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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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