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. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
