9 May
2006
9 May
'06
3:24 a.m.
On Tuesday 09 May 2006 01:08, Lee Revell wrote:
Not really. The problem with making the softirq timer thread high
priority is that lots of random code gets run from softirq timer
context, including rt_secret_rebuild() which is a well known latency
killer.
Ah, ok. Now well known to me, too ;-)
On Tue, 2006-05-09 at 00:47 +0200, Florian Paul
Schmidt wrote:
> On Mon, 8 May 2006 21:54:53 +0200
>
> Wolfgang Hoffmann <woho(a)woho.de> wrote:
> > Apropos: on your page on -rt setup (excellent page, btw., many thanks!
> > :-), you suggest raising "softirq-timer/0" to prio 99, to make
sleep()
> > function right (http://tapas.affenbande.org/?page_id=40 sleep()
> > based/system timer).
> >
> > I did so, and got strange latencies (> 40 ms) exactly once every 10
> > minutes, caused by some routing-related action (rt_secret_rebuild)
> > being run by the softirq-timer/0 thread. Don't you get bit by that,
> > too? Kernel is 2.6.16-rt16
>
> I haven't been bitten by that. Do you also get xruns with [i suppose so,
> just asking to make sure]? I haven't had as much time as before to play
> around and test things, so maybe it has crept into the kernel recently
> or maybe i just always had high-res timers enabled.
To be more precise, I get xruns, not scheduling latencies. Scheduling latency
is low (~50us), but softirq-timer/0 preempts all other threads when boosted
to prio 99, leading to xruns in jackd.
It's not a scheduler or -rt latency problem, it's a priority problem, since
softirq-timer/0 runs different jobs that would require different priorities:
wakeup sleep() at highest prio, rt_secret_rebuild et. al. at low prio.
With CONFIG_HIGH_RES_TIMERS=y it seems thatt softirq-timer/0 is no longer
needed for waking up sleep()ing threads, so it can be left at it's default
low prio (or even demoted to SCHED_OTHER?).
My solution is to configure with
CONFIG_HIGH_RES_TIMERS=y. Then,
sleep() wakes up correctly even with softirq-timer/0 being low-priority
(SCHED_FIFO 1 or even SCHED_OTHER).
In general I find adjusting priorities of the various softirq kernel
threads a bit of secret art. I can't find much documentation about
"what kernel thread runs which job" that would help making some proper
decisions here. I found my desktop "feels" most responsive when
demoting all softirq thread to SCHED_OTHER. I did so after seeing that
with a non-rt kernel, bottom-half handler don't run SCHED_FIFO/_RR at
all. So -rt now gives me robust low latencies for jackd, and still
proper desktop feeling.
Well, maybe this is getting off-topic for this list. But it seems to me
trimming priorities between kernel and userland threads is a bit like
no man's land.
I agree. Maybe Lee Revell knows more [CC'ing him]. Lee, you know
something about all these softirq threads? What exactly do they do? Lately I've been focused on getting mainline
usable for audio - I have
not run -rt in a while.
It is a bit of a black art because you need lots of knowledge of the
kernel to know how to set the priorities. But that's the nature of hard
realtime - you really need to know a lot about the system to get the
priorities right.
I've never had to alter the priority of the softirq timer thread myself
- does it work OK if you leave the softirq thread alone?
With CONFIG_HIGH_RES_TIMERS=y, yes.
With CONFIG_HIGH_RES_TIMERS=n, no. Then, high-prio theads don't wake up from
sleep() if a mid-prio CPU hog is running. Typical scenario is a high-prio
watchdog like Florians rt_watchdog (http://tapas.affenbande.org/?page_id=38),
which fails to work unless softirq-timer/0 runs with higher prio than the CPU
hog. It seems that softirq-timer/0 has to be scheduled for any thread to wake
up. It's a bit like priority inversion.
If not try setting the softirq thread to a lower RT
priority than your
soundcard, JACK, and all other interrupt threads.
Yes, that should cure the xruns. But I wonder if jackd's watchdog can do it's
job when a jack client is running wild? Probably not. softirq-timer/0 would
not get to run then, so the watchdog wouldn't wake up.
I've added Ingo to the cc: list, maybe he has some
input.
That would be helpful. Also adding Steve to cc: as he mentioned writing a
section in an O'Reilly book about how to use the -rt patch in
http://www.ussg.iu.edu/hypermail/linux/kernel/0604.2/0340.html
Wolfgang