On 1 June 2016 at 03:03, Yuyang Du <yuyang...@intel.com> wrote: > 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.
Why should we not do anything else ? The current default configuration is to only use the wake_affine path. With your changes, the default configuration will try to use wake affine and will fall back to long load balance sequence if wake affine doesn't find a sched_domain That's a major changes in the behavior > > So I think this is a combined case better than either of the "only > wake_affine" > or "only full" cases. Make sense?
