linux-kernel - Re: [PATCH v4 2/3] sched/rt: Fix wrong SMP scheduler behavior for equal prio cases

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Date:	Mon, 16 Feb 2015 10:35:06 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Xunlei Pang <xlpang@....com>
Cc:	linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Juri Lelli <juri.lelli@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dan Streetman <ddstreet@...e.org>,
	Xunlei Pang <pang.xunlei@...aro.org>
Subject: Re: [PATCH v4 2/3] sched/rt: Fix wrong SMP scheduler behavior for
 equal prio cases

On Mon, 16 Feb 2015 17:32:23 +0800
Xunlei Pang <xlpang@....com> wrote:

> From: Xunlei Pang <pang.xunlei@...aro.org>
> 
> Currently, SMP RT scheduler has some trouble in dealing with
> equal prio cases.
> 
> For example, in check_preempt_equal_prio():
> When RT1(current task) gets preempted by RT2, if there is a
> migratable RT3 with same prio, RT3 will be pushed away instead
> of RT1 afterwards, because RT1 will be enqueued to the tail of
> the pushable list when going through succeeding put_prev_task_rt()
> triggered by resched. This broke FIFO.
> 
> Furthermore, this is also problematic for normal preempted cases
> if there're some rt tasks queued with the same prio as current,
> because current will be put behind these tasks in the pushable
> queue.
> 
> So, if a task is running and gets preempted by a higher priority
> task (or even with same priority for migrating), this patch ensures
> that it is put before any existing task with the same priority in
> the pushable queue.
> 
> Signed-off-by: Xunlei Pang <pang.xunlei@...aro.org>

I'd love to review this now, but unfortunately I need to get ready for
my trip to Linux Collab. I'll get back to this next week. Feel free to
ping me then. If I have time, I might review this while at the
conference, but don't place any bets that I will.

-- Steve
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