Jack clients cannot use real-time scheduling with kernel 5.4.0-25-generic

[Tristan Tarrant]

With kernels 5.4.0-24-generic and 5.4.0-25-generic Jack client applications can no longer use real-time scheduling. Kernel 5.4.0-23-generic works.
For example starting guitarix or ardour produces the following logs on stderr:

Cannot use real-time scheduling (RR/5)(1: Operation not permitted)
JackClient::AcquireSelfRealTime error

The following is a strace from an older (working) kernel:

sched_get_priority_min(SCHED_FIFO) = 1
sched_get_priority_max(SCHED_FIFO) = 99
sched_get_priority_min(SCHED_FIFO) = 1
sched_get_priority_max(SCHED_FIFO) = 99
mmap(NULL, 8392704, PROT_NONE, MAP_PRIVATE|MAP_ANONYMOUS|MAP_STACK, -1, 0) = 0x7fd5c307c000
mprotect(0x7fd5c307d000, 8388608, PROT_READ|PROT_WRITE) = 0
clone(child_stack=0x7fd5c387b9f0, flags=CLONE_VM|CLONE_FS|CLONE_FILES|CLONE_SIGHAND|CLONE_THREAD|CL
ONE_SYSVSEM|CLONE_SETTLS|CLONE_PARENT_SETTID|CLONE_CHILD_CLEARTID, parent_tid=[47564], tls=0x7fd5c3
87c700, child_tidptr=0x7fd5c387c9d0) = 47564
sched_setscheduler(47564, SCHED_FIFO, [1]) = 0

Compare against an strace with an affected kernel:

sched_get_priority_min(SCHED_FIFO) = 1
sched_get_priority_max(SCHED_FIFO) = 99
sched_get_priority_min(SCHED_FIFO) = 1
sched_get_priority_max(SCHED_FIFO) = 99
mmap(NULL, 8392704, PROT_NONE, MAP_PRIVATE|MAP_ANONYMOUS|MAP_STACK, -1, 0) = 0x7f8d827ee000
mprotect(0x7f8d827ef000, 8388608, PROT_READ|PROT_WRITE) = 0
clone(child_stack=0x7f8d82fed9f0, flags=CLONE_VM|CLONE_FS|CLONE_FILES|CLONE_SIGHAND|CLONE_THREAD|CL
ONE_SYSVSEM|CLONE_SETTLS|CLONE_PARENT_SETTID|CLONE_CHILD_CLEARTID, parent_tid=[21592], tls=0x7f8d82
fee700, child_tidptr=0x7f8d82fee9d0) = 21592
sched_setscheduler(21592, SCHED_FIFO, [1]) = -1 EPERM (Operation not permitted)

[Tristan Tarrant (tristan-tarrant-gmail)]

Apparently a duplicate of https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1873315

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.