Re: [PATCH] sched: wake up task on prev_cpu if not in SD_WAKE_AFFINE domain with cpu

[Mike Galbraith]

On Fri, 2014-05-09 at 01:27 -0400, Rik van Riel wrote: 
> On Thu, 08 May 2014 22:20:25 -0400
> Rik van Riel <riel@xxxxxxxxxx> wrote:
> 
> > Looks like SD_BALANCE_WAKE is not gotten from the sd flags at
> > all, but passed into select_task_rq by try_to_wake_up, as a
> > hard coded sd_flags argument.
> 
> > Should we do that, if SD_WAKE_BALANCE is not set for any sched domain?
> 
> I answered my own question. The sd_flag SD_WAKE_BALANCE simply means
> "this is a wakeup of a previously existing task, please place it
> properly".
> 
> However, it appears that the current code will fall back to the large
> loop with select_idlest_group and friends, if prev_cpu and cpu are not
> part of the same SD_WAKE_AFFINE sched domain. That is a bug...

ttwu(): cpu = select_task_rq(p, p->wake_cpu, SD_BALANCE_WAKE, wake_flags);

We pass SD_BALANCE_WAKE for a normal wakeup, so sd will only be set if
we encounter a domain during traversal where Joe User has told us to do
(expensive) wake balancing before we hit a domain shared by waker/wakee.

The user can turn SD_WAKE_AFFINE off beyond socket, and we'll not pull
cross node on wakeup.

Or, you could create an override button to say despite SD_WAKE_AFFINE
perhaps having been set during domain construction (because of some
pseudo-random numbers), don't do that if we have a preferred node, or
just make that automatically part of having numa scheduling enabled, and
don't bother wasting cycles if preferred && this != preferred.

-Mike  

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