Re: [PATCH 03/14] sched: pack small tasks

[Vincent Guittot <vincent.guittot@linaro.org>]

On 26 April 2013 14:30, Peter Zijlstra <peterz@infradead.org> wrote:
> On Thu, Apr 25, 2013 at 07:23:19PM +0200, Vincent Guittot wrote:
>> During the creation of sched_domain, we define a pack buddy CPU for each CPU
>> when one is available. We want to pack at all levels where a group of CPUs can
>> be power gated independently from others.
>> On a system that can't power gate a group of CPUs independently, the flag is
>> set at all sched_domain level and the buddy is set to -1. This is the default
>> behavior.
>>
>> On a dual clusters / dual cores system which can power gate each core and
>> cluster independently, the buddy configuration will be :
>>
>>       | Cluster 0   | Cluster 1   |
>>       | CPU0 | CPU1 | CPU2 | CPU3 |
>> -----------------------------------
>> buddy | CPU0 | CPU0 | CPU0 | CPU2 |
>>
>> If the cores in a cluster can't be power gated independently, the buddy
>> configuration becomes:
>>
>>       | Cluster 0   | Cluster 1   |
>>       | CPU0 | CPU1 | CPU2 | CPU3 |
>> -----------------------------------
>> buddy | CPU0 | CPU1 | CPU0 | CPU0 |
>>
>> Small tasks tend to slip out of the periodic load balance so the best place
>> to choose to migrate them is during their wake up. The decision is in O(1) as
>> we only check again one buddy CPU
>
>
> So I really don't get the point of this buddy stuff, even for light load non
> performance impact stuff you want to do.
>
> The moment you judge cpu0 busy you'll bail, even though its perfectly doable
> (and desirable afaict) to continue stacking light tasks on cpu1 instead of
> waking up cpu2/3.
>
> So what's wrong with keeping a single light-wake target cpu selection and
> updating it appropriately?

I have tried to follow the same kind of tasks migration as during load
balance: 1 CPU in a power domain group migrates tasks with other
groups.

>
> Also where/how does the nohz balance cpu criteria not match the light-wake
> target criteria?

The nohz balance cpu is an idle cpu but it doesn't mean that it's the
target cpu which will be sometime busy with the light tasks.