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SubjectRe: [PATCH] linux-2.5.67_lost-tick-fix_A2
john stultz wrote:
> 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.

I think that if you look at the generated code you may find that there
are NO div in the asm code. The C folks know about scaling to avoid
div especially when the divisor is a constant :)

-g
>
> 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)
>
>
>
>
>
>

--
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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