[LAD] JACK and vanilla 3.19.2 startup issue

Tito Latini tito.01beta at gmail.com
Sat Jun 6 21:08:38 UTC 2015


On Sat, Jun 06, 2015 at 03:35:16PM +0100, Harry van Haaren wrote:
> On Sat, Apr 4, 2015 at 8:13 PM, Adrian Knoth <adi at drcomp.erfurt.thur.de>
> wrote:
> 
> > Not enough information. I recommend starting jackd with strace
> >
> Done - apologies for the delay. Strace output available[1], but the most
> interesting part copied here:
> 
> sched_get_priority_min(SCHED_FIFO)      = 1
> sched_setscheduler(0, SCHED_FIFO, { 1 }) = -1 EPERM (Operation not
> permitted)
> write(2, "\nJACK is running in realtime mod"..., 87
> JACK is running in realtime mode, but you are not allowed to use realtime
> scheduling.
> ) = 87
> 
> 
> 
> > This code tries to call sched_setscheduler() with SCHED_FIFO. My bet is
> > that your "almost vanilla kernel" fails to fulfil this request, but
> > strace will tell you for sure.
> >
> By "almost vanilla" I meant the Arch linux packaged version - I didn't
> change the config myself (and Arch aims to be true to vanilla). A very
> accurace prediction though - indeed sched_setscheduler() is causing a
> return of -1. This is running as root though - so something is wrong here.
> 
> 
> > The question is why said call should fail. The only thing that comes to
> > my mind are CGROUPS. Maybe your old kernel comes without, the new kernel
> > supports them and the configuration is set in a way that disables
> > SCHED_FIFO by default.
> >
> Perhaps - I'm not experienced with cgroups or such, if anybody has any test
> ideas for me I'll try them out?

No tested but perhaps the follow lines in __sched_setscheduler
(linux-3.19.8/kernel/sched/core.c) are relevant:

#ifdef CONFIG_RT_GROUP_SCHED
		/*
		 * Do not allow realtime tasks into groups that have no runtime
		 * assigned.
		 */
		if (rt_bandwidth_enabled() && rt_policy(policy) &&
				task_group(p)->rt_bandwidth.rt_runtime == 0 &&
				!task_group_is_autogroup(task_group(p))) {
			task_rq_unlock(rq, p, &flags);
			return -EPERM;
		}
#endif
#ifdef CONFIG_SMP
		if (dl_bandwidth_enabled() && dl_policy(policy)) {
			cpumask_t *span = rq->rd->span;

			/*
			 * Don't allow tasks with an affinity mask smaller than
			 * the entire root_domain to become SCHED_DEADLINE. We
			 * will also fail if there's no bandwidth available.
			 */
			if (!cpumask_subset(span, &p->cpus_allowed) ||
			    rq->rd->dl_bw.bw == 0) {
				task_rq_unlock(rq, p, &flags);
				return -EPERM;
			}
		}
#endif


However I'm using 3.10 compiled without CONFIG_RT_GROUP_SCHED and the
second `#ifdef' (CONFIG_SMP) is not present in the source code.

Hypothesis: the second condition fails

  => disable the bandwidth management logic (the default is 950000):

     echo -1 > /proc/sys/kernel/sched_rt_runtime_us

     More info about sched_rt_runtime_us:

       scheduler/sched-rt-group.txt: "2.1 System wide settings"
       scheduler/sched-deadline.txt: "4.1 System wide settings"

If it continues to fail and your kernel is compiled with
CONFIG_RT_GROUP_SCHED, you could recompile it without this
configuration option (if you don't use realtime group scheduling).

Oh, if it continues to fail, see the other branches with `return -EPERM'
in __sched_setscheduler.


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