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SubjectRe: [RFC][PATCH] linux-2.5.64_monotonic-clock_A1
john,

Some comments below on the scaling.

On a related note, I would like to extend the CLOCK_MONOTONIC code to
the same res as CLOCK_REALTIME in the POSIX clocks and timers patch.
The patch uses jiffies_64 for CLOCK_MONOTONIC, so what I would like to
do is use get_offset() to fill in the sub_jiffies part. Is this
function available (i.e. timer->get_offset()) on all archs?

It seems to me that the lost jiffies should be rolled into
get_offset(). Have you considered doing this?

-g

john stultz wrote:
> <sigh> Patch below.
>
> thanks
> -john
>
>
> diff -Nru a/arch/i386/kernel/time.c b/arch/i386/kernel/time.c
> --- a/arch/i386/kernel/time.c Tue Mar 11 11:24:04 2003
> +++ b/arch/i386/kernel/time.c Tue Mar 11 11:24:04 2003
> @@ -138,6 +138,19 @@
> clock_was_set();
> }
>
> +unsigned long long monotonic_clock(void)
> +{
> + unsigned long long ret;
> + unsigned long seq;
> + do {
> + seq = read_seqbegin(&xtime_lock);
> + ret = timer->monotonic_clock();
> + } while (read_seqretry(&xtime_lock, seq));
> + return ret;
> +}
> +EXPORT_SYMBOL(monotonic_clock);
> +
> +
> /*
> * In order to set the CMOS clock precisely, set_rtc_mmss has to be
> * called 500 ms after the second nowtime has started, because when
> 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 Tue Mar 11 11:24:04 2003
> +++ b/arch/i386/kernel/timers/timer_cyclone.c Tue Mar 11 11:24:04 2003
> @@ -27,19 +27,24 @@
> #define CYCLONE_MPMC_OFFSET 0x51D0
> #define CYCLONE_MPCS_OFFSET 0x51A8
> #define CYCLONE_TIMER_FREQ 100000000
> -
> +#define CYCLONE_TIMER_MASK (((u64)1<<40)-1) /*40 bit mask*/
> int use_cyclone = 0;
>
> static u32* volatile cyclone_timer; /* Cyclone MPMC0 register */
> -static u32 last_cyclone_timer;
> +static u32 last_cyclone_low;
> +static u32 last_cyclone_high;
> +static unsigned long long monotonic_base;
>
> static void mark_offset_cyclone(void)
> {
> int count;
> + unsigned long long this_offset, last_offset;
> + last_offset = ((unsigned long long)last_cyclone_high<<32)|last_cyclone_low;
> +
> spin_lock(&i8253_lock);
> /* quickly read the cyclone timer */
> - if(cyclone_timer)
> - last_cyclone_timer = cyclone_timer[0];
> + last_cyclone_high = cyclone_timer[1];
> + last_cyclone_low = cyclone_timer[0];
>
> /* calculate delay_at_last_interrupt */
> outb_p(0x00, 0x43); /* latch the count ASAP */
> @@ -50,6 +55,10 @@
>
> count = ((LATCH-1) - count) * TICK_SIZE;
> delay_at_last_interrupt = (count + LATCH/2) / LATCH;
> +
> + /* 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;
> }
>
> static unsigned long get_offset_cyclone(void)
> @@ -63,7 +72,7 @@
> offset = cyclone_timer[0];
>
> /* .. relative to previous jiffy */
> - offset = offset - last_cyclone_timer;
> + offset = offset - last_cyclone_low;
>
> /* convert cyclone ticks to microseconds */
> /* XXX slow, can we speed this up? */
> @@ -73,6 +82,21 @@
> return delay_at_last_interrupt + offset;
> }
>
> +static unsigned long long monotonic_clock_cyclone(void)
> +{
> +
> + u32 now_low = cyclone_timer[0];
> + u32 now_high = cyclone_timer[1];
