On Fri, Feb 08, 2013 at 06:44:08AM -0800, Tejun Heo wrote: > (cc'ing Andrew) > > On Wed, Dec 26, 2012 at 06:00:02PM -0800, Kent Overstreet 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. > > > > Signed-off-by: Kent Overstreet <koverstreet@xxxxxxxxxx> > > What's the status of this series? The percpu-refcnt part is still > going through review and the merge window is opening up pretty soon. > Kent, Andrew? I don't think the percpu-refcount stuff should be blocking, the existing code in Andrew's tree isn't ideal w.r.t. the api but it is stable and tested. I have patches on top of that that incorporate most of your ideas and are definite improvements, I've just been holding off on them because I want to come up with a good torture test (mainly because of the bias mechanism for teardown). -- 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
