On Mon, 2013-04-22 at 13:01 +0200, Vincent Guittot wrote:
I'm not quite getting things.. what's wrong with adding this flags thing to sched_domain itself? That's already RCU destroyed so why
add a
second RCU layer?
We need one flags for all sched_domain so if we add it into sched_domain struct, we have to define which one will handle the flags for all other and find it in the sched_domain tree when we need it.
Just pick rq->sd -- if the root_domain thing doesn't work out.
In addition, the flags in other sched_domain will be a waste of space. The RCU in sched_domain might become useless as it is protected by the one that is in sched_domain_rq
I'm all for wasting space instead over adding extra pointer chasing all over the place. But also, look at pahole -C sched_domain, there's plenty of 4 byte holes in there where we can stuff a single bit.
We also have the root_domain for things that don't need to go in a hierarchy but are once per cpu -- it sounds like this is one of
those
things; iirc the root_domain life-time is the same as the entire sched_domain tree so adding it to the root_domain is also an option.
AFAICT, it doesn't share the same RCU object and as a result the same lifecycle than sched_domain so there is a time window where sched_domain and flags could lost their synchronization. Nevertheless, i'm going to have a look at root_domain
They're set under the same write side lock at the same time rq->sd it set, but yes I suppose that since its a separate pointer there might be a tiny window where we could go wrong.