On Wed, 26 Dec 2012 18:00:02 -0800 Kent Overstreet <koverstreet@xxxxxxxxxx> wrote: > This implements a refcount with similar semantics to > atomic_get()/atomic_dec_and_test(), that starts out as just an atomic_t > but dynamically switches to per cpu refcounting when the rate of > gets/puts becomes too high. > > It also implements two stage shutdown, as we need it to tear down the > percpu counts. Before dropping the initial refcount, you must call > percpu_ref_kill(); this puts the refcount in "shutting down mode" and > switches back to a single atomic refcount with the appropriate barriers > (synchronize_rcu()). > > It's also legal to call percpu_ref_kill() multiple times - it only > returns true once, so callers don't have to reimplement shutdown > synchronization. > > For the sake of simplicity/efficiency, the heuristic is pretty simple - > it just switches to percpu refcounting if there are more than x gets > in one second (completely arbitrarily, 4096). > > It'd be more correct to count the number of cache misses or something > else more profile driven, but doing so would require accessing the > shared ref twice per get - by just counting the number of gets(), we can > stick that counter in the high bits of the refcount and increment both > with a single atomic64_add(). But I expect this'll be good enough in > practice. I still don't "get" why this code exists. It is spectacularly, stunningly undocumented and if someone were to ask me "under what circumstances should I use percpu-refcount", I would not be able to help them. -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
