Re: [PATCH] sched/fair: Do not decay new task load on first enqueue

From: Dietmar Eggemann
Date: Thu Sep 29 2016 - 12:15:32 EST

On 28/09/16 14:13, Vincent Guittot wrote:
> Le Wednesday 28 Sep 2016 à 05:27:54 (-0700), Vincent Guittot a écrit :
>> On 28 September 2016 at 04:31, Dietmar Eggemann
>> <dietmar.eggemann@xxxxxxx> wrote:
>>> On 28/09/16 12:19, Peter Zijlstra wrote:
>>>> On Wed, Sep 28, 2016 at 12:06:43PM +0100, Dietmar Eggemann wrote:
>>>>> On 28/09/16 11:14, Peter Zijlstra wrote:
>>>>>> On Fri, Sep 23, 2016 at 12:58:08PM +0100, Matt Fleming wrote:


> IIUC the problem raised by Matt, he see a regression because we now remove
> during the dequeue the exact same load as during the enqueue so
> cfs_rq->runnable_load_avg is null so we select a cfs_rq that might already have
> a lot of hackbench blocked thread.

This is my understanding as well.

> The fact that runnable_load_avg is null, when the cfs_rq doesn't have runnable
> task, is quite correct and we should keep it. But when we look for the idlest
> group, we have to take into account the blocked thread.
> That's what i have tried to do below


> + /*
> + * In case that we have same runnable load (especially null
> + * runnable load), we select the group with smallest blocked
> + * load
> + */
> + min_avg_load = avg_load;
> + min_runnable_load = runnable_load;

Setting 'min_runnable_load' wouldn't be necessary here.

> idlest = group;
> }
> +
> } while (group = group->next, group != sd->groups);
> - if (!idlest || 100*this_load < imbalance*min_load)
> + if (!idlest || 100*this_load < imbalance*min_runnable_load)
> return NULL;
> return idlest;

On the Hikey board (ARM64) (2 cluster, each 4 cpu's, so MC and DIE), the
first f_i_g (on DIE) is still based on rbl_load. So if the first
hackbench task (spawning all the worker task) runs on cluster1, and the
former worker p_X already blocks f_i_g returns cluster2, if p_X still
runs, it returns idlest=NULL and we continue with cluster1 for second
f_i_g on MC.

The additional 'else if' condition doesn't seem to help much because of
occurrences where an idle cpu (which never took a worker) still has a
small value of rbl_load (shouldn't actually happen, weighted_cpuload()
should be 0) so it is never chosen or it has even a negative impact in
the case where an idle cpu (which never took a worker) is not chosen
because its load (cfs->avg.load_avg) hasn't been updated for a long time
so another cpu with rbl_load = 0 and a smaller load is used (even though
a lot of worker where already placed on it).

There are also episodes where we 'pack' workers onto the cpu which is
initially picked in f_i_c (on DIE) because (100*this_load <
imbalance*min_load) is true in f_i_g on MC. Maybe we can get rid of this
for !sd->child ?