From 0f87493487d81522df481bcce7fc8eb5f3134d8d Mon Sep 17 00:00:00 2001
From: Kevin O'Connor <kevin@koconnor.net>
Date: Fri, 14 Oct 2016 00:39:46 -0400
Subject: [PATCH] timer: Rework AVR timer priority code to use time instead of
 event count

Rework the timer prioritization code so that is compares against the
current time instead of the number of repeat timers in a given
interrupt.  This makes the code slightly faster and it should provide
better protection against task starvation.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>
---
 src/avr/timer.c | 34 +++++++++++++++++++++++-----------
 1 file changed, 23 insertions(+), 11 deletions(-)

diff --git a/src/avr/timer.c b/src/avr/timer.c
index e4d7b942..a7e832bc 100644
--- a/src/avr/timer.c
+++ b/src/avr/timer.c
@@ -42,6 +42,14 @@ timer_set_clear(uint16_t next)
     TIFR1 = 1<<OCF1A;
 }
 
+static inline void
+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;
+}
+
 ISR(TIMER1_COMPA_vect)
 {
     sched_timer_kick();
@@ -128,9 +136,8 @@ timer_set_next(uint32_t next)
     return 1;
 }
 
-static uint8_t timer_repeat;
-#define TIMER_MAX_REPEAT 40
-#define TIMER_MAX_NEXT_REPEAT 15
+#define TIMER_IDLE_REPEAT_TICKS 8000
+#define TIMER_REPEAT_TICKS 3000
 
 #define TIMER_MIN_TRY_TICKS 60 // 40 ticks to exit irq; 20 ticks of progress
 #define TIMER_DEFER_REPEAT_TICKS 200
@@ -147,11 +154,12 @@ timer_try_set_next(uint32_t target)
         goto done;
 
     // Next timer is in the past or near future - can't reschedule to it
-    uint8_t tr = timer_repeat-1;
-    if (likely(tr)) {
+    if (!(TIFR1 & (1<<OCF1B))) {
+        // Can run more timers from this irq; briefly allow irqs
         irq_enable();
-        timer_repeat = tr;
+        asm("nop");
         irq_disable();
+
         while (diff >= 0) {
             // Next timer is in the near future - wait for time to occur
             now = timer_get();
@@ -161,13 +169,13 @@ timer_try_set_next(uint32_t target)
         }
         return 0;
     }
-
-    // Too many repeat timers from a single interrupt - force a pause
-    timer_repeat = TIMER_MAX_NEXT_REPEAT;
-    next = now + TIMER_DEFER_REPEAT_TICKS;
     if (diff < (int16_t)(-timer_from_us(1000)))
         goto fail;
 
+    // Too many repeat timers - force a pause so tasks aren't starved
+    timer_repeat_set(now + TIMER_REPEAT_TICKS);
+    next = now + TIMER_DEFER_REPEAT_TICKS;
+
 done:
     timer_set(next);
     return 1;
@@ -175,9 +183,13 @@ fail:
     shutdown("Rescheduled timer in the past");
 }
 
+// Periodic background task that temporarily boosts priority of
+// timers.  This helps prioritize timers when tasks are idling.
 static void
 timer_task(void)
 {
-    timer_repeat = TIMER_MAX_REPEAT;
+    irq_disable();
+    timer_repeat_set(timer_get() + TIMER_IDLE_REPEAT_TICKS);
+    irq_enable();
 }
 DECL_TASK(timer_task);