On Mon, Jan 28, 2013 at 10:55:52AM -0800, Tejun Heo wrote: > 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.
Yeah, I tend to agree. > > 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. Could do that too, but then teardown gets really messy for the user - we need two synchronize_rcu()s: state := dying synchronize_rcu() /* Now nothing's changing the per cpu counters */ Add per cpu counters to atomic counter counter /* Atomic counter is now consistent */ state := dead synchronize_rcu() /* Now percpu_ref_put will check for ref == 0 */ /* Drop initial ref */ percpu_ref_put() And note that the first synchronize_rcu() is only needed when we had allocated per cpu counters, my current code skips it otherwise. (which is a reason for keeping dynamic allocation I hadn't thought of - if we don't ever allocate percpu counters, teardown is faster) -- 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/
