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[PARISC] Further updates to timer_interrupt() 

This version (relative to the current tree):
o eliminates "while (ticks_elapsed)" loop. It's not needed.
o drop "ticks_elapsed" completely from timer_interrupt().
o Estimates elapsed cycles (based on HZ) to see which kind of
  math we want to use to calculate "cycles_remainder".
o Fixes a bug where we would loose a tick if we decided
  we wanted to skip one interrupt.

Signed-off-by: Grant Grundler <grundler@parisc-linux.org>
Signed-off-by: Kyle McMartin <kyle@parisc-linux.org>
This commit is contained in:
Grant Grundler 2006-09-10 12:57:55 -07:00 committed by Matthew Wilcox
parent 6b799d9222
commit 6e5dc42b5a
1 changed files with 65 additions and 62 deletions
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127
arch/parisc/kernel/time.c
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@ -43,12 +43,11 @@ irqreturn_t timer_interrupt(int irq, void *dev_id, struct pt_regs *regs)
unsigned long now;
unsigned long next_tick;
unsigned long cycles_elapsed;
unsigned long cycles_remainder;
unsigned long ticks_elapsed = 1; /* at least one elapsed */
int cpu = smp_processor_id();
unsigned long cycles_remainder;
unsigned int cpu = smp_processor_id();
/* gcc can optimize for "read-only" case with a local clocktick */
unsigned long local_ct = clocktick;
unsigned long cpt = clocktick;
profile_tick(CPU_PROFILING, regs);
@ -63,28 +62,16 @@ irqreturn_t timer_interrupt(int irq, void *dev_id, struct pt_regs *regs)
cycles_elapsed = now - next_tick;
/* Determine how much time elapsed. */
if (now < next_tick) {
/* Scenario 2: CR16 wrapped after clock tick.
* 1's complement will give us the "elapse cycles".
*
* This "cr16 wrapped" cruft is primarily for 32-bit kernels.
* So think "unsigned long is u32" when reading the code.
* And yes, of course 64-bit will someday wrap, but only
* every 198841 days on a 1GHz machine.
if ((cycles_elapsed >> 5) < cpt) {
/* use "cheap" math (add/subtract) instead
* of the more expensive div/mul method
*/
cycles_elapsed = ~cycles_elapsed; /* off by one cycle - don't care */
}
if (likely(cycles_elapsed < local_ct)) {
/* ticks_elapsed = 1 -- We already assumed one tick elapsed. */
cycles_remainder = cycles_elapsed;
while (cycles_remainder > cpt) {
cycles_remainder -= cpt;
}
} else {
/* more than one tick elapsed. Do "expensive" math. */
ticks_elapsed += cycles_elapsed / local_ct;
/* Faster version of "remainder = elapsed % clocktick" */
cycles_remainder = cycles_elapsed - (ticks_elapsed * local_ct);
cycles_remainder = cycles_elapsed % cpt;
}
/* Can we differentiate between "early CR16" (aka Scenario 1) and
@ -94,51 +81,65 @@ irqreturn_t timer_interrupt(int irq, void *dev_id, struct pt_regs *regs)
* cycles after the IT fires. But it's arbitrary how much time passes
* before we call it "late". I've picked one second.
*/
if (ticks_elapsed > HZ) {
/* aproximate HZ with shifts. Intended math is "(elapsed/clocktick) > HZ" */
#if HZ == 1000
if (cycles_elapsed > (cpt << 10) )
#elif HZ == 250
if (cycles_elapsed > (cpt << 8) )
#elif HZ == 100
if (cycles_elapsed > (cpt << 7) )
#else
#warn WTF is HZ set to anyway?
if (cycles_elapsed > (HZ * cpt) )
#endif
{
/* Scenario 3: very long delay? bad in any case */
printk (KERN_CRIT "timer_interrupt(CPU %d): delayed!"
" ticks %ld cycles %lX rem %lX"
" cycles %lX rem %lX "
" next/now %lX/%lX\n",
cpu,
ticks_elapsed, cycles_elapsed, cycles_remainder,
cycles_elapsed, cycles_remainder,
next_tick, now );
}
/* convert from "division remainder" to "remainder of clock tick" */
cycles_remainder = cpt - cycles_remainder;
/* Determine when (in CR16 cycles) next IT interrupt will fire.
* We want IT to fire modulo clocktick even if we miss/skip some.
* But those interrupts don't in fact get delivered that regularly.
