Re: [RFC][PATCH] new timeofday core subsystem (v.A0)

George Anzinger
2004-09-15T01:01:58Z

Christoph Lameter wrote:
> Here is a rev on the concept of a time source subsystem taking into
> account some of the feedback:
>
> 1. add mask
> 2. rename time source so that its different from xtime
> 3. provide set_time functionality
> 4. add monotonic time support
> 5. Give some example of time sources including a function time source.
>
> note that the original concept already includes the ability to switch time
> sources. I would think that this would be necessary especially if the CPU
> time sources frequency switches or if the system goes into sleep mode.
>
> /****************************************************************
> * A concept for a new timer module
> * V2
> *
> * Christoph Lameter, September 14, 2003
> ***************************************************************/
>
> struct time_source_t {
> u8 type;
> u8 shift;
> u32 multiply;
> void *address;
> u64 mask;
> };
>
> #define TIME_SOURCE_CPU 0
> #define TIME_SOURCE_MMIO32 1
> #define TIME_SOURCE_MMIO64 2
> #define TIME_SOURCE_FUNCTION 15
>
> struct seqlock_t time_lock; /* Protects access to time_base time_source time_mono_offset and time_source_at_base */
> u64 time_base;
> u64 time_mono_offset;
> struct time_source_t *time_source;
> u64 time_source_at_base; /* time source value at time time_base */
>
> u64 inline time_source_get(struct time_source_t *s) {
> switch (s->type) {
> case TIME_SOURCE_CPU : return cycles();
> case TIME_SOURCE_MMIO32 : return readl(s->address);
> case TIME_SOURCE_MMIO64 : return readq(s->address);
> case TIME_SOURCE_FUNCTION: {
> u64 (*x)(void);
> x = s->address;
> return x();
> }
> }
>
> u64 time_source_to_ns(u64 x) {
> return (((x-time_source_at_base) & time_source->mask)*time_source->multiply) >> time_source->shift;
> }

This seems to assume that the time souce is incrementing. On some archs, I
think, it decrements...
>
> inline u64 now(void) {
> u64 t;
> unsigned long seq;
>
> do {
> seq = read_seqbegin(&time_lock);
> t = time_base + time_source_to_ns(time_source_get(time_source));
> } while (unlikely(seq_retry(&time_lock,seq)));
> return t;
> }
>
> #define now_mono() (now()+time_mono_offset)
>
> /* Time adjustment (only possible forward, to go backwards adjust time_source->multiply and wait ...) */
> void time_adjust_skip(u64 ns) {
> u64 c;
>
> write_seqlock(&time_lock);
> c = time_source_get(time_source);
> time_base += ns + time_source_to_ns(c);
> time_source_at_base = c;
> write_sequnlock(&time_lock);
> }

So we would do "time_adjust_skip(0);" to update time_source_at_base?
>
> void time_adjust_slower(void) {
> u64 c;
>
> write_seqlock(&time_lock);
> c = time_source_get(time_source);
> time_base += time_source_to_ns(c);
> time_source_at_base = c;
> time_source->multiply--;
> write_sequnlock(&time_lock);
> }
>
If we do a "good" job of choosing <multiply> and <shift> this will be a "very"
small change. Might be better to pass in a "delta" to change it by. Then you
would only need one function.

> void time_adjust_faster(void) {
> u64 c;
>
> write_seqlock(&time_lock);
> c = time_source_get(time_source);
> time_base += time_source_to_ns(c);
> time_source_at_base = c;
> time_source->multiply++;
> write_sequnlock(&time_unlock);
> }
>
> /* Switch to another time source */
> void new_time_source(struct time_source_t *s) {
> u64 c_old;
> u64 c_new;
>
> write_seqlock(&time_lock);
> c_old = time_source_get(time_source);
> c_new = time_source_get(s);
>
> time_base += time_source_to_ns(c_old);
>
> time_source_at_base = c_new;
> time_source = s;
>
> write_sequnlock(&time_lock);
> }
>
> struct time_source_t *make_time_source(u64 freq, int t,void *a, int bits) {
> struct time_source_t *s = kmalloc(sizeof(struct time_source_t));
>
> s->shift = 64 - bits;
> s->multiply = (NSEC_PER_SEC << s->shift) / freq;
> s->address = a;
> s->type = t;
> s->mask = 1 << bits -1;
> }

