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

Peter Zijlstra
2008-08-19T18:39:29Z

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@chello.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.