linux-kernel - Re: [PATCH] sched: Update task->on_rq when tasks are moving between runqueues

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Message-ID: <20151029015852.GF11242@worktop.amr.corp.intel.com>
Date:	Thu, 29 Oct 2015 02:58:52 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Olav Haugan <ohaugan@...eaurora.org>
Cc:	mingo@...hat.com, linux-kernel@...r.kernel.org,
	linux-arm-msm@...r.kernel.org
Subject: Re: [PATCH] sched: Update task->on_rq when tasks are moving between
 runqueues

On Wed, Oct 28, 2015 at 05:57:10PM -0700, Olav Haugan wrote:
> On 15-10-25 11:09:24, Peter Zijlstra wrote:
> > On Sat, Oct 24, 2015 at 11:01:02AM -0700, Olav Haugan wrote:
> > > Task->on_rq has three states:
> > > 	0 - Task is not on runqueue (rq)
> > > 	1 (TASK_ON_RQ_QUEUED) - Task is on rq
> > > 	2 (TASK_ON_RQ_MIGRATING) - Task is on rq but in the process of being
> > > 	migrated to another rq
> > > 
> > > When a task is moving between rqs task->on_rq state should be
> > > TASK_ON_RQ_MIGRATING
> > 
> > Only when not holding both rq locks..
> 
> IMHO I think we should keep the state of p->on_rq updated with the correct state
> all the time unless I am incorrect in what p->on_rq represent. The task
> is moving between rq's and is on the rq so the state should be
> TASK_ON_RQ_MIGRATING right? I do realize that the code is currently not
> broken. However, in the future someone might come along and change
> set_task_cpu() and the code change might rely on an accurate p->on_rq value. It
> would be good design to keep this value correct.

At the same time; we should also provide lean and fast code. Is it
better to add assertions about required state than to add superfluous
code for just in case scenarios.
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