On Wed, Jun 01, 2016 at 11:36:39AM +0200, Mike Galbraith 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. > > Nope, those two have different meanings. We pass SD_BALANCE_WAKE to > identify a ttwu() wakeup, just as we pass SD_BALANCE_FORK to say we're > waking a child. SD_WAKE_AFFINE means exactly what it says, but is only > applicable to ttwu() wakeups. I don't disagree, but want to add that, SD_WAKE_AFFINE has no meaning that is so special and so important for anyone to use the flag to tune anything. If you want to do any SD_BALANCE_*, waker CPU is a valid candidate if allowed, that is it.
IIUC your XXX mark and your comment "Prefer wake_affine over balance flags", you said the same thing: SD_WAKE_AFFINE should be part of SD_BALANCE_WAKE, and should be part of all SD_BALANCE_* flags, > > (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. > > SD_WAKE_AFFINE being a separate domain flag, the user can turn it > on/off... separately :) Sure, that is very true, :) But turning it off for what, that is a big question mark. We don't want a flag unless the flag is for goodness, and not a flag with big question mark. > > (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. > > If wake_wide() says we do not want an affine wakeup, but you apply > SD_WAKE_AFFINE meaning to SD_BALANCE_WAKE and turn it on in ->flags, > we'll give the user a free sample of full balance cost, no? Yes, and otherwise we don't select anything? That is just bad engough whether worse or not. So the whole fuss I made is really that this is a right thing to start with. :)
