On Tuesday 09 May 2006 01:08, Lee Revell wrote: > 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@xxxxxxx> 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? > > 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 ;-) > 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
