Re: softirq buggy [Re: Serial port latency]

Manfred Spraul
2001-04-08T18:16:38Z

From: <kuznet@ms2.inr.ac.ru>
To: "Manfred Spraul" <manfred@colorfullife.com>
Cc: <linux-kernel@vger.redhat.com>
Sent: Sunday, April 08, 2001 7:58 PM
Subject: Re: softirq buggy [Re: Serial port latency]

> Hello!
>
> > But with a huge overhead. I'd prefer to call it directly from within
the
> > idle functions, the overhead of schedule is IMHO too high.
>
>
> + if (current->need_resched) {
> + return 0;
> ^^^^^^^^
> + }
> + if (softirq_active(smp_processor_id()) &
softirq_mask(smp_processor_id())) {
> + do_softirq();
> + return 0;
> ^^^^^^^^^
> You return one value in both casesand I decided it means "schedule".
8)
> Apparently you meaned return 1 in the first case. 8)
>
No, the code is correct. 0 means "don't stop the cpu".
The pm_idle function pointer will return to cpu_idle()
(arch/i386/kernel/process.c), and that function contains another

while(!current->need_resched)
idle();

loop ;-)

> But in this case it becomes wrong. do_softirq() can raise need_reshed
> and moreover irqs arrive during it. Order of check should be
different.
>
Yes, I'll correct that.

>
> BTW what's about overhead... I suspect it is _lower_ in the case
> of schedule(). In the case of networking at least, when softirq
> most likely wakes some socket.
>
I'm not sure - what if the computer is just a router?
But OTHO: the cpu is idle, so it doesn't matter at all if the idle cpu
spends it's time within schedule() or within safe_hlt(), I'll change my
patch.

I have another question:
I added cpu_is_idle() into <linux/interrupt.h>. Is that acceptable, or
is there a better header file for such a function?

--
Manfred