*/
next_tick = now + (local_ct - cycles_remainder);
next_tick = now + cycles_remainder;
cpu_data[cpu].it_value = next_tick;
/* Skip one clocktick on purpose if we are likely to miss next_tick.
* We'll catch what we missed on the tick after that.
* We should never need 0x1000 cycles to read CR16, calc the
* new next_tick, then write CR16 back. */
if (!((local_ct - cycles_remainder) >> 12))
next_tick += local_ct;
* We want to avoid the new next_tick being less than CR16.
* If that happened, itimer wouldn't fire until CR16 wrapped.
* We'll catch the tick we missed on the tick after that.
*/
if (!(cycles_remainder >> 13))
next_tick += cpt;
/* Program the IT when to deliver the next interrupt. */
/* Only bottom 32-bits of next_tick are written to cr16. */
cpu_data[cpu].it_value = next_tick;
mtctl(next_tick, 16);
/* Now that we are done mucking with unreliable delivery of interrupts,
* go do system house keeping.
/* Done mucking with unreliable delivery of interrupts.
* Go do system house keeping.
*/
while (ticks_elapsed--) {
#ifdef CONFIG_SMP
smp_do_timer(regs);
smp_do_timer(regs);
#else
update_process_times(user_mode(regs));
update_process_times(user_mode(regs));
#endif
if (cpu == 0) {
write_seqlock(&xtime_lock);
do_timer(1);
write_sequnlock(&xtime_lock);
}
if (cpu == 0) {
write_seqlock(&xtime_lock);
do_timer(regs);
write_sequnlock(&xtime_lock);
}
/* check soft power switch status */
if (cpu == 0 && !atomic_read(&power_tasklet.count))
tasklet_schedule(&power_tasklet);
@ -164,14 +165,12 @@ unsigned long profile_pc(struct pt_regs *regs)
EXPORT_SYMBOL(profile_pc);
/*** converted from ia64 ***/
/*
* Return the number of micro-seconds that elapsed since the last
* update to wall time (aka xtime). The xtime_lock
* must be at least read-locked when calling this routine.
*/
static inline unsigned long
gettimeoffset (void)
static inline unsigned long gettimeoffset (void)
{
#ifndef CONFIG_SMP
/*
@ -185,36 +184,40 @@ gettimeoffset (void)
unsigned long elapsed_cycles;
unsigned long usec;
unsigned long cpuid = smp_processor_id();
unsigned long local_ct = clocktick;
unsigned long cpt = clocktick;
next_tick = cpu_data[cpuid].it_value;
now = mfctl(16); /* Read the hardware interval timer. */
prev_tick = next_tick - local_ct;
prev_tick = next_tick - cpt;
/* Assume Scenario 1: "now" is later than prev_tick. */
elapsed_cycles = now - prev_tick;
if (now < prev_tick) {
/* Scenario 2: CR16 wrapped!
* ones complement is off-by-one. Don't care.
*/
elapsed_cycles = ~elapsed_cycles;
}
if (elapsed_cycles > (HZ * local_ct)) {
/* aproximate HZ with shifts. Intended math is "(elapsed/clocktick) > HZ" */
#if HZ == 1000
if (elapsed_cycles > (cpt << 10) )
#elif HZ == 250
if (elapsed_cycles > (cpt << 8) )
#elif HZ == 100
if (elapsed_cycles > (cpt << 7) )
#else
#warn WTF is HZ set to anyway?
if (elapsed_cycles > (HZ * cpt) )
#endif
{
/* Scenario 3: clock ticks are missing. */
printk (KERN_CRIT "gettimeoffset(CPU %d): missing ticks!"
"cycles %lX prev/now/next %lX/%lX/%lX clock %lX\n",
cpuid,
elapsed_cycles, prev_tick, now, next_tick, local_ct);
printk (KERN_CRIT "gettimeoffset(CPU %ld): missing %ld ticks!"
" cycles %lX prev/now/next %lX/%lX/%lX clock %lX\n",
cpuid, elapsed_cycles / cpt,
elapsed_cycles, prev_tick, now, next_tick, cpt);
}
/* FIXME: Can we improve the precision? Not with PAGE0. */
usec = (elapsed_cycles * 10000) / PAGE0->mem_10msec;
/* add in "lost" jiffies */
usec += local_ct * (jiffies - wall_jiffies);
usec += cpt * (jiffies - wall_jiffies);
return usec;
#else
return 0;