On 22 April 2013 11:30, Peter Zijlstra <[email protected]> wrote: > On Fri, 2013-04-19 at 15:10 +0200, Vincent Guittot wrote: >> As suggested by Frederic Weisbecker, another solution is to have the >> same >> rcu lifecycle for both NOHZ_IDLE and sched_domain struct. I have >> introduce >> a new sched_domain_rq struct that is the entry point for both >> sched_domains >> and objects that must follow the same lifecycle like NOHZ_IDLE flags. >> They >> will share the same RCU lifecycle and will be always synchronized. >> >> The synchronization is done at the cost of : >> - an additional indirection for accessing the first sched_domain >> level >> - an additional indirection and a rcu_dereference before accessing to >> the >> NOHZ_IDLE flag. > >> diff --git a/include/linux/sched.h b/include/linux/sched.h >> index d35d2b6..61ad5f1 100644 >> --- a/include/linux/sched.h >> +++ b/include/linux/sched.h >> @@ -959,6 +959,18 @@ struct sched_domain { >> unsigned long span[0]; >> }; >> >> +/* >> + * Some flags must stay synchronized with fields of sched_group_power >> and as a >> + * consequence they must follow the same lifecycle for the lockless >> scheme. >> + * sched_domain_rq encapsulates those flags and sched_domains in one >> RCU >> + * object. >> + */ >> +struct sched_domain_rq { >> + struct sched_domain *sd; >> + unsigned long flags; >> + struct rcu_head rcu; /* used during destruction */ >> +}; > > 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. 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 > > 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 > -- 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/
