Re: [RFC PATCH] check_preempt_tick should not compare vruntime with wall time

Mike Galbraith
2010-09-14T02:10:29Z

On Mon, 2010-09-13 at 12:16 -0400, Mathieu Desnoyers wrote:
> * Peter Zijlstra (peterz@infradead.org) wrote:
> > On Mon, 2010-09-13 at 09:56 -0400, Mathieu Desnoyers wrote:
> [...]
> > > > static void
> > > > check_preempt_tick(struct cfs_rq *cfs_rq, struct sched_entity *curr)
> > > > {
> > > > - unsigned long ideal_runtime, delta_exec;
> > > > + unsigned long slice = sched_slice(cfs_rq, curr);
> > >
> > > So you still compute the sched_slice(), based on sched_period(), based on
> > > sysctl_sched_min_granularity *= nr_running when there are more than nr_latency
> > > running threads.
> >
> > What's wrong with that? I keep asking you, you keep not giving an
> > answer. Stop focussing on nr_latency, its an by produce not a
> > fundamental entity.
> >
> > period := max(latency, min_gran * nr_running)
> >
> > See, no nr_latency -- the one and only purpose of nr_latency is avoiding
> > that multiplication when possible.
>
> OK, the long IRC discussions we just had convinced me that the current scheme
> takes things into account by adapting the granularity dynamically, but also got
> me to notice that check_preempt seems to compare vruntime with wall time, which
> is utterly incorrect. So maybe all my patch was doing was to expose this bug:

It's not wall time, it's just a distance. But I'm not attached to it by
any means, if something else works better, do that :)

-Mike