Re: [PATCH 6/6] sched: disabled rt-bandwidth by default

Andi Kleen
2008-08-28T11:06:59Z

> Even if the system has multiple CPUs, and even if just a single CPU is
> fully utilized by an RT task, without the rt-limit the system will still
> lock up in practice due to various other factors: workqueues and tasks
> being 'stuck' on CPUs that host an RT hog.

The load balancer will not notice that a particular CPU is busy
with real time tasks?

> While there's obviously CPU
> time available on other CPUs, you cannot run 'top', the desktop will
> freeze, work flows of the system can be stuck, etc, etc..

I had such a situation at least once in the past (not due
run away RT but due a kernel bug) and even with 2 out of 4 CPUs blocked
the system was still quite usable. top/kill definitely worked. The system
didn't have a desktop, but I didn't notice many problems in shell use.
Ok it's just one sample.

That said I don't think having such a limit by default is a bad idea actually.

Just handling it in the scheduler anyways is also probably good because
it can happen even due to other issues than just run away RT tasks.

-Andi