linux-kernel - Re: [PATCH 1/2] sched: check for pushing rt tasks after all scheduling

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Date:	Wed, 29 Jul 2009 10:41:24 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Gregory Haskins <gregory.haskins@...il.com>,
	Steven Rostedt <srostedt@...hat.com>
Subject: Re: [PATCH 1/2] sched: check for pushing rt tasks after all
 scheduling

On Wed, 2009-07-29 at 00:21 -0400, Steven Rostedt wrote:
> plain text document attachment
> (0001-sched-check-for-pushing-rt-tasks-after-all-schedulin.patch)
> From: Steven Rostedt <srostedt@...hat.com>
> 
> The current method for pushing RT tasks after scheduling only
> happens after a context switch. But we found cases where a task
> is set up on a run queue to be pushed but the push never happens
> because the schedule chooses the same task.
> 
> This bug was found with the help of Gregory Haskins and the use of
> ftrace (trace_printk). It tooks several days for both of us analyzing
> the code and the trace output to find this.


> +               if (current->sched_class->needs_post_schedule)
> +                       post_schedule = current->sched_class->needs_post_schedule(rq);


> +       if (post_schedule)
> +               current->sched_class->post_schedule(rq);


Why can't we omit that first call, and do the second unconditionally,
using storage in the class rq to save state?
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