Current kernel preemption code opens a small window between the check
for need_resched in schedule and the setting of preempt_count to zero in
preempt_schedule. While this window is generally short (a few cycles)
the resulting period of non-preemptibility could be as long as the next
timer tick - much longer, in fact, if a lock is obtained in the interim.
This patch checks for need_resched in preempt_schedule after setting
preempt_count back to zero, before returning. The overhead is
negligible and it is crucial to never miss a preemption opportunity.
Also fixes/clarifies some comments. Patch is against 2.5.8 ... enjoy,
Robert Love
diff -urN linux-2.5.8/kernel/sched.c linux/kernel/sched.c
--- linux-2.5.8/kernel/sched.c Sun Apr 14 15:18:47 2002
+++ linux/kernel/sched.c Mon Apr 15 15:49:44 2002
@@ -765,8 +765,8 @@
spin_lock_irq(&rq->lock);
/*
- * if entering from preempt_schedule, off a kernel preemption,
- * go straight to picking the next task.
+ * if entering off of a kernel preemption go
+ * straight to picking the next task.
*/
if (unlikely(preempt_get_count() & PREEMPT_ACTIVE))
goto pick_next_task;
@@ -842,7 +842,9 @@
#ifdef CONFIG_PREEMPT
/*
- * this is is the entry point to schedule() from in-kernel preemption.
+ * this is is the entry point to schedule() from in-kernel preemption
+ * off of preempt_enable. Preemptions off return-from-interrupt are
+ * handled directly in that codepath.
*/
asmlinkage void preempt_schedule(void)
{
@@ -851,10 +853,15 @@
if (unlikely(ti->preempt_count))
return;
+need_resched:
ti->preempt_count = PREEMPT_ACTIVE;
schedule();
ti->preempt_count = 0;
barrier();
+
+ /* we could miss a preemption between schedule() and now */
+ if (unlikely(test_thread_flag(TIF_NEED_RESCHED)))
+ goto need_resched;
}
#endif /* CONFIG_PREEMPT */
-
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This archive was generated by hypermail 2b29 : Mon Apr 15 2002 - 22:00:25 EST