> + unsigned long long last_offset, this_offset;
> + unsigned long long ret;
> + last_offset = ((unsigned long long)last_cyclone_high<<32)|last_cyclone_low;
> + this_offset = ((unsigned long long)now_high<<32)|now_low;
> +
> + ret = monotonic_base + ((this_offset - last_offset)&CYCLONE_TIMER_MASK);
> + ret = ret * (1000000000 / CYCLONE_TIMER_FREQ);
> + return ret;
> +}
> +
> static int init_cyclone(void)
> {
> u32* reg;
> @@ -190,5 +214,6 @@
> .init = init_cyclone,
> .mark_offset = mark_offset_cyclone,
> .get_offset = get_offset_cyclone,
> + .monotonic_clock = monotonic_clock_cyclone,
> .delay = delay_cyclone,
> };
> diff -Nru a/arch/i386/kernel/timers/timer_none.c b/arch/i386/kernel/timers/timer_none.c
> --- a/arch/i386/kernel/timers/timer_none.c Tue Mar 11 11:24:04 2003
> +++ b/arch/i386/kernel/timers/timer_none.c Tue Mar 11 11:24:04 2003
> @@ -15,6 +15,11 @@
> return 0;
> }
>
> +static unsigned long long monotonic_clock_none(void)
> +{
> + return 0;
> +}
> +
> static void delay_none(unsigned long loops)
> {
> int d0;
> @@ -33,5 +38,6 @@
> .init = init_none,
> .mark_offset = mark_offset_none,
> .get_offset = get_offset_none,
> + .monotonic_clock = monotonic_clock_none,
> .delay = delay_none,
> };
> diff -Nru a/arch/i386/kernel/timers/timer_pit.c b/arch/i386/kernel/timers/timer_pit.c
> --- a/arch/i386/kernel/timers/timer_pit.c Tue Mar 11 11:24:04 2003
> +++ b/arch/i386/kernel/timers/timer_pit.c Tue Mar 11 11:24:04 2003
> @@ -27,6 +27,11 @@
> /* nothing needed */
> }
>
> +static unsigned long long monotonic_clock_pit(void)
> +{
> + return 0;
> +}
> +
> static void delay_pit(unsigned long loops)
> {
> int d0;
> @@ -141,5 +146,6 @@
> .init = init_pit,
> .mark_offset = mark_offset_pit,
> .get_offset = get_offset_pit,
> + .monotonic_clock = monotonic_clock_pit,
> .delay = delay_pit,
> };
> 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 Tue Mar 11 11:24:04 2003
> +++ b/arch/i386/kernel/timers/timer_tsc.c Tue Mar 11 11:24:04 2003
> @@ -23,6 +23,38 @@
> static int delay_at_last_interrupt;
>
> static unsigned long last_tsc_low; /* lsb 32 bits of Time Stamp Counter */
> +static unsigned long last_tsc_high; /* msb 32 bits of Time Stamp Counter */
> +static unsigned long long monotonic_base;
> +
> +
> +/* convert from cycles(64bits) => nanoseconds (64bits)
> + * basic equation:
> + * ns = cycles / (freq / ns_per_sec)
> + * ns = cycles * (ns_per_sec / freq)
> + * ns = cycles * (10^9 / (cpu_mhz * 10^6))
> + * ns = cycles * (10^3 / cpu_mhz)
> + *
> + * Then we use scaling math (suggested by george@mvista.com) to get:
> + * ns = cycles * (10^3 * SC / cpu_mhz) / SC
> + * ns = cycles * cyc2ns_scale / SC
> + *
> + * And since SC is a constant power of two, we can convert the div
> + * into a shift.
> + * -johnstul@us.ibm.com "math is hard, lets go shopping!"
> + */
> +static unsigned long cyc2ns_scale;
> +#define CYC2NS_SCALE_FACTOR 10 /* 2^10, carefully chosen*/
> +
> +static inline set_cyc2ns_scale(unsigned long cpu_mhz)
> +{
> + cyc2ns_scale = (1000 * (1<<CYC2NS_SCALE_FACTOR))/cpu_mhz;
The function would be:
div_sc_n(const N, unsigned long a, unsigned long b);
returns (a << N) / b

