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