Hey, Kent. On Mon, Jan 28, 2013 at 10:49:33AM -0800, Kent Overstreet wrote: > Yeah. It'd be really nice if it was doable without synchronize_rcu(), > but it'd definitely make get/put heavier. > > Though, re. close() - considering we only need a synchronize_rcu() if > the ref was in percpu mode, I wonder if that would be a dealbreaker. I > have no clue myself.
The problem is that the performance drop (or latency increase) in patheological cases would be catastrophic. We're talking about possibly quite a few millisecs of delay between each close(). When done sequentially for large number of files, it gets ugly. It becomes a dangerous optimization to make. > Getting rid of synchronize_rcu would basically require turning get and > put into cmpxchg() loops - even in the percpu fastpath. However, percpu > mode would still be getting rid of the shared cacheline contention, we'd > just be adding another branch that can be safely marked unlikely() - and > my current version has one of those already, so two branches instead of > one in the fast path. Or offer an asynchrnous interface so that high-frequency users don't end up inserting synchronize_sched() between each call. It makes the interface more complex and further away from simple atomic_t replacement tho. Thanks. -- tejun -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
