[RFC][PATCH] linux-2.5.59_lost-tick_A0

From: john stultz (johnstul@us.ibm.com)
Date: Tue Jan 21 2003 - 17:59:22 EST

        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.


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 @@
+/*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 @@
+ detect_lost_tick();
         do_timer_interrupt(irq, NULL, regs);

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