The mask and the shift value are not really related. The mask is a function of
the number of bits the hardware provides. The shift is related to the value of
freq. Me thinks they should not be tied together here.
>
> void time_set(u64 ns) {
> u64 c;
>
> write_seqlock(&time_lock);
> c = time_source_get(time_source);
> /* Adjust monotonic time base */
> time_mono_offset += time_base + time_source_to_ns(c) - ns;
> /* Setup new time base */
> time_base = ns;
> time_source_at_base = c;
> write_sequnlock(&time_lock);
> }
>
> void time_init(struct time_source_t *s) {
> {
> write_seqlock(&time_lock);
> time_base = 0;
> time_mono_offset = 0;
> time_source_at_base = time_source_get(s);
> time_source = s;
> write_sequnlock(&time_lock);
> }
>
> /* Values in use in the kernel and how they may be derived from xtime */
> #define jiffies (now()/1000000)

This assumes HZ=1000. (Assuming there is an HZ any more, that is.) Not all
archs will want this value. Possibly:
#define jiffies ((now() * HZ) / 1000000000)

> #define seconds(x) ((x)/1000000000)
> #define nanoseconds_within_second(x) ((x)%1000000000)
> #define microseconds_within_second(x) (nanoseconds_within_second(x) / 1000
>
> u32 time(void) {
> return seconds(now());
> }
>
> void gettimeofday(struct timeval *p) {
> u64 t=now();
>
> p->tv_sec = seconds(t);
> p->tv_usec = microseconds_within_second(t);
> }
>
> void clock_gettime(int clock, struct timespec *p) {
> u64 t=now();
>
> p->tv_sec = seconds(t);
> p->tv_nsec = nanoseconds_within_second(t);
> }
>
> /* Exampe of a CPU time time source */
>
> make_time_source(1200000000, TIME_SOURCE_CPU, NULL, 44);
>
> /* A memory based time source at 20Mhz 55 bits wide */
>
> make_time_source(20000000, TIME_SOURCE_MMIO64, &my_timer_address, 55);
>
> /* Example of handling a difficult time source. In SMP systems the CPU time sources are notoriously difficult to
> * sync. If we have such a problem then insure at least sure that the time source never goes backward.
> */
>
> u64 time_source_last;
>
> u64 get_cpu_time_filtered() {
> u64 x;
> u64 l;
This will need to be "static";
>
> do {
> l = time_source_last;
> x = cycles();
> if (x<l) return l;
> /* the cmpxchg is going to hurt in terms of scalability ! */
> } while (cmpxchg(&time_source_last, l, x) != l);
> return x;
> }
>
> Generate the time_source with
>
> make_time_source(1200000000, TIME_SOURCE_FUNCTION, cpu_time_filtered, 44);
> -

Ok, so now lets hook this up with interval timers:

#define ns_per_jiffie (NSEC_PER_SEC / HZ)
#define jiffies_to_ns(jiff) (jiff * ns_per_jiffie)

This function is a request to interrupt at the next jiffie after the passed
reference jiffie. If that time is passed return true, else false.

int schedule_next_jiffie(unsigned long ref_jiffie)
{
u64 remains;
u64 now_frozen = now();
u32 jiffies_frozen = ((now_frozen * HZ) / 1000000000);

if (jiffies_frozen > ref_jiffies)
return 1;

remains = now_frozen - jiffies_to_ns(ref_jiffie + 1);
/*
* set_timer_to_interrupt(u64 ns) is provided by the arch layer
*/
set_timer_to_interrupt_in(remains);
return 0;
}

On interrupt:

time_adjust_skip(0);
do_timer();

Or, possibly do_timer() does the time_adjust_skip(0);

do_timer() will set up the softirq to run the timer list. The timer list could
could call schedule_next_jiffie, but there are SMP issues here.

--
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