linux-kernel - Re: [RFC PATCH] sched: rt: fix selecting runqueue for task to be pushed

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Date:	Tue, 11 Dec 2012 21:46:14 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Hillf Danton <dhillf@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH] sched: rt: fix selecting runqueue for task to be
 pushed

On Tue, 2012-12-11 at 20:54 -0500, Steven Rostedt wrote:

> I'm actually thinking that that test should always fail. The
> cpupri_find() does a scan of all priorities up to but not including the
> current task's priority. If cpupri_find() finds a mask, it means that it
> found CPUs that are running only tasks of lower priority than the task
> we are checking. Which means, it should never include the task's CPU, as
> that CPU should have a higher priority than what is being returned by
> lowest_mask. If it can't find a set of CPUs of lower priority, it should
> return false, and the find_lowest_rq() should exit.
> 
> I'll add a WARN_ON_ONCE() there, and see if I can trigger it. :-/

Ah, for select_task_rq_rt() it can get that CPU, because it's called in
the wakeup path before the task is added into the CPUs priority. And we
definitely want the current CPU in that case.

For pushing, the local CPU should never be returned. I'll check to make
sure that's the case too.

-- Steve


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