linux-kernel - Re: CFS and suspend2: hang in atomic copy (was: [Announce] [patch] Modular Scheduler Core and Completely Fair Scheduler [CFS])

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Message-ID: <20070419101131.GB32274@elte.hu>
Date:	Thu, 19 Apr 2007 12:11:31 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Esben Nielsen <nielsen.esben@...glemail.com>
Cc:	Christian Hesse <mail@...thworm.de>, linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Con Kolivas <kernel@...ivas.org>,
	Nick Piggin <npiggin@...e.de>, Mike Galbraith <efault@....de>,
	Arjan van de Ven <arjan@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	suspend2-devel@...ts.suspend2.net
Subject: Re: CFS and suspend2: hang in atomic copy (was: [Announce] [patch] Modular Scheduler Core and Completely Fair Scheduler [CFS])


* Esben Nielsen <nielsen.esben@...glemail.com> wrote:

> >+	/*
> >+	 * Temporarily insert at the last position of the tree:
> >+	 */
> >+	p->fair_key = LLONG_MAX;
> >+	__enqueue_task_fair(rq, p);
> >	p->on_rq = 1;
> >+
> >+	/*
> >+	 * Update the key to the real value, so that when all other
> >+	 * tasks from before the rightmost position have executed,
> >+	 * this task is picked up again:
> >+	 */
> >+	p->fair_key = rq->fair_clock - p->wait_runtime + p->nice_offset;
> 
> I don't think it safe to change the key after inserting the element in 
> the tree. You end up with an unsorted tree giving where new entries 
> end up in wrong places "randomly".

yeah, indeed. I hoped that once this rightmost entry is removed (as soon 
as it gets scheduled next time) the tree goes back to a correct shape, 
but that's not the case - the left sub-tree and the right sub-tree is 
merged by the rbtree code with the assumption that the entry had a 
correct key.

> I think a better approach would be to keep track of the rightmost 
> entry, set the key to the rightmost's key +1 and then simply insert it 
> there.

yeah. I had that implemented at a stage but was trying to be too clever 
for my own good ;-)

	Ingo
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