On Wed, Jun 01, 2016 at 10:32:53AM +0200, Vincent Guittot wrote: > > Yup. Up to this point, we don't have any disagreement. And I don't think we > > have any disagreement conceptually. What the next patch really does is: > > > > (1) we don't remove SD_BALANCE_WAKE as an important sched_domain flag, on > > the contrary, we strengthen it. > > > > (2) the semantic of SD_BALANCE_WAKE is currently represented by > > SD_WAKE_AFFINE, > > we actually remove this representation. > > > > (3) regarding the semantic of SD_WAKE_AFFINE, it is really not about > > selecting > > waker CPU or about the fast path. Conceptually, it is just saying the > > waker > > CPU is a valid and important candidate if SD_BALANCE_WAKE, which is > > just so > > obvious, so I don't think it deserves to be a separate sched_domain > > flag. > > > > (4) the outcome is, if SD_BALANCE_WAKE, we definitely will/should try waker > > CPU, > > and if !SD_BALANCE_WAKE, we don't try waker CPU. So nothing functional > > is > > changed. > > > AFAIU, there is 4 possible cases during wake up: > - we don't want any balance at wake so we don't have SD_BALANCE_WAKE > nor SD_WAKE_AFFINE in sched_domain->flags > - we only want wake affine balance check so we only have > SD_WAKE_AFFINE in sched_domain->flags > - we want wake_affine and full load balance at wake so we have both > SD_BALANCE_WAKE and SD_WAKE_AFFINE in sched_domain->flags > - we want full load balance but want to skip wake affine fast path so > we only have SD_BALANCE_WAKE in sched_domain->flags > > I'm not sure that we can still do only wake_affine or only full > load_balance with your changes whereas these sequences are valid ones
So with the patch, we will have a little bit semantic change, SD_BALANCE_WAKE implies SD_WAKE_AFFINE if allowed, and will favor "fast path" if possible. I don't think we should do anything otherwise. So I think this is a combined case better than either of the "only wake_affine" or "only full" cases. Make sense?
