I've gotten a report from Rik van Riel that the patch below improves PPP
throughput and makes MP3s less prone to skipping. Not too many people
seem to have tried it, so I'm reposting with a subject that'll hopefully
get some attention. This is different from the first version of the patch
in that it only allows each bottom half to be run at most once during a
successful run through do_bottom_halves (eg one in which we manage to
obtain the locks). Comments or suggestions for a better heuristic will be
appreciated, as stock 2.2.x bottom half handling needs to be improved.
-ben
--- softirq.c.orig Sun Mar 21 10:22:00 1999
+++ softirq.c Thu May 6 21:30:47 1999
@@ -26,45 +26,50 @@
void (*bh_base[32])(void);
/*
- * This needs to make sure that only one bottom half handler
- * is ever active at a time. We do this without locking by
- * doing an atomic increment on the intr_count, and checking
- * (nonatomically) against 1. Only if it's 1 do we schedule
- * the bottom half.
- *
- * Note that the non-atomicity of the test (as opposed to the
- * actual update) means that the test may fail, and _nobody_
- * runs the handlers if there is a race that makes multiple
- * CPU's get here at the same time. That's ok, we'll run them
- * next time around.
+ * Repeatedly run over the bottom halves until there are no more, but only run
+ * each bottom half at most once. If we don't loop, if one bottom half triggers
+ * another, it might get delayed a long time. If we loop indefinately, then the
+ * system might completely when unable to handle the irq load. -bcrl
*/
-static inline void run_bottom_halves(void)
-{
- unsigned long active;
- void (**bh)(void);
-
- active = get_active_bhs();
- clear_active_bhs(active);
- bh = bh_base;
- do {
- if (active & 1)
- (*bh)();
- bh++;
- active >>= 1;
- } while (active);
-}
-
asmlinkage void do_bottom_half(void)
{
int cpu = smp_processor_id();
+ unsigned long left = ~0UL;
+ unsigned long active;
+ void (**bh)(void);
+again:
if (softirq_trylock(cpu)) {
if (hardirq_trylock(cpu)) {
__sti();
- run_bottom_halves();
+
+ while ((active = left & get_active_bhs())) {
+ left &= ~active;
+ clear_active_bhs(active);
+ bh = bh_base;
+ do {
+ if (active & 1)
+ (*bh)();
+ bh++;
+ active >>= 1;
+ } while (active);
+ }
+
__cli();
hardirq_endlock(cpu);
+ softirq_endlock(cpu);
+
+ /*
+ * Avoid the race by checking if any bottom halves
+ * are active after releasing all locks. This is a
+ * rare race, but should inexpensive to check. -bcrl
+ */
+ rmb();
+ if (get_active_bhs() & left)
+ goto again;
+ return;
}
softirq_endlock(cpu);
}
}
+
-
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