On Tue, 2003-01-21 at 14:59, john stultz wrote:
> All,
> This patch addresses the following problem: Linux cannot properly
> handle the case where interrupts are disabled for longer then two ticks.
>
> Quick background:
> gettimeofday() calculates wall time using roughly the following
> equation: xtime + (jiffies - wall_jiffies) + timer->get_offset()
>
> When a tick is lost, the system is able to compensate short-term because
> even though jiffies is not incremented, timer->get_offset() (which is
> hardware based) continues to increase. However this falls apart, because
> if after 3 or so lost-ticks an interrupt does occur, jiffies is
> incremented only once and we reset timer->get_offset() effectively
> loosing the time we had been compensating for. This causes brief
> inconsistencies in time, in addition to causing system time to drift
> behind that of other systems.
>
> This patch solves the problem by checking when an interrupt occurs if
> timer->get_offset() is a value greater then 2 ticks. If so, it
> increments jiffies appropriately.
>
> Comments, flames and suggestions are requested and welcome.
>
> thanks
> -john
>
> diff -Nru a/arch/i386/kernel/time.c b/arch/i386/kernel/time.c
> --- a/arch/i386/kernel/time.c Tue Jan 21 14:14:18 2003
> +++ b/arch/i386/kernel/time.c Tue Jan 21 14:14:18 2003
> @@ -265,6 +265,21 @@
> #endif
> }
>
> +/*Lost tick detection and compensation*/
> +void detect_lost_tick(void)
> +{
> + /*read time since last interrupt*/
> + unsigned long delta = timer->get_offset();
> +
> + /*check if delta is greater then two ticks*/
> + if(delta > 2*(1000000/HZ)){
> + /*calculate number of missed ticks*/
> + delta = delta/(1000000/HZ)-1;
> + jiffies += delta;
> + }
> +
> +}
> +
> /*
> * This is the same as the above, except we _also_ save the current
> * Time Stamp Counter value at the time of the timer interrupt, so that
> @@ -281,6 +296,7 @@
> */
> write_lock(&xtime_lock);
>
> + detect_lost_tick();
> timer->mark_offset();
>
> do_timer_interrupt(irq, NULL, regs);
>
>
>
> -
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-- Stephen Hemminger <shemminger@osdl.org> Open Source Devlopment Lab- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/