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avr: Rework timer irq handler to check for tasks pending 

Allow the timer dispatch irq handler to run for extended periods if
there are no tasks pending.  This reduces the amount of lost cpu time
spent entering and exiting the irq handler.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>
This commit is contained in:
Kevin O'Connor 2017-08-07 12:49:41 -04:00
parent 2c272f99a3
commit f886212b44
1 changed files with 33 additions and 39 deletions
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72
src/avr/timer.c
View File

@ -67,7 +67,10 @@ timer_repeat_set(uint16_t next)
{
// Timer1B is used to limit the number of timers run from a timer1A irq
OCR1B = next;
TIFR1 = 1<<OCF1B;
// This is "TIFR1 = 1<<OCF1B" - gcc handles that poorly, so it's hand coded
uint8_t dummy;
asm volatile("ldi %0, %2\n out %1, %0"
: "=d"(dummy) : "i"(&TIFR1 - 0x20), "i"(1<<OCF1B));
}
// Activate timer dispatch as soon as possible
@ -162,52 +165,43 @@ ISR(TIMER1_COMPA_vect)
// Run the next software timer
next = sched_timer_dispatch();
int16_t diff = timer_get() - next;
if (likely(diff >= 0)) {
// Another timer is pending - briefly allow irqs to fire
for (;;) {
int16_t diff = timer_get() - next;
if (likely(diff >= 0)) {
// Another timer is pending - briefly allow irqs and then run it
irq_enable();
if (unlikely(TIFR1 & (1<<OCF1B)))
goto check_defer;
irq_disable();
break;
}
if (likely(diff <= -TIMER_MIN_TRY_TICKS))
// Schedule next timer normally
goto done;
irq_enable();
if (unlikely(TIFR1 & (1<<OCF1B)))
// Too many repeat timers - must exit irq handler
goto force_defer;
goto check_defer;
irq_disable();
continue;
}
if (likely(diff <= -TIMER_MIN_TRY_TICKS))
// Schedule next timer normally
goto done;
// Next timer in very near future - wait for it to be ready
do {
irq_enable();
if (unlikely(TIFR1 & (1<<OCF1B)))
goto force_defer;
check_defer:
// Check if there are too many repeat timers
irq_disable();
diff = timer_get() - next;
} while (diff < 0);
uint16_t now = timer_get();
if ((int16_t)(next - now) < (int16_t)(-timer_from_us(1000)))
shutdown("Rescheduled timer in the past");
if (sched_tasks_busy()) {
timer_repeat_set(now + TIMER_REPEAT_TICKS);
next = now + TIMER_DEFER_REPEAT_TICKS;
goto done;
}
timer_repeat_set(now + TIMER_IDLE_REPEAT_TICKS);
timer_set(now);
}
}
force_defer:
// Too many repeat timers - force a pause so tasks aren't starved
irq_disable();
uint16_t now = timer_get();
if ((int16_t)(next - now) < (int16_t)(-timer_from_us(1000)))
shutdown("Rescheduled timer in the past");
timer_repeat_set(now + TIMER_REPEAT_TICKS);
next = now + TIMER_DEFER_REPEAT_TICKS;
done:
timer_set(next);
return;
}
// Periodic background task that temporarily boosts priority of
// timers. This helps prioritize timers when tasks are idling.
void
timer_task(void)
{
irq_disable();
timer_repeat_set(timer_get() + TIMER_IDLE_REPEAT_TICKS);
irq_enable();
}
DECL_TASK(timer_task);