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SubjectRe: [RFCv3 PATCH 44/48] sched: Tipping point from energy-aware to conventional load balancing
On 24/03/15 15:26, Peter Zijlstra wrote:
> On Wed, Feb 04, 2015 at 06:31:21PM +0000, Morten Rasmussen wrote:
>> From: Dietmar Eggemann <dietmar.eggemann@arm.com>
>>
>> Energy-aware load balancing bases on cpu usage so the upper bound of its
>> operational range is a fully utilized cpu. Above this tipping point it
>> makes more sense to use weighted_cpuload to preserve smp_nice.
>> This patch implements the tipping point detection in update_sg_lb_stats
>> as if one cpu is over-utilized the current energy-aware load balance
>> operation will fall back into the conventional weighted load based one.
>>
>> cc: Ingo Molnar <mingo@redhat.com>
>> cc: Peter Zijlstra <peterz@infradead.org>
>>
>> Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
>> ---
>> kernel/sched/fair.c | 4 ++++
>> 1 file changed, 4 insertions(+)
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index 6b79603..4849bad 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -6723,6 +6723,10 @@ static inline void update_sg_lb_stats(struct lb_env *env,
>> sgs->sum_weighted_load += weighted_cpuload(i);
>> if (idle_cpu(i))
>> sgs->idle_cpus++;
>> +
>> + /* If cpu is over-utilized, bail out of ea */
>> + if (env->use_ea && cpu_overutilized(i, env->sd))
>> + env->use_ea = false;
>> }
>
> I don't immediately see why this is desired. Why would a single
> overloaded CPU be reason to quit? It could be the cpus simply aren't
> 'balanced' right and the group as a whole is still under utilized.

We want to play it safe here.

E.g. in a >2 cluster system, this over-utilized cpu could run >1 high
priority tasks on a cluster with energy efficient cpus and this cluster
could still not be the lb src on DIE level because a not over-utilized
cluster with less energy-efficient cpus (burning more energy) could be
chosen instead. We could construct cases where the other cpus in this
energy efficient cluster can't help the over-utilized cpu during lb on
MC level.

I can see that using per-cpu data in code which deals w/ sg's is against
the sd scalability design where we should rely on per-sg and not per-cpu
data though.

By bailing out in such a scenario we at least guarantee smpnice provided
by conv. CFS.

We could also favor an sg with an over-utilized cpu to become the src
but which one do we pick if there're multiple potential src sg's w/ an
over-utilized cpu?

>
> In that case we want to continue the balance pass to reach this
> equilibrium.
>



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