linux-kernel - Re: [BUG] How to get real-time priority using idle priority

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Message-Id: <1232015423.13856.5.camel@marge.simson.net>
Date:	Thu, 15 Jan 2009 11:30:23 +0100
From:	Mike Galbraith <efault@....de>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Brian Rogers <brian@...w.org>, Ingo Molnar <mingo@...e.hu>,
	linux-kernel@...r.kernel.org
Subject: Re: [BUG] How to get real-time priority using idle priority

On Thu, 2009-01-15 at 11:14 +0100, Peter Zijlstra wrote:
> On Thu, 2009-01-15 at 10:28 +0100, Mike Galbraith wrote:

> OK, so why did I write it like that to begin with...
> 
> Aah, yes.
> 
> Say we've just dequeued current
> 
> schedule
>   deactivate_task(prev)
>     dequeue_entity
>       update_min_vruntime
> 
> Then we'll set
> 
>   vruntime = cfs_rq->min_vruntime;
> 
> we find !cfs_rq->curr, but do find someone in the tree. Then we _must_
> do vruntime = se->vruntime, because
> 
>  vruntime = min_vruntime(vruntime := cfs_rq->min_vruntime, se->vruntime)
> 
> will not advance vruntime, and cause lags the other way around (which we
> fixed with that initial patch: 1af5f730fc1bf7c62ec9fb2d307206e18bf40a69
> (sched: more accurate min_vruntime accounting).
> 
> Which leads me to suggest the following
> 
> ---
> diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
> index 8e1352c..f2d2d94 100644
> --- a/kernel/sched_fair.c
> +++ b/kernel/sched_fair.c
> @@ -283,7 +283,7 @@ static void update_min_vruntime(struct cfs_rq
> *cfs_rq)
>  						   struct sched_entity,
>  						   run_node);
>  
> -		if (vruntime == cfs_rq->min_vruntime)
> +		if (!cfs_rq->curr)
>  			vruntime = se->vruntime;
>  		else
>  			vruntime = min_vruntime(vruntime, se->vruntime);

Aha.  Yeah, I'll re-test with that instead.

> The below can be split into 3 patches:
> 
>  - the idle weight change (do we really need that? why?)

I saw idle tasks slamming extremely far.  I'll verify, less is more.

	-Mike

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