Messages in this thread | | | From | Vincent Guittot <> | Date | Wed, 1 Oct 2014 09:02:18 +0200 | Subject | Re: [PATCH] sched: fix spurious active migration |
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On 30 September 2014 20:41, Peter Zijlstra <peterz@infradead.org> wrote: > On Tue, Sep 30, 2014 at 10:41:08AM +0200, Vincent Guittot wrote: >> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c >> index 2a1e6ac..adad532 100644 >> --- a/kernel/sched/fair.c >> +++ b/kernel/sched/fair.c >> @@ -6425,13 +6425,14 @@ static struct sched_group *find_busiest_group(struct lb_env *env) >> >> if (env->idle == CPU_IDLE) { >> /* >> - * This cpu is idle. If the busiest group load doesn't >> - * have more tasks than the number of available cpu's and >> - * there is no imbalance between this and busiest group >> - * wrt to idle cpu's, it is balanced. >> + * This cpu is idle. If the busiest group is not overloaded >> + * and there is no imbalance between this and busiest group >> + * wrt to idle cpus, it is balanced. The imbalance becomes >> + * significant if the diff is greater than 1 otherwise we >> + * might end up to just move the imbalance on another group >> */ >> - if ((local->idle_cpus < busiest->idle_cpus) && >> - busiest->sum_nr_running <= busiest->group_weight) >> + if ((local->idle_cpus <= (busiest->idle_cpus + 1)) && > > So I'm thick and I don't get this one.. In fact I don't seem to > understand the existing code either.
My understand of the original code is that if a group is overloaded (wrt capacity_factor) but has less tasks than CPUs (so overloaded because of rt) and the local group has more idle CPUs then it's worth balancing tasks and load.
I have changed it into : if the busiest group is overloaded or the local has more than 1 idle CPU than the busiest, it makes sense to try to balance tasks in order to balance the avg_load of the groups. But if the local group has only 1 more idle CPU than the busiest, it's probably not possible to leverage the average load load of the groups. We will only move the imbalance from 1 group to another one
> > If we're idle, and busiest is overloaded, we want to have tasks. Why > would we care about number of idle cpus etc.. > >> + !(busiest->group_type == group_overloaded)) > > Would not: busiest->group_type != group_overloaded, read more natural? > Also, would it make sense to make this the first condition?
that's fair for both remark
> >> goto out_balanced; >> } else { > >
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