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 10:22 -0400, Rik van Riel wrote: 
> On 05/09/2014 03:34 AM, Mike Galbraith wrote:
> > 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.
> 
> That's not the problem.
> 
> The problem is that if we do not do an affine wakeup, due to
> SD_WAKE_AFFINE not being set on a top level domain, we will
> not try to run p on prev_cpu, but we will fall through into
> the loop with find_idlest_group, etc...

If no ->flags & SD_BALANCE_WAKE is encountered during traversal, sd
remains NULL, we fall through to return prev_cpu. 

-Mike



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