linux-kernel - Re: [PATCH 2/6] sched: rt-bandwidth accounting fix

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Date:	Tue, 19 Aug 2008 20:38:24 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Max Krasnyansky <maxk@...lcomm.com>
Cc:	linux-kernel@...r.kernel.org,
	Stefani Seibold <stefani@...bold.net>,
	Dario Faggioli <raistlin@...ux.it>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH 2/6] sched: rt-bandwidth accounting fix

On Tue, 2008-08-19 at 11:33 -0700, Max Krasnyansky wrote:
> Peter Zijlstra wrote:
> > It fixes an accounting bug where we would continue accumulating runtime
> > even though the bandwidth control is disabled. This would lead to very long
> > throttle periods once bandwidth control gets turned on again.
> > 
> > Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> > ---
> >  kernel/sched_rt.c |   11 +++++------
> >  1 file changed, 5 insertions(+), 6 deletions(-)
> > 
> > Index: linux-2.6/kernel/sched_rt.c
> > ===================================================================
> > --- linux-2.6.orig/kernel/sched_rt.c
> > +++ linux-2.6/kernel/sched_rt.c
> > @@ -438,9 +438,6 @@ static int sched_rt_runtime_exceeded(str
> >  {
> >  	u64 runtime = sched_rt_runtime(rt_rq);
> >  
> > -	if (runtime == RUNTIME_INF)
> > -		return 0;
> > -
> >  	if (rt_rq->rt_throttled)
> >  		return rt_rq_throttled(rt_rq);
> >  
> > @@ -491,9 +488,11 @@ static void update_curr_rt(struct rq *rq
> >  		rt_rq = rt_rq_of_se(rt_se);
> >  
> >  		spin_lock(&rt_rq->rt_runtime_lock);
> > -		rt_rq->rt_time += delta_exec;
> > -		if (sched_rt_runtime_exceeded(rt_rq))
> > -			resched_task(curr);
> > +		if (sched_rt_runtime(rt_rq) != RUNTIME_INF) {
> > +			rt_rq->rt_time += delta_exec;
> > +			if (sched_rt_runtime_exceeded(rt_rq))
> > +				resched_task(curr);
> > +		}
> >  		spin_unlock(&rt_rq->rt_runtime_lock);
> >  	}
> >  }
> 
> This will make 'disabled' case more expensive, will it not ?
> I mean now we'll have to run balance_runtime() even if throttling is 
> disabled.

It should not, its cheaper now. We should never end up in
balance_runtime as we'll never exceed and hit the throttle path.

> Do you guys mind if I make this stuff configurable ? ie Just like 
> CONFIG_RT_GROUP_SCHED we could add CONFIG_RT_BANDWIDTH_THROTTLE.

Yeah - please don't do that, its ifdef fest in there - we really should
reduce the clutter, not add to it.

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