[BUG+PATCH] RT-Preempt: IRQ threads running at prio 0 SCHED_OTHER

["Remy Bohmer" <l.pinguin@gmail.com>]

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Hello Ingo,

I am using your yum-distributed kernel 2.6.19.1-rt15, and
unfortunately I experienced very worse latencies.
It turned out that ALL the IRQ threads were all running at prio 0, SCHED_OTHER.

Looking at the current code in kernel/irq/manage.c, the goal was to
put them at MAX_RT_PRIO, but the call to sys_sched_setscheduler()
fails with EINVAL. I have attached a patch to set them to
(MAX_RT_PRIO-1). This works.

Further I believe that each application of the RT-kernel requires a
different configuration of these thread-priorities and I prefer to
reconfigure these prios from userland during boot. As these
threadnames contain whitespaces in its name, they make the
shell-scripts unnecessary complex that I use to reconfigure the thread
priorities.
So, I would prefer a slight modification of the names: The attached
patch also changes the names from [IRQ nn] to [IRQ-nn]. I hope that
you agree with me here. (If not, I stick to do this patch each time
myself ;-) )

Kind Regards,

Remy Böhmer

[-- Attachment #2: fix-kernel-irq-thread-prio.patch --]
[-- Type: application/octet-stream, Size: 803 bytes --]

--- linux-2.6.19.i686/kernel/irq/manage.c.orig	2006-12-18 22:59:59.000000000 +0100
+++ linux-2.6.19.i686/kernel/irq/manage.c	2006-12-19 21:22:09.000000000 +0100
@@ -692,9 +692,9 @@
 	current->flags |= PF_NOFREEZE | PF_HARDIRQ;
 
 	/*
-	 * Scale irq thread priorities from prio 50 to prio 25
+	 * Scale irq thread priorities to prio 99
 	 */
-	param.sched_priority = MAX_RT_PRIO;
+	param.sched_priority = MAX_RT_PRIO-1;
 
 	sys_sched_setscheduler(current->pid, SCHED_FIFO, &param);
 
@@ -725,7 +725,7 @@
 	if (desc->thread || !ok_to_create_irq_threads)
 		return 0;
 
-	desc->thread = kthread_create(do_irqd, desc, "IRQ %d", irq);
+	desc->thread = kthread_create(do_irqd, desc, "IRQ-%d", irq);
 	if (!desc->thread) {
 		printk(KERN_ERR "irqd: could not create IRQ thread %d!\n", irq);
 		return -ENOMEM;