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Subject[PATCH] linux-2.5.67_lost-tick-fix_A2
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Andrew, All,

This patch fixes a race in the timer_interrupt code caused by
detect_lost_tick(). Since we're doing lost-tick compensation outside
timer->mark_offset, time can pass between time-source reads which can
cause gettimeofday inconsistencies.

Additionally detect_lost_tick() was broken for the PIT case, since the
whole point of detect_lost_tick() is to interpolate between two time
sources to find inconsistencies. Additionally this could cause
xtime_lock seq_lock reader starvation which has been causing machine
hangs for SMP boxes that use the PIT as a time source.

This patch fixes the described race by removing detect_lost_tick() and
instead implementing the lost tick detection code inside mark_offset().

Some of the divs and mods being added here might concern folks, but by
not calling timer->get_offset() in detect_lost_tick() we eliminate much
of the same math. I did some simple cycle counting and the new code
comes out on average equivalent or faster.

Please consider for inclusion.

thanks
-john


diff -Nru a/arch/i386/kernel/time.c b/arch/i386/kernel/time.c
--- a/arch/i386/kernel/time.c Wed Apr 16 14:53:59 2003
+++ b/arch/i386/kernel/time.c Wed Apr 16 14:53:59 2003
@@ -279,41 +279,6 @@
}

/*
- * Lost tick detection and compensation
- */
-static inline void detect_lost_tick(void)
-{
- /* read time since last interrupt */
- unsigned long delta = timer->get_offset();
- static unsigned long dbg_print;
-
- /* check if delta is greater then two ticks */
- if(delta >= 2*(1000000/HZ)){
-
- /*
- * only print debug info first 5 times
- */
- /*
- * AKPM: disable this for now; it's nice, but irritating.
- */
- if (0 && dbg_print < 5) {
- printk(KERN_WARNING "\nWarning! Detected %lu "
- "micro-second gap between interrupts.\n",
- delta);
- printk(KERN_WARNING " Compensating for %lu lost "
- "ticks.\n",
- delta/(1000000/HZ)-1);
- dump_stack();
- dbg_print++;
- }
- /* 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
* we later on can estimate the time of day more exactly.
@@ -329,7 +294,6 @@
*/
write_seqlock(&xtime_lock);

- detect_lost_tick();
timer->mark_offset();

do_timer_interrupt(irq, NULL, regs);
diff -Nru a/arch/i386/kernel/timers/timer_cyclone.c b/arch/i386/kernel/timers/timer_cyclone.c
--- a/arch/i386/kernel/timers/timer_cyclone.c Wed Apr 16 14:53:59 2003
+++ b/arch/i386/kernel/timers/timer_cyclone.c Wed Apr 16 14:53:59 2003
@@ -18,6 +18,7 @@
#include <asm/fixmap.h>

extern spinlock_t i8253_lock;
+extern unsigned long jiffies;
extern unsigned long calibrate_tsc(void);

/* Number of usecs that the last interrupt was delayed */
@@ -46,6 +47,8 @@

static void mark_offset_cyclone(void)
{
+ unsigned long lost, delay;
+ unsigned long delta = last_cyclone_low;
int count;
unsigned long long this_offset, last_offset;

@@ -62,6 +65,15 @@
count |= inb(0x40) << 8;
spin_unlock(&i8253_lock);

+ /* lost tick compensation */
+ delta = last_cyclone_low - delta;
+ delta /=(CYCLONE_TIMER_FREQ/1000000);
+ delta += delay_at_last_interrupt;
+ lost = delta/(1000000/HZ);
+ delay = delta%(1000000/HZ);
+ if(lost >= 2)
+ jiffies += lost-1;
+
/* update the monotonic base value */
this_offset = ((unsigned long long)last_cyclone_high<<32)|last_cyclone_low;
monotonic_base += (this_offset - last_offset) & CYCLONE_TIMER_MASK;
@@ -70,6 +82,12 @@
/* calculate delay_at_last_interrupt */
count = ((LATCH-1) - count) * TICK_SIZE;
delay_at_last_interrupt = (count + LATCH/2) / LATCH;
+
+ /* catch corner case where tick rollover
+ * occured between cyclone and pit reads
+ */
+ if(abs(delay - delay_at_last_interrupt) > 900)
+ jiffies++;
}

static unsigned long get_offset_cyclone(void)
diff -Nru a/arch/i386/kernel/timers/timer_tsc.c b/arch/i386/kernel/timers/timer_tsc.c
--- a/arch/i386/kernel/timers/timer_tsc.c Wed Apr 16 14:53:59 2003
+++ b/arch/i386/kernel/timers/timer_tsc.c Wed Apr 16 14:53:59 2003
@@ -18,6 +18,7 @@
int tsc_disable __initdata = 0;

extern spinlock_t i8253_lock;
+extern unsigned long jiffies;

static int use_tsc;
/* Number of usecs that the last interrupt was delayed */
@@ -112,6 +113,8 @@

static void mark_offset_tsc(void)
{
+ unsigned long lost,delay;
+ unsigned long delta = last_tsc_low;
int count;
int countmp;
static int count1=0, count2=LATCH;
@@ -156,6 +159,23 @@
}
}

+ /* lost tick compensation */
+ delta = last_tsc_low - delta;
+ {
+ register unsigned long eax, edx;
+ eax = delta;
+ __asm__("mull %2"
+ :"=a" (eax), "=d" (edx)
+ :"rm" (fast_gettimeoffset_quotient),
+ "0" (eax));
+ delta = edx;
+ }
+ delta += delay_at_last_interrupt;
+ lost = delta/(1000000/HZ);
+ delay = delta%(1000000/HZ);
+ if(lost >= 2)
+ jiffies += lost-1;
+
/* update the monotonic base value */
this_offset = ((unsigned long long)last_tsc_high<<32)|last_tsc_low;
monotonic_base += cycles_2_ns(this_offset - last_offset);
@@ -164,6 +184,12 @@
/* calculate delay_at_last_interrupt */
count = ((LATCH-1) - count) * TICK_SIZE;
delay_at_last_interrupt = (count + LATCH/2) / LATCH;
+
+ /* catch corner case where tick rollover
+ * occured between tsc and pit reads
+ */
+ if(abs(delay - delay_at_last_interrupt) > 900)
+ jiffies++;
}

static void delay_tsc(unsigned long loops)




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