>>whole time is covered by a write_seqlock on xtime_lock, so how can this
>>even happen? Seems like it is only possible when we are locked and we then
>>throw the whole thing away.
>>
>>A test I would like to see is to put this in the code AFTER the read unlock:
>>
>>if (lost )
>> printk("Lost is %d\n", lost);
>>
>>(need to pull " unsigned long lost;" out of the do{}while loop to do
>>this)
>>
>>In short, I think we are beating a dead issue.
>
>
> There are other issues too: the 'lost' calculation is a prediction
> over the next 'lost' number of ticks. That prediction will be wrong
> if 1) adjtime goes to zero within that interval or, 2) adjtime was
> zero but went nonzero in that interval due to a adjtimex(2) call.
>
> Despite these flaws the patch replaces truly broken code with code
> that is good but slightly inaccurate, which is good enough for now.

Can you prove that "lost" is EVER non-zero in a case we care about? I.e. a case
where the read_seq will exit the loop?

I could be wrong here, but I don't think it can happen. That is why I suggested
the if(lost) test.

--
George Anzinger george@mvista.com
High-res-timers: http://sourceforge.net/projects/high-res-timers/
Preemption patch: http://www.kernel.org/pub/linux/kernel/people/rml

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