Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757759Ab3EOJIZ (ORCPT ); Wed, 15 May 2013 05:08:25 -0400 Received: from mail-pd0-f182.google.com ([209.85.192.182]:45139 "EHLO mail-pd0-f182.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757238Ab3EOJIS (ORCPT ); Wed, 15 May 2013 05:08:18 -0400 Date: Wed, 15 May 2013 02:07:42 -0700 From: Kent Overstreet To: Tejun Heo Cc: linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-aio@kvack.org, akpm@linux-foundation.org, Zach Brown , Felipe Balbi , Greg Kroah-Hartman , Mark Fasheh , Joel Becker , Rusty Russell , Jens Axboe , Asai Thambi S P , Selvan Mani , Sam Bradshaw , Jeff Moyer , Al Viro , Benjamin LaHaise , Oleg Nesterov , Christoph Lameter , Ingo Molnar Subject: Re: [PATCH 04/21] Generic percpu refcounting Message-ID: <20130515090742.GD16164@moria.home.lan> References: <1368494338-7069-1-git-send-email-koverstreet@google.com> <1368494338-7069-5-git-send-email-koverstreet@google.com> <20130514215945.GA2334@mtj.dyndns.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20130514215945.GA2334@mtj.dyndns.org> 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 Content-Length: 2630 Lines: 84 On Tue, May 14, 2013 at 02:59:45PM -0700, Tejun Heo wrote: > A couple more things. > > On Mon, May 13, 2013 at 06:18:41PM -0700, Kent Overstreet wrote: > ... > > +/** > > + * percpu_ref_put - decrement a dynamic percpu refcount > > + * > > + * Returns true if the result is 0, otherwise false; only checks for the ref > > + * hitting 0 after percpu_ref_kill() has been called. Analagous to > > + * atomic_dec_and_test(). > > + */ > > +static inline int percpu_ref_put(struct percpu_ref *ref) > > bool? Was int to match atomic_dec_and_test(), but switching to bool. > > > +{ > > + unsigned __percpu *pcpu_count; > > + int ret = 0; > > + > > + preempt_disable(); > > + > > + pcpu_count = ACCESS_ONCE(ref->pcpu_count); > > + > > + if (pcpu_count) > > We probably want likely() here. Yeah, I suppose so. > > > + __this_cpu_dec(*pcpu_count); > > + else > > + ret = atomic_dec_and_test(&ref->count); > > + > > + preempt_enable(); > > + > > + return ret; > > With likely() added, I think the compiler should be able to recognize > that the branch on pcpu_count should exclude later branch in the > caller to test for the final put in most cases but I'm a bit worried > whether that would always be the case and wonder whether ->release > based interface would be better. Another concern is that the above > interface is likely to encourage its users to put the release > implementation in the same function. e.g. I... don't follow what you mean hear at all - what exactly would the compiler do differently? and how would passing a release function matter? > void my_put(my_obj) > { > if (!percpu_ref_put(&my_obj->ref)) > return; > destroy my_obj; > free my_obj; > } > > Which in turn is likely to nudge the developer or compiler towards not > inlining the fast path. I'm kind of skeptical partial inlining would be worth it for just an atomic_dec_and_test()... > So, while I do like the simplicity of put() returning %true on the > final put, I suspect it's more likely to slowing down fast paths due > to its interface compared to having separate ->release function > combined with void put(). Any ideas? Oh, you mean having one branch instead of two when we're in percpu mode. Yeah, that is a good point. I bet with the likely() added the compiler is going to generate the same code either way, but I suppose I can have a look at what gcc actually does... -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/