Re: [PATCH 5/6] sched, fair: Make group power more consitent

[Preeti U Murthy <preeti@linux.vnet.ibm.com>]

Hi Peter,
Thank you for the clarification.

On 08/19/2013 04:00 PM, Peter Zijlstra wrote:
> On Mon, Aug 19, 2013 at 09:47:47AM +0530, Preeti U Murthy wrote:
>> Hi Peter,
>>
>> On 08/16/2013 03:42 PM, Peter Zijlstra wrote:
>>
>> I have a few comments and clarification to seek.
>>
>> 1. How are you ensuring from this patch that sgs->group_power does not
>> change over the course of load balancing?
> 
> Well, we only set it the one time when creating the sgs data.
> 
>>     The only path to update_group_power() where sg->sgp->power gets
>> updated, is from update_sg_lb_stats(). You are updating sgs->group_power
>> in update_sg_lb_stats(). Any change to group->sgp->power will get
>> reflected in sgs->group_power as well right?
> 
> Nope, we set it to whatever value group->sgp->power is at the time of
> sgs 'creation' and live with that value from then on. We do this after
> the possible update_group_power() call.
> 
> That said, it is very rare to have group->sgp->power change during the
> load-balance pass, we would have to trigger the time_after case for
> NEWLY_IDLE and then get a concurrent !NEWLY_IDLE load-balance pass.
> 
> This patch takes away that possibility and uses a consistent group power
> reading for the entire load-balance invocation as well as does away with
> that double dereference all the time.

Fair enough. I overlooked the fact that "group" can be manipulated by
multiple load balancing passes whereas "sgs" is created during every
load balance pass.

> 
>> 2. This point is aside from your patch. In the current implementation,
>> each time the cpu power gets updated in update_cpu_power(), should not
>> the power of the sched_groups comprising of that cpu also get updated?
>> Why wait till the load balancing is done at the sched_domain level of
>> that group, to update its group power?
> 
> What would be the advantage of doing so? We also take snapshots of
> cpu/group/domain load, we don't update those either. Having all that
> dynamically update during the load-balance pass would make it an
> impossible situation. 
> 
> You'd fail to meet progress guarantees that way since you'd never be
> able to pin-point a 'busiest' group/cpu because by the time you've made
> any decision you have to go back to make it again because things might
> have changed again.
> 
> So instead what we do is we take a snapshot and live with that state. If
> the values change so fast that our load-balance pass is invalid by the
> time we're finished, too bad, better luck next time.

Agree. Thank you for the clarifications.

Regards
Preeti U Murthy
> 
>