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

Mathieu Desnoyers
2010-09-13T16:16:44Z

* 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:

---
kernel/sched_fair.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

Index: linux-2.6-lttng.git/kernel/sched_fair.c
===================================================================
--- linux-2.6-lttng.git.orig/kernel/sched_fair.c
+++ linux-2.6-lttng.git/kernel/sched_fair.c
@@ -869,7 +869,7 @@ check_preempt_tick(struct cfs_rq *cfs_rq
struct sched_entity *se = __pick_next_entity(cfs_rq);
s64 delta = curr->vruntime - se->vruntime;

- if (delta > ideal_runtime)
+ if (delta > calc_delta_fair(ideal_runtime, curr))
resched_task(rq_of(cfs_rq)->curr);
}
}

--
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.