From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1946459Ab3BHOoT (ORCPT ); Fri, 8 Feb 2013 09:44:19 -0500 Received: from mail-da0-f46.google.com ([209.85.210.46]:58996 "EHLO mail-da0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753194Ab3BHOoS (ORCPT ); Fri, 8 Feb 2013 09:44:18 -0500 Date: Fri, 8 Feb 2013 06:44:08 -0800 From: Tejun Heo To: Kent Overstreet Cc: linux-kernel@vger.kernel.org, linux-aio@kvack.org, linux-fsdevel@vger.kernel.org, zab@redhat.com, bcrl@kvack.org, jmoyer@redhat.com, axboe@kernel.dk, viro@zeniv.linux.org.uk, tytso@mit.edu, Andrew Morton Subject: Re: [PATCH 23/32] Generic dynamic per cpu refcounting Message-ID: <20130208144408.GY2875@htj.dyndns.org> References: <1356573611-18590-1-git-send-email-koverstreet@google.com> <1356573611-18590-26-git-send-email-koverstreet@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1356573611-18590-26-git-send-email-koverstreet@google.com> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org (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 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? Thanks. -- tejun