linux-kernel - Re: RSDL v0.31

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Date:	Fri, 23 Mar 2007 09:03:20 +1100
From:	Con Kolivas <kernel@...ivas.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Mike Galbraith <efault@....de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Willy Tarreau <w@....eu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Xavier Bestel <xavier.bestel@...e.fr>, Mark Lord <lkml@....ca>,
	Al Boldi <a1426z@...ab.com>, ck@....kolivas.org,
	Serge Belyshev <belyshev@...ni.sinp.msu.ru>,
	linux-kernel@...r.kernel.org, Nicholas Miell <nmiell@...cast.net>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: RSDL v0.31

All code reviews are most welcome indeed!

On Thursday 22 March 2007 20:18, Ingo Molnar wrote:
> * Mike Galbraith <efault@....de> wrote:
> > Actually, the numbers are an interesting curiosity point, but not as
> > interesting as the fact that the deadline mechanism isn't kicking in.
>
> it's not just the scheduling accounting being off, RSDL also seems to be

I'll look at that when I have time.

> accessing stale data here:
> > >From pull_task():
> >
> > 	/*
> > 	 * If this task has already been running on src_rq this priority
> > 	 * cycle, make the new runqueue think it has been on its cycle
> > 	 */
> > 	if (p->rotation == src_rq->prio_rotation)
> > 		p->rotation = this_rq->prio_rotation;
> >
> > The intent here is clearly that this task continue on the new cpu as
> > if nothing has happened.  However, when the task was dequeued,
> > p->array was left as it was, points to the last place it was queued.
> > Stale data.

I don't think this is a problem because immediately after this in pull_task it 
calls enqueue_task() which always updates p->array in recalc_task_prio(). 
Every enqueue_task always calls recalc_task_prio on non-rt tasks so the array 
should always be set no matter where the entry point to scheduling is from 
unless I have a logic error in setting the p->array in recalc_task_prio() or 
there is another path to schedule() that I've not accounted for by making 
sure recalc_task_prio is done.

> it might point to a hot-unplugged CPU's runqueue as well. Which might
> work accidentally, but we want this fixed nevertheless.

The hot unplugged cpu's prio_rotation will be examined, and then it sets the 
prio_rotation from this runqueue's value. That shouldn't lead to any more 
problems than setting the timestamp based on the hot unplug cpus timestamp 
lower down also in pull_task()

p->timestamp = (p->timestamp - src_rq->most_recent_timestamp) +  
this_rq->most_recent_timestamp;

Thanks for looking!

-- 
-ck
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