The only advantage to this would be the ability to handle a u64 as a
result of the (a<<b) (assumes the div moves all significant bits to a
u32).
+
> +}
> +
> +static inline unsigned long long cycles_2_ns(unsigned long long cyc)
> +{
> + return (cyc * cyc2ns_scale) >> CYC2NS_SCALE_FACTOR;

I think this is a problem. cyc is u64 so you are doing a 64 bit mpy
which C will do by draging in the 64-bit math lib, something we don't
want in the kernel.

Since you want u64=u64*u32 the best the package can do is:
mpy_ll_X_l_ll(unsigned long long mpy1, unsigned long mpy2);
returns (unsigned long long)(mpy1 * mpy2)

This requires two mpy in the best of cases. The actual code leans on
the mpy_l_X_l_ll() which does u64=u32*u32. Here is what it looks like
in sc_math.h:
mpy_ll_X_l_ll(unsigned long long mpy1, unsigned long mpy2)
{
unsigned long long result = mpy_l_X_l_ll((unsigned long)mpy1, mpy2);
result += (mpy_l_X_l_ll((long)(mpy1 >> 32), mpy2) << 32);
return result;
}



> +}
> +
>
> /* Cached *multiplier* to convert TSC counts to microseconds.
> * (see the equation below).
> @@ -60,11 +92,25 @@
> return delay_at_last_interrupt + edx;
> }
>
> +static unsigned long long monotonic_clock_tsc(void)
> +{
> + unsigned long long last_offset, this_offset;
> + last_offset = ((unsigned long long)last_tsc_high<<32)|last_tsc_low;
> +
> + /* Read the Time Stamp Counter */
> + rdtscll(this_offset);
> +
> + /* return the value in ns */
> + return monotonic_base + cycles_2_ns(this_offset - last_offset);
> +}
> +
> static void mark_offset_tsc(void)
> {
> int count;
> int countmp;
> static int count1=0, count2=LATCH;
> + unsigned long long this_offset, last_offset;
> + last_offset = ((unsigned long long)last_tsc_high<<32)|last_tsc_low;
> /*
> * It is important that these two operations happen almost at
> * the same time. We do the RDTSC stuff first, since it's
> @@ -79,7 +125,7 @@
>
> /* read Pentium cycle counter */
>
> - rdtscl(last_tsc_low);
> + rdtsc(last_tsc_low, last_tsc_high);
>
> spin_lock(&i8253_lock);
> outb_p(0x00, 0x43); /* latch the count ASAP */
> @@ -104,6 +150,11 @@
>
> count = ((LATCH-1) - count) * TICK_SIZE;
> delay_at_last_interrupt = (count + LATCH/2) / LATCH;
> +
> + /* 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);
> +
> }
>
> static void delay_tsc(unsigned long loops)
> @@ -293,6 +344,7 @@
> "0" (eax), "1" (edx));
> printk("Detected %lu.%03lu MHz processor.\n", cpu_khz / 1000, cpu_khz % 1000);
> }
> + set_cyc2ns_scale(cpu_khz/1000);
> return 0;
> }
> }
> @@ -326,5 +378,6 @@
> .init = init_tsc,
> .mark_offset = mark_offset_tsc,
> .get_offset = get_offset_tsc,
> + .monotonic_clock = monotonic_clock_tsc,
> .delay = delay_tsc,
> };
> diff -Nru a/drivers/char/hangcheck-timer.c b/drivers/char/hangcheck-timer.c
> --- a/drivers/char/hangcheck-timer.c Tue Mar 11 11:24:04 2003
> +++ b/drivers/char/hangcheck-timer.c Tue Mar 11 11:24:04 2003
> @@ -78,11 +78,13 @@
> static struct timer_list hangcheck_ticktock =
> TIMER_INITIALIZER(hangcheck_fire, 0, 0);
>
> +extern unsigned long long monotonic_clock(void);
> +
> static void hangcheck_fire(unsigned long data)
> {
> unsigned long long cur_tsc, tsc_diff;
>
> - cur_tsc = get_cycles();
> + cur_tsc = monotonic_clock();
>
> if (cur_tsc > hangcheck_tsc)
> tsc_diff = cur_tsc - hangcheck_tsc;
> @@ -98,7 +100,7 @@
> }
> }
> mod_timer(&hangcheck_ticktock, jiffies + (hangcheck_tick*HZ));
> - hangcheck_tsc = get_cycles();
> + hangcheck_tsc = monotonic_clock();
> }
>
>
> @@ -108,10 +110,10 @@
> VERSION_STR, hangcheck_tick, hangcheck_margin);
>
> hangcheck_tsc_margin = hangcheck_margin + hangcheck_tick;
> - hangcheck_tsc_margin *= HZ;
> - hangcheck_tsc_margin *= current_cpu_data.loops_per_jiffy;
> + hangcheck_tsc_margin *= 1000000000;
> +
>
> - hangcheck_tsc = get_cycles();
> + hangcheck_tsc = monotonic_clock();
> mod_timer(&hangcheck_ticktock, jiffies + (hangcheck_tick*HZ));
>
> return 0;
> diff -Nru a/include/asm-i386/timer.h b/include/asm-i386/timer.h
> --- a/include/asm-i386/timer.h Tue Mar 11 11:24:04 2003
> +++ b/include/asm-i386/timer.h Tue Mar 11 11:24:04 2003
> @@ -14,6 +14,7 @@
> int (*init)(void);
> void (*mark_offset)(void);
> unsigned long (*get_offset)(void);
> + unsigned long long (*monotonic_clock)(void);
> void (*delay)(unsigned long);
> };
>
>
>
>
>

--
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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