Re: [PATCH v6] sched: fix init NOHZ_IDLE flag

[Peter Zijlstra <peterz@infradead.org>]

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.