Re: Serial port latency

[Manfred Spraul <manfred@colorfullife.com>]

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Pavel Machek wrote:
> 
> > >
> > > I've seen similar bugs. If you hook something on schedule_tq and
> > forget
> > > to set current->need_resched, this is exactly what you get.
> > >
> > I'm running with a patch that printk's if cpu_idle() is called while a
> > softirq is pending.
> > If I access the floppy on my K6/200 every track triggers the check, and
> > sometimes the console blanking code triggers it.
> 
> Seems floppy and console is buggy, then.
>

No. The softirq implementation is buggy.
I can trigger the problem with the TASKLET_HI (floppy), and both net rx
and tx (ping -l)

> > What about creating a special cpu_is_idle() function that the idle
> > functions must call before sleeping?
> 
> I'd say just fix all the bugs.
>

Ok, there are 2 bugs that are (afaics) impossible to fix without
checking for pending softirq's in cpu_idle():

a)
queue_task(my_task1, tq_immediate);
mark_bh();
schedule();
;within schedule: do_softirq()
;within my_task1:
mark_bh();
; bh returns, but do_softirq won't loop
; do_softirq returns.
; schedule() clears current->need_resched
; idle thread scheduled.
--> idle can run although softirq's are pending

I assume I trigger this race with the floppy driver.

b)
hw interrupt
do_softirq
within the net_rx handler: another hw interrupt, additional packets are
queued
do_softirq won't loop.
returns to idle thread. --> packets delayed unnecessary.

What about the attached patch? Obviously the other idle cpu must be
converted to use the function as well.

--
	Manfred

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--- 2.4/arch/i386/kernel/process.c	Thu Feb 22 22:28:52 2001
+++ build-2.4/arch/i386/kernel/process.c	Sun Apr  1 00:05:21 2001
@@ -73,6 +73,30 @@
 	hlt_counter--;
 }
 
+/**
+ * cpu_is_idle - helper function for idle functions
+ * 
+ * pm_idle functions must call this function to verify that
+ * the cpu is really idle. It must be called with disabled
+ * local interrupts.
+ * Return values:
+ * 0: cpu was not idle, local interrupts reenabled.
+ * 1: go into power saving mode, local interrupts are
+ *    still disabled.
+*/
+static inline int cpu_is_idle(void)
+{
+	if (current->need_resched) {
+		__sti();
+		return 0;
+	}
+	if (softirq_active(smp_processor_id()) & softirq_mask(smp_processor_id())) {
+		__sti();
+		do_softirq();
+		return 0;
+	}
+	return 1;
+}
 /*
  * We use this if we don't have any better
  * idle routine..
@@ -81,10 +105,8 @@
 {
 	if (current_cpu_data.hlt_works_ok && !hlt_counter) {
 		__cli();
-		if (!current->need_resched)
+		if (cpu_is_idle())
 			safe_halt();
-		else
-			__sti();
 	}
 }