Messages in this thread | | | Date | Mon, 22 Apr 2013 14:05:59 +0200 | Subject | Re: [PATCH v6] sched: fix init NOHZ_IDLE flag | From | Vincent Guittot <> |
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On 22 April 2013 13:39, Peter Zijlstra <peterz@infradead.org> wrote: > 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.
Ok, I'm going to move the flags in sched_domain struct. This should make the fix simpler
> >> > 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. >
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