linux-kernel - Re: [BUG+PATCH] RT-Preempt: IRQ threads running at prio 0 SCHED

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Date:	Tue, 19 Dec 2006 22:24:27 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	linux@...mer.net
Cc:	linux-kernel@...r.kernel.org, Clark Williams <williams@...hat.com>
Subject: Re: [BUG+PATCH] RT-Preempt: IRQ threads running at prio 0 SCHED_OTHER


* Remy Bohmer <l.pinguin@...il.com> wrote:

> 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.

oops - my intention was to set all IRQs and softirqs to SCHED_FIFO prio 
50. I have fixed that now in my tree.

prio 99 is pretty extensive and makes it hard to move tasks 'above' 
hardirq priority, without setting the priority of /every/ IRQ thread. So 
i picked SCHED_FIFO:50 - at exact half way.

> 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.

ok - lets try it. Clark: does this impact the set_kthread_prio utility? 
I've changed "IRQ 123" to "IRQ-123" to make pidof & friends work better.

	Ingo
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